The Loran Navigation System

Line of Position from a Loran System

The name LORAN comes from the words LOng RAnge Navigation. The LORAN radionavigation system was originally designed in World War II as a navigation aid which could provide all weather fixing information for both ships and aircraft at sea. Its main use was in the Pacific theater to assist aircraft navigation in long overwater flights. The original system was known as LORAN A. It was used from 1955 through 1970 throughout the coastal waters of the North Atlantic and North Pacific regions. The last sta­tions remained operational until as recently as the late 1970's.

LORAN is a hyperbolic navigation system. In this hyperbolic system a master station generates a brief pulsed signal which is followed at a axed interval by a short pulsed signal, on the same frequency, from a slave or secondary station. The shipboard receiver, capable of measuring time in microsecond intervals, measures the interval between the arrival of the master signal and the slave signal. The resulting reading of time delay (TD) places the ship somewhere on a hyperbolic line, which represents a line of all possible points where the difference between the times of arrival of master and secondary signals is equal to the receiver reading. The loran lines printed on charts are hyperbolic lines. Each hyperbolic line is actually a line of position for the associated time differ­ence measurement.

A fix results when the receiver measures the time delays of two or more master/secondary combinations. The intersection of the hyperbolic lines representing the time delays measured is a LORAN fix. LORAN readings are considered most accurate when close to the baseline. The dashed lines extending from the base line outward from the master and secondary stations are known as the base line exten­sions. LORAN receiver readings of a station pair are not accurate if the ship is in the area of the base line extension. The typical LORAN C arrangement is for one master station to be grouped with two to four secondary stations (three secondaries is the typical arrangement). Such a grouping is called a chain. The secondaries of a given chain are designated X, Y, and Z. If four secondaries are in a chain, W is also included. Baseline distance between the master and secondary stations can be as much as 1000 miles.

Characteristics of the Loran C Navigation System

All LORAN stations in the system transmit their signals on a common frequency of 100 KHZ. The transmitted signal can travel by means of a sound wave (signal follows ground), or by means of the sky wave (signals reflected off the ionosphere). The ground wave range of LORAN C is about 1200 miles, and one hop sky wave range is about 2300 miles. One hop means one reflection of the transmitted signal off the ionosphere back to earth. The absolute maximum distance over which usable LORAN signals may be received is 3000 miles. A zone where no signal is received, called the shadow zone or skip zone, exists between the maximum range of the ground wave signal and the area where the one hop sky wave returns to the earth's surface. When in an area where only sky wave signals may be expected, sky wave corrections must be applied to the receiver time delay readings in order to correct them to equivalent ground wave values. These corrections (both day and night corrections) are printed on LORAN charts at the intersections of latitude and longitude lines.

The accuracy of positions based on LORAN C lines of position depends on the range of the receiver from the transmitting stations. At a distance of 200 miles from the stations accuracy within +/- 300 feet may be expected. At 1000 miles accuracy diminishes to +/- 500 to 1700 feet. Fixes based skywaves are substantially less accurate than ground wave fixes. As with any electronic navigation system, accuracy and reliability can be effected by interference. In addition, accuracy is decreased if the signal must travel a significant distance over land before reaching the receiver.

The format of the characteristic LORAN C signal is multi-pulse. Multi-pulse operation permits higher signal energy at the receiver while keeping transmitter power relatively low. As originally designed, the arrival of the signal from the master station triggered the signal from the slave.

The multi-pulse signal consists of 9 pulses for the master station and 8 pulses for each secondary, which are separated by 1000 microseconds. The ninth pulse, used to identify the master station, follows the eighth pulse by 2000 microseconds. The time interval between the transmission of master station signals and all secondary station signals is called the group repetition interval or GRI. This interval is unique to a particular LORAN C chain and is also called the LORAN C "rate". When expressing a LORAN C reading such as 797Q-X-1l340, the first four digits indicate the GRI, the letter represents the secondary station, and the last five digits represent the time difference reading in microseconds (millionths of a second).

The master station transmitter's ninth pulse is also used to send what is referred to as "blink." Blinking is used to warn receivers using the chain that there is an error in the transmission of a particular station or stations. An example is when the stations are not properly synchronized. Blinking is accomplished by alternately turning on and off the ninth pulse. The secondary station of the pair also blinks by turning the first two pulses of its group off and on. Blinking triggers an alarm in the shipboard receiver, so that the operator is warned against using the pair until blinking stops.

Loran C Receivers

Most modern LORAN C receivers provide displays of either actual time difference readings or direct latitude and longitude readout. When turned on the receiver should be automatically tracking all loran stations within range within about five minutes. If you are at the outer limits of LORAN C system automatic tracking may take longer. Most receivers have an alarm light that remains on until the receiver is tracking properly, and it will also come on whenever the receiver ceases to track a given station pair.

For the Coast Guard exam you should become familiar with plotting LORAN C fixes on charts with LORAN lines which are printed on the chart. It is also possible to plot LORAN LOP's on charts without a printed overlay by using the LORAN Lattice Tables. These tables are published by the Defense Mapping Agency and numbered in the 221-(XXXX) series. Plotting of LOP's by use of these tables is one of the problems which might be found on the license exam for deck licenses.

The Coast Guard will not have the actual lattice tables for your use during the exam. Instead, they have available a plotting sheet which incorporates the portions of the applicable lattice table necessary to solve the given problem.