Time and frequency of an atomic clock plays an essential role in satellite navigation systems: since the distance can be measured from a time, any error on the measure of time leads to an error on the user's position. Hence, an accurate and reliable prediction of satellite clock offsets is the necessary condition for the satellite based navigation system. The IRNSS Satellites are equipped with three space qualified high quality atomic clocks with Rubidium atomic frequency standards (RAFS) and two Atomic Clock Monitoring Units (ACMU). One RAFS is configured as primary frequency standard which will be used for navigation signal generation purpose and the second RAFS is configured as stand by to primary. The third RAFS is maintained as cold redundant. After the launch, when satellite reaches the desired orbit followed by routine activation and stabilization of all payloads, the primary and secondary RAFS were switched ON. Subsequently the onboard clocks are synchronized with the IRNSS reference time and appropriate frequency correction offsets to active ACMU is given through telecommand. The behavior of these RAFS is being continuously monitored through simultaneous reception of data from IRNSS Range and Integrity Monitoring Stations. The clock solution is obtained in post-processing mode. The other alternative means of evaluating the relative performance of active onboard clocks is done through monitoring telemetry phase meter data.
The paper presents the behavior of available IRNSS satellites in terms of satellite clock offsets and its drift with respect to IRNSS reference time, which are estimated by ground based Navigation Software and uplinked to the respective satellites as one of the primary navigation parameter. The results of on-board clocks show satisfactory performance with respect to the mission specifications.
IRNSS, Satellite Clock, Atomic clock
