NAVTEX is a maritime radio warning system consisting of a series of coast stations transmitting radio teletype (CCIR Recommendation 476 standard narrow band direct printing, sometimes called Sitor or ARQ/FEC) safety messages on the international standard medium frequency 518 kHz. Coast stations transmit during preset time slots so as to minimize interference with one another. Routine messages are normally broadcast four to six times daily. Urgent messages are broadcast upon receipt, provided that an another station is not transmitting. Since the broadcast uses the medium frequency band, a typical station service radius ranges from 100-500 NM day and night. Interference from or receipt of stations farther away occasionally occurs at night.
Each NAVTEX message broadcast contains a four-character header describing identification of station (first character), message content (second character), and message serial number (third and fourth characters). This header allows the microprocessor in the shipborne receiver to screen messages, selecting only those stations relevant to the user, messages of subject categories needed by the user, and messages not previously received by the user. Selected messages are printed on a roll of paper as received, to be read by the mariner at his convenience. Unwanted messages are suppressed. Suppression of unwanted messages is more and more important to the mariner as the number of messages, including rebroadcasts, increases yearly. With NAVTEX, a mariner will no longer have to listen to, or sift through, a large number of irrelevant data to obtain the information necessary for safe navigation.
Vessels regulated by the Safety of Life at Sea (SOLAS) Convention, as amended in 1988 (cargo vessels over 300 tons and passenger vessels, on international voyages), and operating in areas where NAVTEX service is available, have been required to carry NAVTEX receivers since 1993. The USCG voice broadcasts (Ch. 22A), of more inshore and harbor information, will remain unaffected by NAVTEX. Mariners not able to man a radio on a 24-hour basis in order to hear critical warning messages, commercial fishermen should also find a advantage in owning a NAVTEX receiver. NAVTEX coverage is reasonably continuous to 200nm off the U.S East, Gulf, and West Coast, Puerto Rico, Southwest Alaska, Hawaii, and 300 - 400 nm off Guam.
Showing posts with label ELECTRONICS. Show all posts
Showing posts with label ELECTRONICS. Show all posts
Friday, February 15, 2008
Saturday, December 1, 2007
MARINE ELECTRONICS (STEERING SYSTEMS)
AUTOMATIC STEERING SYSTEMS
Most vessels are equipped with some form of automatic steering system which takes input from the master gyro through the repeater system. In some cases, magnetic compasses have been adapted to provide a type of directional signal for use with automatic steering. The auto pilot may be a separate system or connected as part of the electric steering unit. Electric steering units, in turn, may utilize a single helm with dual operational systems for the rudder; they may also supplement an existing hydraulic system.
Generally the steering system, when in automatic mode, accepts signals from the repeater network and compensates for the difference between the required course and actual heading. The more sophisticated the system is, the more varied the performance controls will be. Most basic systems contain a weather adjustment that will slow motion of the of the rudder in heavy seas so the steering system is not overworlwd. There will also be a rudder adjustment to limit the amount of rudder adjustment in returning the vessel to course. Other performance controls may include a speed adjustment or a light loaded vessel adjustment.
Gyro pilots, which are part of the primary electric steering system will also have additional features that should be recognized. They include a power failure alarm for the steering unit or rudder control systems. A system selector to switch between the port and starboard rudder control systems will also be available. Various course indicators may be in the form of an open scale repeater or digital repeater as well as a rudder order indicator. Some gyro pilots have a non-follow-up control system (NFU) capable of direct control of the rudder without use of a wheel. This unit is commonly identified in the form of a remote station containing a joystick that can be placed anywhere in the vessel.
Most vessels are equipped with some form of automatic steering system which takes input from the master gyro through the repeater system. In some cases, magnetic compasses have been adapted to provide a type of directional signal for use with automatic steering. The auto pilot may be a separate system or connected as part of the electric steering unit. Electric steering units, in turn, may utilize a single helm with dual operational systems for the rudder; they may also supplement an existing hydraulic system.
Generally the steering system, when in automatic mode, accepts signals from the repeater network and compensates for the difference between the required course and actual heading. The more sophisticated the system is, the more varied the performance controls will be. Most basic systems contain a weather adjustment that will slow motion of the of the rudder in heavy seas so the steering system is not overworlwd. There will also be a rudder adjustment to limit the amount of rudder adjustment in returning the vessel to course. Other performance controls may include a speed adjustment or a light loaded vessel adjustment.
Gyro pilots, which are part of the primary electric steering system will also have additional features that should be recognized. They include a power failure alarm for the steering unit or rudder control systems. A system selector to switch between the port and starboard rudder control systems will also be available. Various course indicators may be in the form of an open scale repeater or digital repeater as well as a rudder order indicator. Some gyro pilots have a non-follow-up control system (NFU) capable of direct control of the rudder without use of a wheel. This unit is commonly identified in the form of a remote station containing a joystick that can be placed anywhere in the vessel.
MARINE ELECTRONICS (GYRO SYSTEMS)
GYRO REPEATER SYSTEMS
Most modern vessels carry two master gyros providing backup should one of the units fail. Output from the two master units goes to a selector switch which can be operated by the deck officer to select between units. Normally, the master units are switched over daily to ensure that the repeater system works correctly on either master gyro.
From the selector switch or, if the system has a single master gyro, the transmitter, signals go to the transmission amplifier/distribution control. This unit has each repeater on a separate circuit, is normally switched and fused, and provides sufficient power to drive all the vessel's repeaters. Repeaters are located not only in bearing or steering units near or on the vessel's bridge, but also in radars, radio direction finders, automatic steering systems, course recorders, and other similar units. The distribution system also provides input to radionavigation equipment requiring course data information.
There are two common types of repeater systems. The first is the step system, which produces segmented signals driving a repeater motor in three separate "steps." This system is generally associated with the older type of gyro systems. The second is the newer synchro system. This uses a different voltage requirement from the step system with a synchro style operating motor. The movement ofthis type repeater is truer to, the movement of the master compass.
The actual repeaters are found in various forms, including the standard repeater, which exhibits the entire azimuth ring, or the open scale repeater, which shows a larger, but only partial, section of the azimuth ring. The open scale repeater is most commonly used as a steering repeater. Signals from the repeater system can also be utilized to provide other types of heading information such as digital readouts used in radars or radionavigation receivers and graphic displays used in course recorders.
There are two familiar types of course recorders. The older style utilizes an eight-day wind-up clock mechanism with two ink pens, one showing the compass quadrant and the other showing the course. The newer type uses an electrically driven clock mechanism with two electric styluses. This graph mechanism works similarly to the fathometer in that special graph paper is drawn over a metal desk and an electrical signal scores the graph paper to show the quadrant and course.
There are several important points that the watch stander should keep in mind when using the gyro system:
1. Ensure that all repeaters are aligned with the master gyro and if there is a dual gyro system aboard, ensure that any difference in their readings is constantly noted.
2. Azimuths for determining compass error should be taken as frequently as permissible, and the information applied to both master compasses if a dual gyro system is installed.
3. All master gyros are equipped with speed and latitude correctors.
These are necessary because the directive force of the gyro decreases as you get nearer the poles. There is also an error introduced in the master gyro by the vessel's motion along its track. Each master gyro has specific parameters for setting these correctors, but generally they should be kept within 3 degrees oflatitude and 3 knots of speed.
4. When switching from one gyro system to another be sure your repeaters are lined up with the master gyro you are switching to before changing over. Be sure to inspect all equipment frequently. Every system has a manual that details maintenance responsibilities for the unit. Be sure to follow these maintenance plans as required.
5. Errors in the gyro system can be introduced by a ship's fluctuating power supply. The gyro system should be on emergency circuits. If extensive variations in the power source are noted be sure to inform the vessel's engineer so that it may be corrected as soon as possible.
6. Although most manufacturers recommend starting a gyro within only a few hours of use, the prudent operator will bring the system on line in order to provide as much time as possible for problems to become noticeable. The gyro will hunt or oscillate prior to settling out so the gyro should be started as close to the vessel's heading as possible to minimize this settling time. If the vessel is located at the dock, you can get the vessel's heading from a chart. Be sure to set the speed corrector at zero when tied up. Starting the gyro at sea should also be done as close to the heading as possible with the speed corrector set accordingly. Remember that the axis of the rotor aligns itself with the geographic meridian and the gyro is not necessarily north seeking. If you start the gyro up 90 degrees or more from the vessel's heading, your gyro may have a l80-degree error. Every gyro has a specified start-up and shut-down procedure which should be followed. This information is available in the unit's operating manual.
7. Small constant errors in a gyro may be removed by moving the master compass lubber line or shifting the binnacle housing, depending on the style ofthe unit.
8. Be sure to remove the gyro's locking latches or uncage the unit and set its controls in the proper operating mode.
9. Each gyro system has a failure alarm connected with it.
Most modern vessels carry two master gyros providing backup should one of the units fail. Output from the two master units goes to a selector switch which can be operated by the deck officer to select between units. Normally, the master units are switched over daily to ensure that the repeater system works correctly on either master gyro.
From the selector switch or, if the system has a single master gyro, the transmitter, signals go to the transmission amplifier/distribution control. This unit has each repeater on a separate circuit, is normally switched and fused, and provides sufficient power to drive all the vessel's repeaters. Repeaters are located not only in bearing or steering units near or on the vessel's bridge, but also in radars, radio direction finders, automatic steering systems, course recorders, and other similar units. The distribution system also provides input to radionavigation equipment requiring course data information.
There are two common types of repeater systems. The first is the step system, which produces segmented signals driving a repeater motor in three separate "steps." This system is generally associated with the older type of gyro systems. The second is the newer synchro system. This uses a different voltage requirement from the step system with a synchro style operating motor. The movement ofthis type repeater is truer to, the movement of the master compass.
The actual repeaters are found in various forms, including the standard repeater, which exhibits the entire azimuth ring, or the open scale repeater, which shows a larger, but only partial, section of the azimuth ring. The open scale repeater is most commonly used as a steering repeater. Signals from the repeater system can also be utilized to provide other types of heading information such as digital readouts used in radars or radionavigation receivers and graphic displays used in course recorders.
There are two familiar types of course recorders. The older style utilizes an eight-day wind-up clock mechanism with two ink pens, one showing the compass quadrant and the other showing the course. The newer type uses an electrically driven clock mechanism with two electric styluses. This graph mechanism works similarly to the fathometer in that special graph paper is drawn over a metal desk and an electrical signal scores the graph paper to show the quadrant and course.
