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.
