Nowadays the rate of every orbiting GPS clock is adjusted by this "factoryoffset" before launch. But before the first GPS satellite was launched in 1977,although it was recognized that orbiting clocks would require such arelativistic offset, there was uncertainty as to its magnitude, and even its sign.So correcting frequency synthesizers were built into the clocks, spanning alarge enough range around the nominal 10.23 MHz clock frequency toencompass all possibilities. After the satellite's cesium atomic clock wasturned on, it was operated for three weeks to measure its rate. The frequencyshift measured during this initial period was found to be 4.425 parts per tenbillion, agreeing with the relativistic calculation to better than 1%.
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