There are several important points that the watch stander should keep in mind when using the gyro system:
1. Ensure that all repeaters are aligned with the master gyro and if there is a dual gyro system aboard, ensure that any difference in their readings is constantly noted.
2. Azimuths for determining compass error should be taken as frequently as permissible, and the information applied to both master compasses if a dual gyro system is installed.
3. All master gyros are equipped with speed and latitude correctors.
These are necessary because the directive force of the gyro decreases as you get nearer the poles. There is also an error introduced in the master gyro by the vessel's motion along its track. Each master gyro has specific parameters for setting these correctors, but generally they should be kept within 3 degrees oflatitude and 3 knots of speed.
4. When switching from one gyro system to another be sure your repeaters are lined up with the master gyro you are switching to before changing over. Be sure to inspect all equipment frequently. Every system has a manual that details maintenance responsibilities for the unit. Be sure to follow these maintenance plans as required.
5. Errors in the gyro system can be introduced by a ship's fluctuating power supply. The gyro system should be on emergency circuits. If extensive variations in the power source are noted be sure to inform the vessel's engineer so that it may be corrected as soon as possible.
6. Although most manufacturers recommend starting a gyro within only a few hours of use, the prudent operator will bring the system on line in order to provide as much time as possible for problems to become noticeable. The gyro will hunt or oscillate prior to settling out so the gyro should be started as close to the vessel's heading as possible to minimize this settling time. If the vessel is located at the dock, you can get the vessel's heading from a chart. Be sure to set the speed corrector at zero when tied up. Starting the gyro at sea should also be done as close to the heading as possible with the speed corrector set accordingly. Remember that the axis of the rotor aligns itself with the geographic meridian and the gyro is not necessarily north seeking. If you start the gyro up 90 degrees or more from the vessel's heading, your gyro may have a l80-degree error. Every gyro has a specified start-up and shut-down procedure which should be followed. This information is available in the unit's operating manual.
7. Small constant errors in a gyro may be removed by moving the master compass lubber line or shifting the binnacle housing, depending on the style ofthe unit.
8. Be sure to remove the gyro's locking latches or uncage the unit and set its controls in the proper operating mode.
9. Each gyro system has a failure alarm connected with it.
MARINE ELECTRONICS (GYRO COMPASS)
Anything that spins can technically be considered a gyro. There are two forces associated with the gyro that must be understood: gyroscopic inertia, or rigidity in space, and precession.
Gyroscopic inertia is the natural tendency of any rotating body, spinning at a constant speed, to preserve its plane of rotation. The body will continue to spin in this manner until some outside force is applied to change the plane of rotation. The second force, precession, occurs when outside pressure is applied to the spinning body. Precession causes the body to move in a direction nearly perpendicular to the application of the applied force. The function ofthe gyro is also dependent on having three axes of freedom, namely, spinning, vertical, and horizontal. This allows the gyro to move in any manner without interference.
The gyro cannot become a usable compass, however, without utilizing the two natural constant forces of the earth: the earth's rotation and the force of gravity. To make a gyro a compass, a weight is attached to the frame supporting the spinning element of the gyro. As the earth rotates, the gyro has a tendency to remain in its original plane of rotation. The force of gravity acts upon the weight, forcing the axis of the spinning element to precess toward an alignment with the geographic meridian, north and south. It should be noted that gyros are not necessarily north seeking. Depending on the manufacturer, gyros can use one of several methods to apply a weight, which creates the precessive forces. One way is to utilize a ballistic which uses a free-flowing heavy liquid such as mercury or silicone within the element. This is known as a ballistic gyro. Another method is to use a solid weight suspended from the gyro frame. This is the pendulous style gyro.
Again the construction of the gyro varies with its manufacturer, but generally all gyros have five major parts. Commonly known as the sensitive element, contains the spinning rotor, the electric motor that drives the rotor, the rotor housing, and the supportive frame. The second part, containing the ballistic or pendulous weight, is known as the controlling element. The third element of the gyro is composed of the frames and mechanisms that shadow the movement of the sensitive element without interfering with it. This element is known as the phantom element and also contains the compass card from which the heading off the master compass can be read. The fourth is a supporting element which holds all the elements of the master gyro. This part contains the motor that drives the phantom element to follow the movement of the sensitive element and a transmission system that sends compass signals to the repeater system. This element is known as the spider element. There is the compass binnacle itself which contains all the parts ofthe master gyrocompass. This is more accessible in older units, but modern binnacle housings are sealed to prevent tampering and maintain shockproof supports as well as an inert atmosphere to reduce rotor friction.
The master gyro is operated by an electronic control device which regulates the power systems that operate the various elements of the master gyro including the rotor, phantom, azimuth, and transmission systems (see the illustration at the beginning of this chapter). There is also a separate transmission amplifier and distribution control system which receives signals from the master gyro transmitter and distributes this signal to all of the vessel's repeaters.
Gyroscopic inertia is the natural tendency of any rotating body, spinning at a constant speed, to preserve its plane of rotation. The body will continue to spin in this manner until some outside force is applied to change the plane of rotation. The second force, precession, occurs when outside pressure is applied to the spinning body. Precession causes the body to move in a direction nearly perpendicular to the application of the applied force. The function ofthe gyro is also dependent on having three axes of freedom, namely, spinning, vertical, and horizontal. This allows the gyro to move in any manner without interference.
The gyro cannot become a usable compass, however, without utilizing the two natural constant forces of the earth: the earth's rotation and the force of gravity. To make a gyro a compass, a weight is attached to the frame supporting the spinning element of the gyro. As the earth rotates, the gyro has a tendency to remain in its original plane of rotation. The force of gravity acts upon the weight, forcing the axis of the spinning element to precess toward an alignment with the geographic meridian, north and south. It should be noted that gyros are not necessarily north seeking. Depending on the manufacturer, gyros can use one of several methods to apply a weight, which creates the precessive forces. One way is to utilize a ballistic which uses a free-flowing heavy liquid such as mercury or silicone within the element. This is known as a ballistic gyro. Another method is to use a solid weight suspended from the gyro frame. This is the pendulous style gyro.
Again the construction of the gyro varies with its manufacturer, but generally all gyros have five major parts. Commonly known as the sensitive element, contains the spinning rotor, the electric motor that drives the rotor, the rotor housing, and the supportive frame. The second part, containing the ballistic or pendulous weight, is known as the controlling element. The third element of the gyro is composed of the frames and mechanisms that shadow the movement of the sensitive element without interfering with it. This element is known as the phantom element and also contains the compass card from which the heading off the master compass can be read. The fourth is a supporting element which holds all the elements of the master gyro. This part contains the motor that drives the phantom element to follow the movement of the sensitive element and a transmission system that sends compass signals to the repeater system. This element is known as the spider element. There is the compass binnacle itself which contains all the parts ofthe master gyrocompass. This is more accessible in older units, but modern binnacle housings are sealed to prevent tampering and maintain shockproof supports as well as an inert atmosphere to reduce rotor friction.
The master gyro is operated by an electronic control device which regulates the power systems that operate the various elements of the master gyro including the rotor, phantom, azimuth, and transmission systems (see the illustration at the beginning of this chapter). There is also a separate transmission amplifier and distribution control system which receives signals from the master gyro transmitter and distributes this signal to all of the vessel's repeaters.
MARINE ELECTRONICS (SPEED LOGS)
ELECTROMAGNETIC SPEED LOGS
Electromagnetic speed logs are an older method of determining velocity of a vessel through the water. This system utilizes a flow probe that extends out beyond the ship's hull. The probe contains a coil through which an electrical current is passed producing a magnetic field around the probe. As water moves past the probe, the magnetic field becomes altered indicating the vessel's speed through the water. Unfortunately, when measuring speed through the water, the unit cannot compensate for current.
The probes normally retract into the hull where they can be protected when not in use and serviced as necessary. Generally the probe requires regular routine maintenance. This system has become less common since the introduction of the Doppler speed log system.
Electromagnetic speed logs are an older method of determining velocity of a vessel through the water. This system utilizes a flow probe that extends out beyond the ship's hull. The probe contains a coil through which an electrical current is passed producing a magnetic field around the probe. As water moves past the probe, the magnetic field becomes altered indicating the vessel's speed through the water. Unfortunately, when measuring speed through the water, the unit cannot compensate for current.
The probes normally retract into the hull where they can be protected when not in use and serviced as necessary. Generally the probe requires regular routine maintenance. This system has become less common since the introduction of the Doppler speed log system.
MARINE ELECTRONICS (DOPPLER SPEED LOGS)
DOPPLER SPEED LOGS
Doppler speed logs work on the principle of the Doppler effect, which is a shift in frequency between a transmitted signal and a received signal caused by the motion of a vessel over the sea bottom. A transducer broadcasts a continuous beam of sound vibrations at about a 60-degree angle from the keel. A second transducer receives the diffusely reflected signal returning from the seabed. Unlike the fathometer, which times the returning signal, the Doppler speed log registers the change in frequency between the transmitted signal and the received signal and then calculates the velocity of the vessel based on the amount of the frequency shift.
There are several differences between Doppler beams and fathometer beams. Doppler beams are continuous, narrower (about 3 degrees in width), and higher in frequency. In addition to the transducer set facing forward, there is a second transducer set facing aft. This is called a Janus configuration (named for the two-faced Greek god) and allows the system to calculate frequency shift in two directions thus insuring a more accurate speed measurement. The placing of the Janus configuration in a fore and aft direction is known as a single axis system and is used to calculate speed over ground in the forward and after direction. A dual axis system places a second grouping of Janus configured transducers in an athwartships direction allowing for the calculation of a vessel's speed when moving sideways through the water, as in docking. The beam width of the athwartship installation is about 8 degrees to account for the possibility of a vessel's rolling.
The Doppler system calculates speed to within an accuracy of about 0.5 percent of the distance traveled. It functions well for all speeds that modern vessels can attain and works from a minimum depth of about 1.5 feet to a maximum depth of about 600 feet. Frequencies employed are between 100 kHz and 600 kHz. There are primarily four errors to be aware of when using the Doppler system:
1. Transducer orientation error caused when the pitching or rolling of the vessel becomes excessive
2. Vessel motion error caused by excessive vibration of the vessel as it moves through the water
3. Velocity of sound errors due to changes in water temperature or density due to salinity and particle content
4. Signal loss errors caused by attenuation ofthe vibrations during transit through the water or upon reflection from the bottom
The Doppler system normally measures speed over ground to about 600 feet. Mter this depth signals may be returned by a dense, colder layer of water located throughout the oceans called the deep scattering layer (DSL). Signals received off the DSL are not as accurate as signals received from bottom reflections but can still be used to provide an indication of speed through the water instead of speed over ground when bottom tracking. Your unit may have a manual or automatic system which will switch from bottom tracking to water tracking at increased depth.
