Transit is a space-based radiodetermination system consisting of satellites in approximately 600 nm polar orbits. The phasing of the satellites is deliberately staggered to minimize time between fixes for users. In addition, Transit has four ground-based monitors. The monitor stations track each satellite while in view and provide the tracking information necessary to update satellite orbital parameters every 12 hours.
Normally, a minimum of four operational TRANSIT satellites were needed to provide the required frequency of precise navigation fixes. The constellation consisted of two types of spacecraft -- Oscar and Nova. The final constellation consisted of six satellites in a polar orbit with a nominal 600 nautical mile altitude, three ground control stations, and receivers (i.e., the system's users). Of the six satellites, three provided navigation service while three others were "stored-in-orbit" spares. The last Transit satellite launch was in August 1988. The Transit Program terminated navigation service on December 31, 1996.
The satellites broadcast ephemeris information continuously on 150 and 400 MHz. One frequency is required to determine a position. However, by using the two frequencies, higher accuracy can be attained. A receiver measures successive Doppler, or apparent frequency shifts of the signal, as the satellite approaches or passes the user. The receiver then calculates the geographic position of the user based on knowledge of the satellite position that is transmitted from the satellite every two minutes, and knowledge of the doppler shift of the satellite signal.
Predictable positioning accuracy is 500 meters for a single frequency receiver and 25 meters for a dual frequency receiver. Repeatable positioning accuracy is 50 meters for a single frequency and 15 meters for a dual frequency receiver. Relative positioning accuracy of less than 10 meters has been measured through translocation techniques. Navigational accuracy is heavily dependent upon the accuracy to which vessel course, speed, and time are known. A one knot velocity input error can cause up to 0.2 nm fix error.
Availability is better than 99 percent when a Transit satellite is in view. It depends on user latitude, antenna mask angle, user maneuvers during a satellite pass, the number of operational satellites and satellite configuration. The reliability of the Transit satellites is greater than 99 percent. Coverage is worldwide but not continuous due to the relatively low altitude of the Transit satellites and the precession of satellite orbits.
Transit satellites provide a two-dimensional fix. Fix rate varies with latitude, theoretically from an average of 110 minutes at the equator to an average of 30 minutes at 80 degrees. Presently, due to non-uniform orbital precession, the Transit satellites are no longer in evenly spaced orbits. Consequently, a user can occasionally expect a period greater than 6 hours between fixes. This condition exists for less than 5 percent of system availability.
Transit satellite signals are monitored by the Naval Astronautics Group (NAG) at Point Mugu, California, which serves as the satellite constellation ground control facility. Whenever a satellite-transmitted signal is out-of-tolerance or otherwise unsuitable for use, NAG will issue a “SPATRAK” alerting message to all known U.S. Navy Transit users, with an information copy to DMA. DMA then ensures that the alert is entered into the Notice to Mariners system for distribution to civil users. The same procedure is used for scheduled test or preventative maintenance periods on selected satellites. Transit receivers do not possess inherent signal integrity monitoring capabilities, other than the ability to recognize and reject the scrambled signal format broadcast by selected satellites during certain NAG-implemented system tests.
The 110 pound Oscar spacecraft demonstrated a long lifetime, with an average in orbit operating time of 12 years. Oscar 13, which operated for 21 years and eight months before its power system failed, held the record for length of service in the Transit program.
SOOS, an acronym for "Stacked Oscars on Scout," consists of Oscar spacecraft launched in pairs on a single Scout launch vehicle. The stacked launch of two satellites and separation technique placed the two Oscars in virtually the same orbit plane.
A Nova spacecraft is an improved Oscar. The 350-pound "Nova-2" satellite is the last of three Novas in the Transit constellation. Nova-1, -2, and -3 each operated for an average of nine years before succumbing to power system failures.
Day-to-day TRANSIT operations were conducted by the Naval Space Operations Center headquartered at Point Mugu, Calif. The Naval Space Operations Center provided navigation updates and satellite telemetry, tracking, and control to maintain TRANSIT navigation accuracies. John Hopkins Applied Physics Laboratory [JHU APL] was the inventor and design agent for all aspects of Transit. The Laboratory served as the technical resource for enhancements and improvements during the system's more than 34 year lifespan.