Transit

Summary form only given. The US Navy Navigational Satellite System, aka TRANSIT, is the nation´s only operational navigation satellite system at this time. Its primary mission and the reason it was first developed is to supply accurate positions to the Navy´s ballistic missile submarines so they can periodically update their inertial navigation systems. There are currently 7 operational TRANSIT satellites in 1000 km altitude polar orbits, 4 doppler tracking stations in Maine, Minnesota, California and Hawaii and a command, control and orbit computation center in California. It is noteworthy that a single operating satellite is sufficient for the primary mission. Both orbit determination and navigation are accomplished through measurements of the doppler shifts of ultra-stable satellite radio transmissions at 150/400 MHz. These transmissions also carry 2 minute time markers synchronized to within a few microseconds of Universal Time. Single frequency (400 MHz) navigation receivers provide positions accurate to about 0.1 nm. Dual frequency navigation results in accuracies of about 0.05 nm. Dual frequency geodetic receivers using some tens of satellite passes at a fixed location have produced positions to better than 50 cm. The Navy´s Special Projects Office plans to phase out TRANSIT in 1994, when SPO will switch to the Global Positioning System (GPS). Current delays in establishing GPS as a fully operational system (GPS satellites are to be launched by the Shuttle) could delay the TRANSIT/GPS changeover.