The Doppler system can be connected with other electronic navigation systems providing generally accurate speed input. The navigator should be cautioned, that precise speed should be determined not only by using the Doppler but also from careful calculations of distances between accurate navigational fixes.
Doppler speed logs work on the principle of the Doppler effect, which is a shift in frequency between a transmitted signal and a received signal caused by the motion of a vessel over the sea bottom. A transducer broadcasts a continuous beam of sound vibrations at about a 60-degree angle from the keel. A second transducer receives the diffusely reflected signal returning from the seabed. Unlike the fathometer, which times the returning signal, the Doppler speed log registers the change in frequency between the transmitted signal and the received signal and then calculates the velocity of the vessel based on the amount of the frequency shift.
There are several differences between Doppler beams and fathometer beams. Doppler beams are continuous, narrower (about 3 degrees in width), and higher in frequency. In addition to the transducer set facing forward, there is a second transducer set facing aft. This is called a Janus configuration (named for the two-faced Greek god) and allows the system to calculate frequency shift in two directions thus insuring a more accurate speed measurement. The placing of the Janus configuration in a fore and aft direction is known as a single axis system and is used to calculate speed over ground in the forward and after direction. A dual axis system places a second grouping of Janus configured transducers in an athwartships direction allowing for the calculation of a vessel's speed when moving sideways through the water, as in docking. The beam width of the athwartship installation is about 8 degrees to account for the possibility of a vessel's rolling.
The Doppler system calculates speed to within an accuracy of about 0.5 percent of the distance traveled. It functions well for all speeds that modern vessels can attain and works from a minimum depth of about 1.5 feet to a maximum depth of about 600 feet. Frequencies employed are between 100 kHz and 600 kHz. There are primarily four errors to be aware of when using the Doppler system:
1. Transducer orientation error caused when the pitching or rolling of the vessel becomes excessive
2. Vessel motion error caused by excessive vibration of the vessel as it moves through the water
3. Velocity of sound errors due to changes in water temperature or density due to salinity and particle content
4. Signal loss errors caused by attenuation ofthe vibrations during transit through the water or upon reflection from the bottom
The Doppler system normally measures speed over ground to about 600 feet. Mter this depth signals may be returned by a dense, colder layer of water located throughout the oceans called the deep scattering layer (DSL). Signals received off the DSL are not as accurate as signals received from bottom reflections but can still be used to provide an indication of speed through the water instead of speed over ground when bottom tracking. Your unit may have a manual or automatic system which will switch from bottom tracking to water tracking at increased depth.
The Doppler system can be connected with other electronic navigation systems providing generally accurate speed input. The navigator should be cautioned, that precise speed should be determined not only by using the Doppler but also from careful calculations of distances between accurate navigational fixes.
MARINE ELECTRONICS (FATHOMETERS)
FATHOMETERS
All fathometer systems, whether they are indicating or recording, work on the principle of producing short pulses of sound vibrations. These pulses are transmitted vertically to the ocean floor and when they are received, the difference in time between transmission and reception is calculated. Based on the concept that sound travels at a near constant 4,800 feet per second (1,500 meters per second), the fathometer measures the time the signal takes to return and then determines the approximate depth.
Sound vibrations travel in a beam pattern that has an angle of about 12 to 25 degrees in width, perpendicular to the vessel bottom. This beam pattern travels downward with the wave front striking the ocean bottom where the signals may be reflected directly back (specular reflection) or at various angles (diffuse reflection). The bottom surface is usually not perfectly flat and signal return may vary due to the contour of the bottom. It is important to note that the fathometer is dependent on both the diffuse and specular reflection of a signal in order to function. This is also good when a vessel is rolling and the signals are returning at different angles at different times.
Like radio waves, density differences in the ocean cause the fathometer signal to be absorbed, scattered, or reflected. This causes attenuation of the sound vibrations. The effect is not only on the initial emitted signal but also on the returning echo as well. The fathometer compensates for this attenuated signal by utilizing a swept gain, which essentially increases the amplification of the signal at a rate dependent upon the amount of time the signal takes to return. The longer a signal travels, the more it is absorbed. This absorption can be decreased by reducing the frequency of the signal. Fathometer signal frequencies vary from about 55 kHz for shallow depths to about 10 kHz for great depths. The signals are above the range of audible sound; therefore, they are considered ultrasonic. This is done specifically to lock out any noise generated by the vessel. Most fathometers have a range of about 1.5 feet to about 4,500 feet depending on depth. They emit vibrations between 10 and 600 pulses per minute.
Fathometer systems include an indicator unit, which contains an oscillator to create the electrical signal, and a transducer, which converts the electrical signal to ultrasonic vibrations when transmitting. The transducer also converts the returning echoes back into an electrical signal, which then passes to an amplifier that boosts a signal to a usable level where it is read on the indicator or on a recording graph. The indicator unit is essentially the the controlling element of the system, coordinating the sending and receiving of signals, timing pulses, and adjusting frequencies.
There are two types of transducers used in commercial application. The first is the electrostrictive type which converts the electrical signal to sound vibrations by passing the current through two plates which form a sandwich with a nonconductive material. The plates vibrate due to the magnetic field induced. The second type oftransducer is the magnetostrictive type transducer which uses a form of electromagnet creating vibrations in a diaphragm, thus producing the fathometer signal. Most transducers are installed in special hull openings and are in direct contact with the water. The deck officer should be aware of the location of the transducer in order to prevent damage to the unit when dry-docking or to provide protection when doing any hull blasting or hull painting.
There are two types of fathometers: the indicating fathometer provides some form of visual presentation to show depth; the recording fathometer uses a graph leaving a permanent record. Most fathometers are capable of indicating depth in fathoms, feet, or meters. The vessel officer should keep in mind that this indication is depth under the keel of his vessel, not overall depth of the water. Indicating fathometers use various methods of presenting a display including a rotating neon light, digital readout, or cathode-ray tube presentation. Recording fathometers utilize a special graph paper moving over the top of a metal desk. When an electrical signal is sent for transmission a stylus conducts an electric current through the paper to the metal desk. The electrical charge burns a nonconductive coating off the special paper leaving a mark on the graph. When the signal returns, the stylus once again passes an electrical charge through the paper to the metal desk leaving a second mark on the graph. Depending on the way the signal is heard upon its return, the presentation can show the trained eye the type of bottom the vessel is encountering. Clear dark lines indicate a hard bottom. A wider, less distinct line is an indication of a soft bottom. It is essential that the user set the fathometer on the right depth scale based on information taken from a navigational chart. This will ensure that the signal timing and frequency are properly set for the appropriate depth, allowing the system to function efficiently. The depth indication left on the graph paper, which usually displays several depth scales available on the particular unit, is a profile of the seabed along the course ofthe ship. Incorrect scale settings can result in additional tracings on the graph due to over-amplified signals at shallow depths or false soundings as a result of signals returning later than anticipated. Most fathometers also have a manual gain. If turned up too high the unit will pick up noise caused by vibration created by the vessel moving through the water.
Different temperatures and varying water densities due to salinity content can also affect the proper functioning of the fathometer system. Colder, denser water will permit the signal to travel faster, thus indicating a shallower depth than is actually present. Warmer water will slow the passage of the signal and may compromise your safe limits by indicating a greater depth than may be actually present. The user should keep these factors in mind when his vessel encounters extremes in either case.
Colder or denser layers may also return the fathometer signal before it reaches bottom. A dual trace can appear on the fathometer graph. This does not usually compromise the vessel's safety because it indicates a shallower depth than is actually present. Every fathometer whether indicating or recording has its own particular method of operation. The operator needs to consult the manual for the specific unit to determine proper operation.
All fathometer systems, whether they are indicating or recording, work on the principle of producing short pulses of sound vibrations. These pulses are transmitted vertically to the ocean floor and when they are received, the difference in time between transmission and reception is calculated. Based on the concept that sound travels at a near constant 4,800 feet per second (1,500 meters per second), the fathometer measures the time the signal takes to return and then determines the approximate depth.
Sound vibrations travel in a beam pattern that has an angle of about 12 to 25 degrees in width, perpendicular to the vessel bottom. This beam pattern travels downward with the wave front striking the ocean bottom where the signals may be reflected directly back (specular reflection) or at various angles (diffuse reflection). The bottom surface is usually not perfectly flat and signal return may vary due to the contour of the bottom. It is important to note that the fathometer is dependent on both the diffuse and specular reflection of a signal in order to function. This is also good when a vessel is rolling and the signals are returning at different angles at different times.
Like radio waves, density differences in the ocean cause the fathometer signal to be absorbed, scattered, or reflected. This causes attenuation of the sound vibrations. The effect is not only on the initial emitted signal but also on the returning echo as well. The fathometer compensates for this attenuated signal by utilizing a swept gain, which essentially increases the amplification of the signal at a rate dependent upon the amount of time the signal takes to return. The longer a signal travels, the more it is absorbed. This absorption can be decreased by reducing the frequency of the signal. Fathometer signal frequencies vary from about 55 kHz for shallow depths to about 10 kHz for great depths. The signals are above the range of audible sound; therefore, they are considered ultrasonic. This is done specifically to lock out any noise generated by the vessel. Most fathometers have a range of about 1.5 feet to about 4,500 feet depending on depth. They emit vibrations between 10 and 600 pulses per minute.
Fathometer systems include an indicator unit, which contains an oscillator to create the electrical signal, and a transducer, which converts the electrical signal to ultrasonic vibrations when transmitting. The transducer also converts the returning echoes back into an electrical signal, which then passes to an amplifier that boosts a signal to a usable level where it is read on the indicator or on a recording graph. The indicator unit is essentially the the controlling element of the system, coordinating the sending and receiving of signals, timing pulses, and adjusting frequencies.
There are two types of transducers used in commercial application. The first is the electrostrictive type which converts the electrical signal to sound vibrations by passing the current through two plates which form a sandwich with a nonconductive material. The plates vibrate due to the magnetic field induced. The second type oftransducer is the magnetostrictive type transducer which uses a form of electromagnet creating vibrations in a diaphragm, thus producing the fathometer signal. Most transducers are installed in special hull openings and are in direct contact with the water. The deck officer should be aware of the location of the transducer in order to prevent damage to the unit when dry-docking or to provide protection when doing any hull blasting or hull painting.
