QUESTION: I have a question on GPS systems. Where is the satellite's reference clock? Is it on board the satellite or is the information relayed to the satellite via an uplink? Also what kind of clock is it?
ANSWER: GPS satellites all have on-board clocks. The types of clocks on-board vary by the block (vintage) of the satellite. Most satellites have 2 rubidium and 2 cesium clocks. These clocks are monitored and can be corrected from the earth via an uplink from one of the ground stations.
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