All on-board measurements were regulated by signals derived from the on-board clock. The stability of these timings can be checked by comparing the on-board times with the ground-station times. Hipparcos had very limited memory for on-board data storage, and its operations depended on continuous contact with a ground station for immediate downloading of observations. The telemetry was sent to the ground station in telemetry formats, covering 10.667 s of operations (spacecraft and payload). Each format received by the ground station, was time-tagged using a signal from the ground-station clock. With the orbital position of the satellite generally known to within a few hundred meter, the time delay between satellite and ground station could be calculated to the sub-micro second level, thus providing a direct comparison between on-board and ground-station clocks. For obvious reasons, the groundstation clock was considerably more stable than the on-board clock, and drifts between the two could generally be assigned to the on-board clock. What was not always stable, however, was the ground-station time delay: the time passed between the signal reaching the antenna and it being time-tagged. This applied in particular to the Goldstone ground station.
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