There are two types of fathometers: the indicating fathometer provides some form of visual presentation to show depth; the recording fathometer uses a graph leaving a permanent record. Most fathometers are capable of indicating depth in fathoms, feet, or meters. The vessel officer should keep in mind that this indication is depth under the keel of his vessel, not overall depth of the water. Indicating fathometers use various methods of presenting a display including a rotating neon light, digital readout, or cathode-ray tube presentation. Recording fathometers utilize a special graph paper moving over the top of a metal desk. When an electrical signal is sent for transmission a stylus conducts an electric current through the paper to the metal desk. The electrical charge burns a nonconductive coating off the special paper leaving a mark on the graph. When the signal returns, the stylus once again passes an electrical charge through the paper to the metal desk leaving a second mark on the graph. Depending on the way the signal is heard upon its return, the presentation can show the trained eye the type of bottom the vessel is encountering. Clear dark lines indicate a hard bottom. A wider, less distinct line is an indication of a soft bottom. It is essential that the user set the fathometer on the right depth scale based on information taken from a navigational chart. This will ensure that the signal timing and frequency are properly set for the appropriate depth, allowing the system to function efficiently. The depth indication left on the graph paper, which usually displays several depth scales available on the particular unit, is a profile of the seabed along the course ofthe ship. Incorrect scale settings can result in additional tracings on the graph due to over-amplified signals at shallow depths or false soundings as a result of signals returning later than anticipated. Most fathometers also have a manual gain. If turned up too high the unit will pick up noise caused by vibration created by the vessel moving through the water.
Different temperatures and varying water densities due to salinity content can also affect the proper functioning of the fathometer system. Colder, denser water will permit the signal to travel faster, thus indicating a shallower depth than is actually present. Warmer water will slow the passage of the signal and may compromise your safe limits by indicating a greater depth than may be actually present. The user should keep these factors in mind when his vessel encounters extremes in either case.
Colder or denser layers may also return the fathometer signal before it reaches bottom. A dual trace can appear on the fathometer graph. This does not usually compromise the vessel's safety because it indicates a shallower depth than is actually present. Every fathometer whether indicating or recording has its own particular method of operation. The operator needs to consult the manual for the specific unit to determine proper operation.
MARINE ELECTRONICS (RADIO TIME SIGNALS)
RADIO TIME SIGNALS
In the United States, the National Bureau of Standards operates two radio stations providing constant time information that mariners can use for navigational work. The stations are WWV located in Fort Collins, Colorado, and WWVH located in Maui, Hawaii. The stations, in addition to providing continuous time signals, also provide marine storm warnings, geophysical alerts, Omega navigation system status reports, and time corrections when required. There are additional services available, but they are not generally used by the mariner.
Radio station WWV broadcasts on frequencies of 2.5 MHz, 5.0 MHz, 10.0 MHz, 15.0 MHz, and 20.0 MHz. Radio station WWVH broadcasts on frequencies of 2.5 MHz, 5.0 MHz, 10.0 MHz, and 15.0 MHz. The stations can be distinguished from one another because WWV utilizes a male announcer and WWVH utilizes a female announcer. The listener should use signals from the nearest station for time calibration. For additional information on National Bureau of Standards time services and formats, you should consult either Pub. No. 117 A or 117B.
International time signals are also available from many other nations on the shortwave bands. Normally you can pick up the stations operated by the National Bureau of Standards anywhere in the world. There are also stations broadcasting from India, France, Switzerland, Italy, Japan, Argentina, England, Czechoslovakia, the Soviet Union, South Africa, and Canada. Frequencies and periods of operation can be located in any general shortwave listener's guide. In some areas of the western North Atlantic and eastern Pacific, radio station CHU located in Ottawa, Canada, can be picked up more easily than the NBS stations. CHU broadcasts eastern standard time in English and French at 3.33 MHz, 7.335 MHz, and 14.67 MHz continuously.
In the United States, the National Bureau of Standards operates two radio stations providing constant time information that mariners can use for navigational work. The stations are WWV located in Fort Collins, Colorado, and WWVH located in Maui, Hawaii. The stations, in addition to providing continuous time signals, also provide marine storm warnings, geophysical alerts, Omega navigation system status reports, and time corrections when required. There are additional services available, but they are not generally used by the mariner.
Radio station WWV broadcasts on frequencies of 2.5 MHz, 5.0 MHz, 10.0 MHz, 15.0 MHz, and 20.0 MHz. Radio station WWVH broadcasts on frequencies of 2.5 MHz, 5.0 MHz, 10.0 MHz, and 15.0 MHz. The stations can be distinguished from one another because WWV utilizes a male announcer and WWVH utilizes a female announcer. The listener should use signals from the nearest station for time calibration. For additional information on National Bureau of Standards time services and formats, you should consult either Pub. No. 117 A or 117B.
International time signals are also available from many other nations on the shortwave bands. Normally you can pick up the stations operated by the National Bureau of Standards anywhere in the world. There are also stations broadcasting from India, France, Switzerland, Italy, Japan, Argentina, England, Czechoslovakia, the Soviet Union, South Africa, and Canada. Frequencies and periods of operation can be located in any general shortwave listener's guide. In some areas of the western North Atlantic and eastern Pacific, radio station CHU located in Ottawa, Canada, can be picked up more easily than the NBS stations. CHU broadcasts eastern standard time in English and French at 3.33 MHz, 7.335 MHz, and 14.67 MHz continuously.
MARINE ELECTRONICS (DISTRESS FREQUENCY)
CALLING AND DISTRESS FREQUENCY MONITORING
By international agreement the FCC requires that while underway, all vessels maintain a watch on certain designated calling and distress frequencies. These include a 24-hour watch on each of the following:
1. Channel 16, bridge-to-bridge VHF radio.
2. 2182 kHz,(2.182 MHz), SSB radio.
3. 500 kHz, radiotelegraph, if equipment is carried.
The VHF and SSB frequencies are required to be monitored by the deck officer on watch. The radiotelegraph frequency must be monitored by the radio operator or have an approved auto alarm system on the designated frequency to be used when the radio officer is not on watch.
Watch officers should log all voice communications in the VHF logbook, and radio officers must keep a CW log as required. Any distress messages received, if within assistance range or not, are required to be logged.
If a distress message is received, the officer should copy down all information and, if within possible range to render assistance, establish communications with the vessel in distress. The master should always be notified of the reception of a distress message; if a radio officer is aboard, he or she should also be notified to render communications assistance. The vessel should always contact the U.S. Coast Guard or the nearest government agency to notify the authority that they are available to render assistance.
The deck officer should be well aware of the following voice communication prefixes used in marine radio communications:
1. Mayday, mayday, mayday-used in the transmission of distress messages.
2. Pan, pan, pan-used in the broadcast of very urgent safety or navigation information.
3. Securite, securite, securite-used in the broadcast of important navigation or safety information.
By international agreement the FCC requires that while underway, all vessels maintain a watch on certain designated calling and distress frequencies. These include a 24-hour watch on each of the following:
1. Channel 16, bridge-to-bridge VHF radio.
2. 2182 kHz,(2.182 MHz), SSB radio.
3. 500 kHz, radiotelegraph, if equipment is carried.
The VHF and SSB frequencies are required to be monitored by the deck officer on watch. The radiotelegraph frequency must be monitored by the radio operator or have an approved auto alarm system on the designated frequency to be used when the radio officer is not on watch.
Watch officers should log all voice communications in the VHF logbook, and radio officers must keep a CW log as required. Any distress messages received, if within assistance range or not, are required to be logged.
If a distress message is received, the officer should copy down all information and, if within possible range to render assistance, establish communications with the vessel in distress. The master should always be notified of the reception of a distress message; if a radio officer is aboard, he or she should also be notified to render communications assistance. The vessel should always contact the U.S. Coast Guard or the nearest government agency to notify the authority that they are available to render assistance.
The deck officer should be well aware of the following voice communication prefixes used in marine radio communications:
1. Mayday, mayday, mayday-used in the transmission of distress messages.
2. Pan, pan, pan-used in the broadcast of very urgent safety or navigation information.
3. Securite, securite, securite-used in the broadcast of important navigation or safety information.
MARINE ELECTRONICS (SATELLITE COMMUNICATIOS)
Satellite Communications (Satcom)
Reliable worldwide communications can also be achieved through use of the INMARSAT marine satellite system. This network functions as a joint effort by in ternational communication companies utilizing a series of satellites providing coverage in the major shipping areas of the world. Each participating nation or company maintains earth stations that link their particular normal telephone network with the satellite being used. In the United States, Comsat General is the prime operator.
The system utilizes a total of six satellites in geosynchronous orbits.
The satellites cover the Atlantic, Pacific, and Indian oceans with one primary satellite and 1 backup satellite per area. The system provides 339 channels to the user transmitting between 1636.5 MHz and 1645.0 MHz and receiving on frequencies between 1535.0 and 1543.5 MHz. The system is capable of communications by voice, data (coded information relayed over voice communication lines), and telex.
The users select the satellite to be used by the area they are located in.
Once the satellite is selected, the position of the user is entered into the satcom unit so the parabolic antenna can be oriented to the selected satellite. The user can select any earth station or system desired, depending on where the communication link is required.
Satcom is a very reliable and easy system to use. The communications are very clear and comparable to land line reception. Of all the systems integrated into land communication networks, satcom is the most expensive to use.
Reliable worldwide communications can also be achieved through use of the INMARSAT marine satellite system. This network functions as a joint effort by in ternational communication companies utilizing a series of satellites providing coverage in the major shipping areas of the world. Each participating nation or company maintains earth stations that link their particular normal telephone network with the satellite being used. In the United States, Comsat General is the prime operator.
The system utilizes a total of six satellites in geosynchronous orbits.
The satellites cover the Atlantic, Pacific, and Indian oceans with one primary satellite and 1 backup satellite per area. The system provides 339 channels to the user transmitting between 1636.5 MHz and 1645.0 MHz and receiving on frequencies between 1535.0 and 1543.5 MHz. The system is capable of communications by voice, data (coded information relayed over voice communication lines), and telex.
The users select the satellite to be used by the area they are located in.
Once the satellite is selected, the position of the user is entered into the satcom unit so the parabolic antenna can be oriented to the selected satellite. The user can select any earth station or system desired, depending on where the communication link is required.
Satcom is a very reliable and easy system to use. The communications are very clear and comparable to land line reception. Of all the systems integrated into land communication networks, satcom is the most expensive to use.
