Transit satellites provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use. The TRIAD satellite was launched in 1972 to test improvements, including a sensor designed to compensate for atmospheric drag and keep the satellite precisely in its orbit. This sensor used a mass floating inside a cavity. Small thrusters automatically fired to keep the floating mass in the middle of the cavity. Transit was superseded by the Navstar global positioning system. The use of the satellites for navigation was discontinued at the end of 1996, but the satellites continued transmitting and became the Navy Ionospheric Monitoring System (NIMS).

• 2 - McDowell, Jonathan, The United Nations Registry of Space Objects, Harvard University, 1997. HTML when accessed: http://hea-www.harvard.edu/QEDT/jcm/space/un/un.html
• 6 - JPL Mission and Spacecraft Library, Jet Propulsion Laboratory, 1997. HTML when accessed: http://msl.jpl.nasa.gov/home.html
• 279 - McDowell, Jonathan, Launch Log, October 1998. HTML when accessed: http://hea-www.harvard.edu/~jcm/space/log/launch.html