Determination of Ginga Clock Rate

A clock marker is inserted into the Ginga data stream every 8 sec, along with a running cycle count. This clock marker in the real time data stream is compared to a clock at the KSC ground station at the beginning of every contact pass. The time of comparison is corrected for light transit time from the satellite (about 8 ms), to yield the time of generation. The average clock period is obtained by dividing successive times of generation by the number of intervening clock cycles - there are approximately 750 clock cycles during a single satellite orbit of some 97 minutes duration. The average clock period is always within a few microseconds of 8 s, and provides a basis for assigning time of observation to the data by linear interpolation between successive clock comparisons.

In order to investigate the stability of the clock rate, the clock period was computed for a segment of data approximately 20 days in duration, from 1988 August 23 to 1988 September 3. The results of this computation are shown in Figure 1. It can be seen that the clock period drifts by at least 10 µs from the nominal value of 8 sec, and that this drift can be quite rapid, with large changes occurring within the space of five hours. We suspected that a major portion of the variation in clock rate was due to temperature variations in the clock circuit, and so we next investigated temperature variations in the satellite.