MARINE ELECTRONICS (SINGLE-SIDEBAND)
Single-Sideband Ship-to-Shore
For long-range voice communications, single-sideband is the primary marine system worldwide. Frequencies are between 2.0 MHz and 23.0 MHz and like VHF can broadcast in a simplex or duplex mode. Singlesideband works on the principle of taking a standard amplitude modulated (AM) signal, suppressing one side of the signal, and using the energy to increase the efficiency of the remaining sideband signal. Most units are capable of utilizing the upper sideband (USB) or lower sideband (LSB) for communications depending on the need of the user. There are still a few units with double-sideband capability (DSB) which is the same as AM. The AM ship-to-shore/ship-to-ship system was the first type of voice communication method used by mariners but has been almost completely replaced by single-sideband.
Most single-sideband users still must be familiar with designated frequencies for specific uses. The frequencies now have specified channels associated with them called ITU channels or International Telephone Units. ITU channels begin at 401 and go through 2240 with the first one or two numbers of the channel corresponding to the frequency range in megahertz. Not every number in this range has a designated marine use. The international calling and distress frequency for SSB is 2182 kHz or 2.182 MHz. There are no channel designations for the 2-3 MHz band, which is considered medium frequency, and the 4-23 MHz bands, which are considered high frequency.
The U.S. Coast Guard also monitors the SSB calling and distress frequency as well as having several designated working frequencies. High seas radiotelephone service is also available on various frequencies.
For long-range voice communications, single-sideband is the primary marine system worldwide. Frequencies are between 2.0 MHz and 23.0 MHz and like VHF can broadcast in a simplex or duplex mode. Singlesideband works on the principle of taking a standard amplitude modulated (AM) signal, suppressing one side of the signal, and using the energy to increase the efficiency of the remaining sideband signal. Most units are capable of utilizing the upper sideband (USB) or lower sideband (LSB) for communications depending on the need of the user. There are still a few units with double-sideband capability (DSB) which is the same as AM. The AM ship-to-shore/ship-to-ship system was the first type of voice communication method used by mariners but has been almost completely replaced by single-sideband.
Most single-sideband users still must be familiar with designated frequencies for specific uses. The frequencies now have specified channels associated with them called ITU channels or International Telephone Units. ITU channels begin at 401 and go through 2240 with the first one or two numbers of the channel corresponding to the frequency range in megahertz. Not every number in this range has a designated marine use. The international calling and distress frequency for SSB is 2182 kHz or 2.182 MHz. There are no channel designations for the 2-3 MHz band, which is considered medium frequency, and the 4-23 MHz bands, which are considered high frequency.
The U.S. Coast Guard also monitors the SSB calling and distress frequency as well as having several designated working frequencies. High seas radiotelephone service is also available on various frequencies.
MARINE ELECTRONICS (VHF RADIO)
MARINE COMMUNICATIONS
Marine equipment for communications is divided into several specific types. These include the long-range, ship-to-ship/ship-to-shore radiotelegraph, very high frequency (VHF) bridge-to-bridge voice communications, single-sideband (SSB) ship-to-shore communications, and satellite communications (satcom). It should be recognized that some of the other equipment is the responsibility of the radio officer according to the organization of a particular commercial operation. More and more, however, voice communication systems are under the control and responsibility of the deck officer.
VHF Bridge-to-Bridge Radio
Marine VHF is usually found between 156.00 MHz and 162.00 MHz. Frequencies are broken down into a series of channels from 1 through 89. (Not all of these channels are in use and have designated frequencies.) Channels are either simplex (transmitting and receiving on the same frequency); or duplex (transmitting on one frequency and receiving on another frequency). Simplex is usually designated for U.S. use (or suffixed with "A"), and duplex is normally designated for international use.
Each channel has a specific use and all marine frequencies are regulated in the United States by the Federal Communications Commission, and regulated worldwide by international agreement. The international calling and distress frequency for VHF is channel 16 (156.800 MHz). The U.S.Coast Guard monitors channel 16 and utilizes several designated working frequencies. Marine radio operator service is also located in coastal, harbor, and river areas on specified working frequencies. The National Weather Service (through NOAA) maintains weather information services near the marine radio band for use by mariners. All VHF radios are capable of picking up these frequencies, which include channels 29 (162.55 MHz) and 89 (162.40 MHz).
Marine equipment for communications is divided into several specific types. These include the long-range, ship-to-ship/ship-to-shore radiotelegraph, very high frequency (VHF) bridge-to-bridge voice communications, single-sideband (SSB) ship-to-shore communications, and satellite communications (satcom). It should be recognized that some of the other equipment is the responsibility of the radio officer according to the organization of a particular commercial operation. More and more, however, voice communication systems are under the control and responsibility of the deck officer.
VHF Bridge-to-Bridge Radio
Marine VHF is usually found between 156.00 MHz and 162.00 MHz. Frequencies are broken down into a series of channels from 1 through 89. (Not all of these channels are in use and have designated frequencies.) Channels are either simplex (transmitting and receiving on the same frequency); or duplex (transmitting on one frequency and receiving on another frequency). Simplex is usually designated for U.S. use (or suffixed with "A"), and duplex is normally designated for international use.
Each channel has a specific use and all marine frequencies are regulated in the United States by the Federal Communications Commission, and regulated worldwide by international agreement. The international calling and distress frequency for VHF is channel 16 (156.800 MHz). The U.S.Coast Guard monitors channel 16 and utilizes several designated working frequencies. Marine radio operator service is also located in coastal, harbor, and river areas on specified working frequencies. The National Weather Service (through NOAA) maintains weather information services near the marine radio band for use by mariners. All VHF radios are capable of picking up these frequencies, which include channels 29 (162.55 MHz) and 89 (162.40 MHz).
MARINE ELECTRONICS (GPS)
GLOBAL POSITIONING SYSTEM (NAVSTAR)
GPS - The system consist of about 18 satellites in geosynchronous orbit in three separate planes around the earth. Unlike Transit satnav, the system is continuous, receiving signals from at least 6 separate satellites at anyone time on 1227 MHz and 1575 MHz. The key advantage of the system is that it provides highly accurate latitude, longitude, and altitude coordinates. Whereas the military releases only a certain accuracy for commercial use, GPS will have the capability of accurate fixes measured in feet.
GPS - The system consist of about 18 satellites in geosynchronous orbit in three separate planes around the earth. Unlike Transit satnav, the system is continuous, receiving signals from at least 6 separate satellites at anyone time on 1227 MHz and 1575 MHz. The key advantage of the system is that it provides highly accurate latitude, longitude, and altitude coordinates. Whereas the military releases only a certain accuracy for commercial use, GPS will have the capability of accurate fixes measured in feet.
MARINE ELECTRONICS (SATNAV)
TRANSIT SATNAV (NAVSAT/ NAVIGATION SATELLITE SYSTEM)
In 1967 the U.S. Navy released for commercial use signals from its Transit satellite navigation system. The system works in this manner. There are five satellites in polar orbit moving at a constant speed between 400 and 700 miles above the earth. Each satellite, travelling at about 26,000 kilometers per hour, orbits the earth once every 108 minutes for an average of 13.5 passes per day. Signals from the satellites, dictating their orbiting path, are broadcast simultaneously on 150 MHz and 400 MHz to defeat the effects of the ionosphere. The satellites are constantly tracked by earth stations located in Hawaii, California, Minnesota, and Maine. This information, along with time information from the Naval Observatory, is fed to the California station which in turn corrects the orbital path information for the satellite over the next 16 hours. This is necessary due to variations in the orbital path caused by gravity and the satellite's motion through the atmosphere.
A Transit satnav receiver on the surface of the earth picks up the satellite's broadcast when it comes into view. The satellite's broadcasts are synchronized in a two-minute format giving the perimeters of its orbital path and a 400 Hz audible signal. Where as the path of the satellite approximates a line of longitude, the receiver need only determine its offset from the satellite's orbital path to determine its own longitude. It does this by recording the frequency shift of the approaching signal. This is called the Doppler effect. As the satellite approaches, the frequency changes at a certain rate allowing the receiver to calculate how far it is from the satellite's path. This determines the first coordinate, the longitude of the receiver. When the satellite reaches the zenith in relation to the receiver, the frequency shift begins to change in the opposite direction. It is at this point that the receiver determines the second coordinate, its latitude. Because its signals are synchronized through time information provided by the Naval Observatory, the receiver is also able to provide accurate coordinated universal time (UTC/GMT).
The biggest disadvantage of the Transit satnav system is that you cannot get readings from it on a continual basis. There are several factors that the user of Transit should be aware of. Transit satnav can be used in almost all weather, worldwide. It is not subject to groundwave or skywave propagational interference. A vessel's position may be determined from a single satellite, and the system is very accurate within 1/4 mile. Most satellite receivers provide DR information which is dependent upon accurate course and speed inputs. The user should be cautioned not to mistake this tracking capability for a continual accurate fix. Fixes are only available when a satellite is in view, which is when the satellite is above the horizon
at an elevation of more than 10 degrees but less than 70 degrees. When initializing the system you must also provide correct antenna height to the receiver to ensure that the unit calculates the Doppler shift correctly. Normally, the average time between passes is around 2.5 hours with the average maximum time between useful passes not exceeding 4 hours. The closer you get to the poles, the more frequent the passes.
Another disadvantage of the system is that if several satellites are in view, the receiving unit will not be able to distinguish the difference between signals coming from several satellites and you will get an inaccurate or totally useless fix. Most receivers have visual or audible indicators telling when a satellite is in view and when it is tracking. A quality receiver can be programmed to block out signals coming from satellites that are too high or too low, indicating a poor pass. Many units have a memory or printer that will give you the time and position of the last fix and indicate if it was usable. The user should also allow the receiver to take a full set of counts before and after the satellite passes its zenith. This will ensure that the receiver has had sufficient information to calculate the Doppler shift as it approaches and check its calculations as the satellite moves away. As with other radionavigation systems, even though the Transit satnav utilizes higher frequencies, signals should not be trusted in unusual weather conditions, during periods of extensive atmospherics, or during twilight.
In 1967 the U.S. Navy released for commercial use signals from its Transit satellite navigation system. The system works in this manner. There are five satellites in polar orbit moving at a constant speed between 400 and 700 miles above the earth. Each satellite, travelling at about 26,000 kilometers per hour, orbits the earth once every 108 minutes for an average of 13.5 passes per day. Signals from the satellites, dictating their orbiting path, are broadcast simultaneously on 150 MHz and 400 MHz to defeat the effects of the ionosphere. The satellites are constantly tracked by earth stations located in Hawaii, California, Minnesota, and Maine. This information, along with time information from the Naval Observatory, is fed to the California station which in turn corrects the orbital path information for the satellite over the next 16 hours. This is necessary due to variations in the orbital path caused by gravity and the satellite's motion through the atmosphere.
