Today in History – April 13, 1960 – First U.S. navigational satellite. Transit satellites were used by the US Navy to develop the first operational navigation satellite system The Transit satellites provided an accurate, all-weather navigational aid for ballistic missile submarines and surface vessels and aircraft. The system was designed such that any craft could pinpoint its position by using a computer specially programmed to translate coded radio signals beamed from the satellites into latitude and longitude.

Transit 1-B, the first in the series, was placed in a north-south polar orbit on April 13, 1960. It had a 40-month life-span, but it operated for only 89 days. Transit 1-B transmitted on two frequency pairs to test the technique for refraction correction and to determine if the transmitted frequencies should be close together or far apart. It also tested a magnetic torque device for spacecraft attitude control – the first satellite to do so. Three advanced Transit models equipped with nuclear-power generators were launched from June 22, 1960, to Nov. 15, 1961.

Transit provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use. Transit receivers used the known characteristics of the satellites orbit, measured the Doppler shift of the satellite’s radio signal, and thereby calculated the receivers position on the earth. Individual satellites operated for over 10 years. Technical breakthroughs during the program included gravity gradient stabilization, the use of radio-isotope thermoelectric generators (RTG), and navigation satellite technologies used in the later GPS series. The TRIAD satellite was launched in 1972 to test improvements. Transit was superseded by the Navstar global positioning system.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).

Also on this date in 1974, the Westar-1 satellite was launched by Western Union and NASA. This was the first commercially-launched American geosynchronous communications satellite.

See the Engineering Pathway’s educational resources on satellites and navigation. For curricular resources, visit the Aerospace Engineering Education, Electrical Engineering Education and the Surveying and Geomatics Engineering Education community sites.