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In page Transit (satellite):

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The US Air Force also periodically launched short lived satellites equipped with radio beacons of 162 MHz and 324 MHz at much lower orbits to study orbital drag.[citation needed] The Transit ground tracking stations tracked these satellites as well, locating the satellites within their orbits using the same principles. The satellite location data was used to collect orbital drag data, including variations in the upper atmosphere and the Earth's gravitational field.