A Transit satnav receiver on the surface of the earth picks up the satellite's broadcast when it comes into view. The satellite's broadcasts are synchronized in a two-minute format giving the perimeters of its orbital path and a 400 Hz audible signal. Where as the path of the satellite approximates a line of longitude, the receiver need only determine its offset from the satellite's orbital path to determine its own longitude. It does this by recording the frequency shift of the approaching signal. This is called the Doppler effect. As the satellite approaches, the frequency changes at a certain rate allowing the receiver to calculate how far it is from the satellite's path. This determines the first coordinate, the longitude of the receiver. When the satellite reaches the zenith in relation to the receiver, the frequency shift begins to change in the opposite direction. It is at this point that the receiver determines the second coordinate, its latitude. Because its signals are synchronized through time information provided by the Naval Observatory, the receiver is also able to provide accurate coordinated universal time (UTC/GMT).
The biggest disadvantage of the Transit satnav system is that you cannot get readings from it on a continual basis. There are several factors that the user of Transit should be aware of. Transit satnav can be used in almost all weather, worldwide. It is not subject to groundwave or skywave propagational interference. A vessel's position may be determined from a single satellite, and the system is very accurate within 1/4 mile. Most satellite receivers provide DR information which is dependent upon accurate course and speed inputs. The user should be cautioned not to mistake this tracking capability for a continual accurate fix. Fixes are only available when a satellite is in view, which is when the satellite is above the horizon
at an elevation of more than 10 degrees but less than 70 degrees. When initializing the system you must also provide correct antenna height to the receiver to ensure that the unit calculates the Doppler shift correctly. Normally, the average time between passes is around 2.5 hours with the average maximum time between useful passes not exceeding 4 hours. The closer you get to the poles, the more frequent the passes.
Another disadvantage of the system is that if several satellites are in view, the receiving unit will not be able to distinguish the difference between signals coming from several satellites and you will get an inaccurate or totally useless fix. Most receivers have visual or audible indicators telling when a satellite is in view and when it is tracking. A quality receiver can be programmed to block out signals coming from satellites that are too high or too low, indicating a poor pass. Many units have a memory or printer that will give you the time and position of the last fix and indicate if it was usable. The user should also allow the receiver to take a full set of counts before and after the satellite passes its zenith. This will ensure that the receiver has had sufficient information to calculate the Doppler shift as it approaches and check its calculations as the satellite moves away. As with other radionavigation systems, even though the Transit satnav utilizes higher frequencies, signals should not be trusted in unusual weather conditions, during periods of extensive atmospherics, or during twilight.
MARINE ELECTRONICS (OMEGA)
OMEGA
Omega was developed by the U.S. Navy in 1957 to be used as a long-range worldwide continuous radionavigation system. Operating at a very low 10.2 kHz to 13.6 kHz, there are 8 stations in the system labeled A through H. The system utilizes a 10-second broadcast format with each station broadcasting sequentially for approximately 1 second. Like Decca, it establishes its hyperbolic pattern through the use of phase comparison. Each hyperbolic line borders an Omega lane with an approximate distance of 8 miles at the baseline. The radio signals can indicate very accurately where you are within the lane (percent oflane), it is the receiver that must keep track of which lane you are in. This is the major disadvantage of the system.
To utilize the Omega system, you must first begin by synchronizing the receiver with the incoming signal at the origin of the voyage. As the vessel gets underway on its path of travel, the receiver counts the lanes it passes through. As long as the signals are strong and nothing interrupts the functioning of the receiver, the lane count is reasonably accurate. Should something interfere with signal reception, the receiver can lose count of the lanes. Omega navigational receivers were developed with a tracking graph that provided the user with a visual indication that the system was working continually.
The system was difficult to use, and became less commercially viable after Transit satnav was introduced. Because the system operates long range, it was also extensively dependent upon skywaves and readings needed to be corrected from Omega propagation tables. The system still has the primary advantage of being continually available almost anywhere on the face of the earth with an effective working range of not less than 600 miles to a station nor more than 7,000 miles from a station. Application of readings is the same as in Loran and Decca on special overprinted charts. Information concerning the system's operation can be obtained from listening to radio station WWV or WWVH, the same stations that broadcast time information.
Omega was developed by the U.S. Navy in 1957 to be used as a long-range worldwide continuous radionavigation system. Operating at a very low 10.2 kHz to 13.6 kHz, there are 8 stations in the system labeled A through H. The system utilizes a 10-second broadcast format with each station broadcasting sequentially for approximately 1 second. Like Decca, it establishes its hyperbolic pattern through the use of phase comparison. Each hyperbolic line borders an Omega lane with an approximate distance of 8 miles at the baseline. The radio signals can indicate very accurately where you are within the lane (percent oflane), it is the receiver that must keep track of which lane you are in. This is the major disadvantage of the system.
To utilize the Omega system, you must first begin by synchronizing the receiver with the incoming signal at the origin of the voyage. As the vessel gets underway on its path of travel, the receiver counts the lanes it passes through. As long as the signals are strong and nothing interrupts the functioning of the receiver, the lane count is reasonably accurate. Should something interfere with signal reception, the receiver can lose count of the lanes. Omega navigational receivers were developed with a tracking graph that provided the user with a visual indication that the system was working continually.
The system was difficult to use, and became less commercially viable after Transit satnav was introduced. Because the system operates long range, it was also extensively dependent upon skywaves and readings needed to be corrected from Omega propagation tables. The system still has the primary advantage of being continually available almost anywhere on the face of the earth with an effective working range of not less than 600 miles to a station nor more than 7,000 miles from a station. Application of readings is the same as in Loran and Decca on special overprinted charts. Information concerning the system's operation can be obtained from listening to radio station WWV or WWVH, the same stations that broadcast time information.
MARINE ELECTRONICS (DECCA)
DECCA
Decca is a low to medium frequency radionavigation system developed in Europe. It is very similar to Loran in application although the chains are developed in a separate manner. Like Loran, Decca is a continuous broadcasting system designed for coastal navigation. The areas of coverage are usually limited to about 500 miles from the stations. Worldwide Decca chains are available in very specific areas including northwest and southwest Europe, a few areas in Africa, the Persian Gulf, the northern Indian Ocean, Australia, Japan, and Nova Scotia. Each chain consists of a master and three slaves designated as red, green, and purple. The baseline between the master and each slave is broken down into ten zones labeled A through J. Within each zone, there is a designated number oflanes dependent upon which slave you are using. With the red slave, each zone will have 24 lanes per zone; with the green slave there are 18 lanes per zone; and with the purple slave, there are 30 lanes per zone. (The lanes are numbered 0 to 23, 30 to 47, and 50 to 79, respectively.) The lanes are broken down into 100 lane fractions called centilanes. The Decca reading consists of a zone, lane, and centilane reading given by the red, green, and purple slave in order. The system assumes that at least two of the three master slave groups will be readable at any time, thus providing a two line of position fix. The system is dependent upon ground wave reception and skywaves can affect the accuracy of the readings.
Decca uses phase comparison to establish the hyperbolic grid, and once readings are taken, they are applied to the Decca pattern in the same manner as Loran. The signals are presented sequentially and several readings should be taken and averaged out prior to plotting. The biggest drawback of the system is the potential of lane slip where you may get the correct centilane reading but find yourself in an incorrect lane. This can be picked up quickly by maintaining an accurate DR and backing up your position finding with other navigation systems. During the day the usable range of Decca is about 500 miles, but due to skywave reception at night the usable range decreases to about 250 miles. The system is also subject to errors due to land effect as well as being very sensitive to weather conditions, as other lower frequency systems are.
Decca is a low to medium frequency radionavigation system developed in Europe. It is very similar to Loran in application although the chains are developed in a separate manner. Like Loran, Decca is a continuous broadcasting system designed for coastal navigation. The areas of coverage are usually limited to about 500 miles from the stations. Worldwide Decca chains are available in very specific areas including northwest and southwest Europe, a few areas in Africa, the Persian Gulf, the northern Indian Ocean, Australia, Japan, and Nova Scotia. Each chain consists of a master and three slaves designated as red, green, and purple. The baseline between the master and each slave is broken down into ten zones labeled A through J. Within each zone, there is a designated number oflanes dependent upon which slave you are using. With the red slave, each zone will have 24 lanes per zone; with the green slave there are 18 lanes per zone; and with the purple slave, there are 30 lanes per zone. (The lanes are numbered 0 to 23, 30 to 47, and 50 to 79, respectively.) The lanes are broken down into 100 lane fractions called centilanes. The Decca reading consists of a zone, lane, and centilane reading given by the red, green, and purple slave in order. The system assumes that at least two of the three master slave groups will be readable at any time, thus providing a two line of position fix. The system is dependent upon ground wave reception and skywaves can affect the accuracy of the readings.
Decca uses phase comparison to establish the hyperbolic grid, and once readings are taken, they are applied to the Decca pattern in the same manner as Loran. The signals are presented sequentially and several readings should be taken and averaged out prior to plotting. The biggest drawback of the system is the potential of lane slip where you may get the correct centilane reading but find yourself in an incorrect lane. This can be picked up quickly by maintaining an accurate DR and backing up your position finding with other navigation systems. During the day the usable range of Decca is about 500 miles, but due to skywave reception at night the usable range decreases to about 250 miles. The system is also subject to errors due to land effect as well as being very sensitive to weather conditions, as other lower frequency systems are.
MARINE ELECTRONICS (LORAN C)
LORAN-C (LONG-RANGE NAVIGATION)
Although Loran-C is not used this much these days, due to GPS, thought I'd give it a brief mention. Just like Loran A it will become obsolete. Loran-C is a low frequency hyperbolic system broadcasting on a frequency of about 100 kHz. The hyperbolic pattern is established through the measurement of time differences (TD) of pulsed signals. The Loran-C system was designed to cover the U.S. coastal confluence zones and is also present in key coastal areas throughout the Northern Hemisphere. Loran-C coverage is divided into chains composed of 3 to 5 land-based transmitting stations consisting of a master station and 2 to 4 slaves. Each slave is designated as W, X, Y, and Z. Each station transmits a group of pulses at a specific rate called the group repetition interval (GRI). This is partly how the receiver determines the differences between chains as all Loran stations transmit on the same frequency. The master station broadcasts 8 pulse modulated signals followed by each slave station in order, broadcasting 8 signals as well. There is also a 9th pulse broadcast by the master station called the performance pulse. This indicates the efficient operation of the system overall. Broadcasting at a specific GRI, signals are received at the specific rate designated for that chain. The hyperbolic pattern is established between the master and each of the slaves with the baseline between the master and each slave. There is approximately an average separation of 650 nautical miles between the master and each slave and the difference between each hyperbolic line (TD) is expressed in microseconds.
The navigator reads the microsecond difference off his receiver and finds the closest TDs on either side of his or her reading. He or she then measures the gradient between the two TDs and interpolates where the LORAN-C (LONG-RANGE NAVIGATION) Loran-C is a low frequency hyperbolic system broadcasting on a frequency of about 100 kHz. The hyperbolic pattern is established through the measurement of time differences (TD) of pulsed signals. The Loran-C system was designed to cover the U.S. coastal confluence zones and is also present in key coastal areas throughout the Northern Hemisphere (see figures 6-3 and 64). Loran-C coverage is divided into chains composed of 3 to 5 land-based transmitting stations consisting of a master station and 2 to 4 slaves. Each slave is designated as W, X, Y, and Z. Each station transmits a group of pulses at a specific rate called the group repetition interval (GRI). This is partly how the receiver determines the differences between chains as all Loran stations transmit on the same frequency. The master station broadcasts 8 pulse modulated signals followed by each slave station in order, broadcasting 8 signals as well. There is also a 9th pulse broadcast by the master station called the performance pulse. This indicates the efficient operation of the system overall. Broadcasting at a specific GRI, signals are received at the specific rate designated for that chain. The hyperbolic pattern is established between the master and each of the slaves with the baseline between the master and each slave. There is approximately an average separation of 650 nautical miles between the master and each slave and the difference between each hyperbolic line (TD) is expressed in microseconds.
The navigator reads the microsecond difference off his receiver and finds the closest TDs on either side of his or her reading. He or she then measures the gradient between the two TDs and interpolates where the
Although Loran-C is not used this much these days, due to GPS, thought I'd give it a brief mention. Just like Loran A it will become obsolete. Loran-C is a low frequency hyperbolic system broadcasting on a frequency of about 100 kHz. The hyperbolic pattern is established through the measurement of time differences (TD) of pulsed signals. The Loran-C system was designed to cover the U.S. coastal confluence zones and is also present in key coastal areas throughout the Northern Hemisphere. Loran-C coverage is divided into chains composed of 3 to 5 land-based transmitting stations consisting of a master station and 2 to 4 slaves. Each slave is designated as W, X, Y, and Z. Each station transmits a group of pulses at a specific rate called the group repetition interval (GRI). This is partly how the receiver determines the differences between chains as all Loran stations transmit on the same frequency. The master station broadcasts 8 pulse modulated signals followed by each slave station in order, broadcasting 8 signals as well. There is also a 9th pulse broadcast by the master station called the performance pulse. This indicates the efficient operation of the system overall. Broadcasting at a specific GRI, signals are received at the specific rate designated for that chain. The hyperbolic pattern is established between the master and each of the slaves with the baseline between the master and each slave. There is approximately an average separation of 650 nautical miles between the master and each slave and the difference between each hyperbolic line (TD) is expressed in microseconds.
The navigator reads the microsecond difference off his receiver and finds the closest TDs on either side of his or her reading. He or she then measures the gradient between the two TDs and interpolates where the LORAN-C (LONG-RANGE NAVIGATION) Loran-C is a low frequency hyperbolic system broadcasting on a frequency of about 100 kHz. The hyperbolic pattern is established through the measurement of time differences (TD) of pulsed signals. The Loran-C system was designed to cover the U.S. coastal confluence zones and is also present in key coastal areas throughout the Northern Hemisphere (see figures 6-3 and 64). Loran-C coverage is divided into chains composed of 3 to 5 land-based transmitting stations consisting of a master station and 2 to 4 slaves. Each slave is designated as W, X, Y, and Z. Each station transmits a group of pulses at a specific rate called the group repetition interval (GRI). This is partly how the receiver determines the differences between chains as all Loran stations transmit on the same frequency. The master station broadcasts 8 pulse modulated signals followed by each slave station in order, broadcasting 8 signals as well. There is also a 9th pulse broadcast by the master station called the performance pulse. This indicates the efficient operation of the system overall. Broadcasting at a specific GRI, signals are received at the specific rate designated for that chain. The hyperbolic pattern is established between the master and each of the slaves with the baseline between the master and each slave. There is approximately an average separation of 650 nautical miles between the master and each slave and the difference between each hyperbolic line (TD) is expressed in microseconds.
The navigator reads the microsecond difference off his receiver and finds the closest TDs on either side of his or her reading. He or she then measures the gradient between the two TDs and interpolates where the
MARINE ELECTRONICS (HYPERBOLIC RADIO SYSTEMS)
HYPERBOLIC RADIONAVIGATION SYSTEMS
Loran, Decca, Omega, and are hyperbolic radionavigational systems. Hyperbolic navigation patterns are created by a baseline between two transmitters, with each transmitter emitting radio waves toward one another. Where the radio waves intersect and have a constant difference of distance from the two transmitters a locus of points is established. At the midpoint of the baseline is a perpendicular line created by this locus of points called the centerline. As we approach either transmitter, the subsequent hyperbolic lines become curved toward that transmitter, creating the overall pattern. Because radio transmissions are constant in reception, the pattern they form can be determined and a chart overprint created for the purposes of navigation. Each hyperbolic line will represent a specific point on the pattern which can be used by navigators for ascertaining a line of position coinciding with and drawn parallel to that hyperbolic line.
The distance between each hyperbolic line represents a specific distance in nautical miles known as the gradient. The baseline may be extended beyond the transmitter; signals received on the baseline extension are not accurate and should not be used for navigational purposes. The most accurate fixes are along the baseline at the centerline. Each radionavigational system establishes its hyperbolic pattern in a separate manner. In the case of Loran, the time difference between received signals creates the pattern. In the case of Omega and Decca, it is phase comparison that is the prime factor. Phase comparison is a method in which the difference in received cycles of radio waves is measured at their reception point.
Once the hyperbolic pattern is established, a series of transmitters can create additional patterns with these patterns crisscrossing an area. The use of two or more patterns and the subsequent lines of position determined from them allows the navigator to fix a position. It should be kept in mind that radionavigational receivers are like all other radio receivers with system signals subject to the forces and errors that affect all radio transmissions
Loran, Decca, Omega, and are hyperbolic radionavigational systems. Hyperbolic navigation patterns are created by a baseline between two transmitters, with each transmitter emitting radio waves toward one another. Where the radio waves intersect and have a constant difference of distance from the two transmitters a locus of points is established. At the midpoint of the baseline is a perpendicular line created by this locus of points called the centerline. As we approach either transmitter, the subsequent hyperbolic lines become curved toward that transmitter, creating the overall pattern. Because radio transmissions are constant in reception, the pattern they form can be determined and a chart overprint created for the purposes of navigation. Each hyperbolic line will represent a specific point on the pattern which can be used by navigators for ascertaining a line of position coinciding with and drawn parallel to that hyperbolic line.
The distance between each hyperbolic line represents a specific distance in nautical miles known as the gradient. The baseline may be extended beyond the transmitter; signals received on the baseline extension are not accurate and should not be used for navigational purposes. The most accurate fixes are along the baseline at the centerline. Each radionavigational system establishes its hyperbolic pattern in a separate manner. In the case of Loran, the time difference between received signals creates the pattern. In the case of Omega and Decca, it is phase comparison that is the prime factor. Phase comparison is a method in which the difference in received cycles of radio waves is measured at their reception point.
Once the hyperbolic pattern is established, a series of transmitters can create additional patterns with these patterns crisscrossing an area. The use of two or more patterns and the subsequent lines of position determined from them allows the navigator to fix a position. It should be kept in mind that radionavigational receivers are like all other radio receivers with system signals subject to the forces and errors that affect all radio transmissions
MARINE ELECTRONICS (RDF NAVIGATION)
RADIO DIRECTION FINDING
The system of worldwide marine radio beacons is one of the oldest and most frequently used radionavigation systems in the world. There remain many advantages to the use of radio direction finders in navigation. Bearings can be taken off any transmitting station if the location of the antenna is indicated on a chart. A position can be fixed using two or more radio beacons. A radio bearing can be used to back up a visual bearing as well as any other type of navigational fix. There are over 900 radio beacons worldwide, a radio directional finder can be used for coastal navigation just about anywhere. Prime importance is the ability of one vessel to take a bearing on a second vessel that is transmitting a distress message.
All radio beacons are found in the 285 kHz to 315 kHz range. Modern radio direction finders are also equipped with the capability of receiving 500 kHz, the international calling and distress frequency for radiotelegraph, as well as 2182 kHz, the international calling and distress frequency for single-sideband. With the exception of calibration beacons, special low-power transmitters used for calibrating RDF receivers, most marine radio beacon transmitting stations operate 24 hours a day. In the United States and Canada the radio beacons are sequenced. Six stations broadcast on the same frequency, one at a time, in a order. This allows the you to get sufficient bearings for a fix without changing frequencies.
There are two types of receivers found aboard ship. They are the traditional radio direction finding receiver (RDF), also known as an aural null indicator, and the auto directional finder. The older style RDF receiver depends on the ability of the user to listen and, sometimes with visual assistance, locate the null of a transmitted signal to determine direction. The auto direction finder locks onto the signal's carrier wave to determine direction. It should be understood that in the case of radio direction finding it is the null of the signal, not its strongest point, that one uses to determine a station's direction. This is because you can more precisely locate the null rather than the strongest signal, usually to within plus or minus 2 degrees. At times the radio direction finder will have a visual indicator such as a meter or split beam tube to assist the user in more accurately determining the null. The modern auto direction finder can be used in a manual mode, as an RDF, or in an auto mode. When in auto mode, the unit searches for the direction of the carrier wave and then locks onto the broadcast signal.
There are three types of antennas used with radio beacon receivers.
One is the older and now less common movable loop antenna which is a circular antenna connected to a hand wheel above the receiver. This is turned by the user to find the signal. The second is the more common crossed loop antenna, known as a Bellini-Tosi antenna. This type of antenna has two fixed loops that are mounted rigidly together usually on a mast. The third type is the rotating loop antenna which is a circular antenna that rotates at a rapid speed within a protected case. There is a fourth type of antenna more commonly found on small portable radio direction finders. This is the bar antenna discussed earlier. RDF loop antennas work on a principle similar to water traveling over a waterwheel. RF energy is picked up by the antenna and induces a current which moves in a circular pattern around the loop. The loop corresponds to the wave of energy passing by it thus creating a current flow in a single direction. When the leading edge of the antenna is aligned with the direction of the transmitter, the signal is the strongest. When the loop is perpendicular to the direction ofthe transmitter, the RF energy will strike the antenna face and flow in two directions at the same time. When the current reaches the receiver it is in the form of different polarities. The current literally cancels itself out; it is at this point that the null is indicated. A direction indicator is aligned with this point on the loop.
While the earlier style directional loops employed this system, the newer crossed loop antenna employs the same principle but determines direction in a slightly different manner. Because there are two loops perpendicular to each other, the antenna as a whole will pick up the signal at varying strengths in different areas of the antenna. The signals then pass to the receiver where a device called a goniometer separates the incoming signals and by measuring their varying strengths determines direction. A direction indicator is connected to the goniometer.
An RDF loop antenna will pick up a signal when either edge of the loop is aligned with the transmitter; the user can have a potential 180-degree error in his radio bearing. This is called ambiguity, and is eliminated by the simple addition of a sense system to the unit. The sense system consists of a second whip antenna that is connected to the receiving circuit. A switch allows the current flow from the second antenna to mix with the flow from the loop antenna. Ifthe polarities are the same the signal direction will be indicated by the stronger mixed signal. If the polarities are different, the input from the sense antenna will cancel the input from the loop antenna and no signal will be heard, thus indicating the loop is facing the wrong direction. All loop antennas have a definite positive and negative side which allows the system to function. The user should carefully consult the instructions for his or her particular unit to become familiar with the specific manner in which the sense system works.
Auto direction finders operate in a similar manner. The unit has a goniometer with a motor attached. The goniometer focuses in on the null point ofthe received signals, and the motor moves the direction indicator to allow the user to determine a bearing to the station.
Rotating antenna systems operate slightly differently with the moving loop picking up a signal as it passes through the RF field. The rotating loop receiver utilizes a cathode-ray tube indicator to show the RF pattern as perceived by the rotating antenna. The RF pattern can be narrowed to determine quite accurately the direction of the transmitter.
The system of worldwide marine radio beacons is one of the oldest and most frequently used radionavigation systems in the world. There remain many advantages to the use of radio direction finders in navigation. Bearings can be taken off any transmitting station if the location of the antenna is indicated on a chart. A position can be fixed using two or more radio beacons. A radio bearing can be used to back up a visual bearing as well as any other type of navigational fix. There are over 900 radio beacons worldwide, a radio directional finder can be used for coastal navigation just about anywhere. Prime importance is the ability of one vessel to take a bearing on a second vessel that is transmitting a distress message.
All radio beacons are found in the 285 kHz to 315 kHz range. Modern radio direction finders are also equipped with the capability of receiving 500 kHz, the international calling and distress frequency for radiotelegraph, as well as 2182 kHz, the international calling and distress frequency for single-sideband. With the exception of calibration beacons, special low-power transmitters used for calibrating RDF receivers, most marine radio beacon transmitting stations operate 24 hours a day. In the United States and Canada the radio beacons are sequenced. Six stations broadcast on the same frequency, one at a time, in a order. This allows the you to get sufficient bearings for a fix without changing frequencies.
There are two types of receivers found aboard ship. They are the traditional radio direction finding receiver (RDF), also known as an aural null indicator, and the auto directional finder. The older style RDF receiver depends on the ability of the user to listen and, sometimes with visual assistance, locate the null of a transmitted signal to determine direction. The auto direction finder locks onto the signal's carrier wave to determine direction. It should be understood that in the case of radio direction finding it is the null of the signal, not its strongest point, that one uses to determine a station's direction. This is because you can more precisely locate the null rather than the strongest signal, usually to within plus or minus 2 degrees. At times the radio direction finder will have a visual indicator such as a meter or split beam tube to assist the user in more accurately determining the null. The modern auto direction finder can be used in a manual mode, as an RDF, or in an auto mode. When in auto mode, the unit searches for the direction of the carrier wave and then locks onto the broadcast signal.
There are three types of antennas used with radio beacon receivers.
One is the older and now less common movable loop antenna which is a circular antenna connected to a hand wheel above the receiver. This is turned by the user to find the signal. The second is the more common crossed loop antenna, known as a Bellini-Tosi antenna. This type of antenna has two fixed loops that are mounted rigidly together usually on a mast. The third type is the rotating loop antenna which is a circular antenna that rotates at a rapid speed within a protected case. There is a fourth type of antenna more commonly found on small portable radio direction finders. This is the bar antenna discussed earlier. RDF loop antennas work on a principle similar to water traveling over a waterwheel. RF energy is picked up by the antenna and induces a current which moves in a circular pattern around the loop. The loop corresponds to the wave of energy passing by it thus creating a current flow in a single direction. When the leading edge of the antenna is aligned with the direction of the transmitter, the signal is the strongest. When the loop is perpendicular to the direction ofthe transmitter, the RF energy will strike the antenna face and flow in two directions at the same time. When the current reaches the receiver it is in the form of different polarities. The current literally cancels itself out; it is at this point that the null is indicated. A direction indicator is aligned with this point on the loop.
While the earlier style directional loops employed this system, the newer crossed loop antenna employs the same principle but determines direction in a slightly different manner. Because there are two loops perpendicular to each other, the antenna as a whole will pick up the signal at varying strengths in different areas of the antenna. The signals then pass to the receiver where a device called a goniometer separates the incoming signals and by measuring their varying strengths determines direction. A direction indicator is connected to the goniometer.
An RDF loop antenna will pick up a signal when either edge of the loop is aligned with the transmitter; the user can have a potential 180-degree error in his radio bearing. This is called ambiguity, and is eliminated by the simple addition of a sense system to the unit. The sense system consists of a second whip antenna that is connected to the receiving circuit. A switch allows the current flow from the second antenna to mix with the flow from the loop antenna. Ifthe polarities are the same the signal direction will be indicated by the stronger mixed signal. If the polarities are different, the input from the sense antenna will cancel the input from the loop antenna and no signal will be heard, thus indicating the loop is facing the wrong direction. All loop antennas have a definite positive and negative side which allows the system to function. The user should carefully consult the instructions for his or her particular unit to become familiar with the specific manner in which the sense system works.
Auto direction finders operate in a similar manner. The unit has a goniometer with a motor attached. The goniometer focuses in on the null point ofthe received signals, and the motor moves the direction indicator to allow the user to determine a bearing to the station.
Rotating antenna systems operate slightly differently with the moving loop picking up a signal as it passes through the RF field. The rotating loop receiver utilizes a cathode-ray tube indicator to show the RF pattern as perceived by the rotating antenna. The RF pattern can be narrowed to determine quite accurately the direction of the transmitter.
MARINE ELECTRONICS
MARINE ELECTRONICS
Nothing has changed the maritime industry more than the introduction and use of electronic systems. There has been development of new equipment in nearly every area of the marine field including navigation, cargo control, communications, collision avoidance, gyrocompass, speed indicating systems, and many others.
All modern marine electronic equipment can be classified in several major categories. These include the areas of communications covering very high frequency (VHF) bridge-to-bridge radio, single-sideband (SSB) ship to shore high seas radio, satellite communications (satcom), and the traditional radiotelegraph system. In addition to communication systems, the modern vessel operator also has come to rely on the general areas of shortwave broadcast reception for time and general navigation information as well as equipment developed for the reception of weather facsimile maps.
The next major area includes the familiar radionavigation systems:
Loran-C, Decca, Omega, which can be further classified as hyperbolic radionavigation systems. Also included in this area are the satellite navigation systems including Transit satnav and the Global Positioning System (GPS). The earliest of radio navigation systems, the worldwide network of marine radio beacons designed for use with radio direction finders (RDF) and automatic direction finders (ADF) are still quite useful.
The use and design of radar has increased dramatically since its introduction to the commercial industry in the late forties. This equipment includes the 3 centimeter (cm) and 10 centimeter (cm) wavelength radars, now coupled with automatic plotting aids or collision avoidance systems.
Equipment used to determine depth or a vessel's speed, the next category of equipment, includes the indicating and recording fathometer, electromagnetic speed log, and Doppler speed log systems. Electromechanical equipment provides us with the main type of vessel control systems; within this category are the master gyrocompass, associated repeater system, gyro pilot, ship control systems, and all the various types of equipment that have become associated with these systems. These can include auto trackers and plotters, course recorders, and other similar equipment.
Then there is the computer. With the application of computers to these and many other systems, the operation of the modern commercial vessel has become almost fully automatic. Computers not only aid us in communications, navigation, and all of the other mentioned systems but they have added new dimension to the handling of cargo, supplies, personnel, and all of the other areas in the marine field that have always been slower.
Nothing has changed the maritime industry more than the introduction and use of electronic systems. There has been development of new equipment in nearly every area of the marine field including navigation, cargo control, communications, collision avoidance, gyrocompass, speed indicating systems, and many others.
All modern marine electronic equipment can be classified in several major categories. These include the areas of communications covering very high frequency (VHF) bridge-to-bridge radio, single-sideband (SSB) ship to shore high seas radio, satellite communications (satcom), and the traditional radiotelegraph system. In addition to communication systems, the modern vessel operator also has come to rely on the general areas of shortwave broadcast reception for time and general navigation information as well as equipment developed for the reception of weather facsimile maps.
The next major area includes the familiar radionavigation systems:
Loran-C, Decca, Omega, which can be further classified as hyperbolic radionavigation systems. Also included in this area are the satellite navigation systems including Transit satnav and the Global Positioning System (GPS). The earliest of radio navigation systems, the worldwide network of marine radio beacons designed for use with radio direction finders (RDF) and automatic direction finders (ADF) are still quite useful.
The use and design of radar has increased dramatically since its introduction to the commercial industry in the late forties. This equipment includes the 3 centimeter (cm) and 10 centimeter (cm) wavelength radars, now coupled with automatic plotting aids or collision avoidance systems.
Equipment used to determine depth or a vessel's speed, the next category of equipment, includes the indicating and recording fathometer, electromagnetic speed log, and Doppler speed log systems. Electromechanical equipment provides us with the main type of vessel control systems; within this category are the master gyrocompass, associated repeater system, gyro pilot, ship control systems, and all the various types of equipment that have become associated with these systems. These can include auto trackers and plotters, course recorders, and other similar equipment.
Then there is the computer. With the application of computers to these and many other systems, the operation of the modern commercial vessel has become almost fully automatic. Computers not only aid us in communications, navigation, and all of the other mentioned systems but they have added new dimension to the handling of cargo, supplies, personnel, and all of the other areas in the marine field that have always been slower.
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