Investigation of the Gps block iir time keeping system (TKS) anomalies caused by the voltage-controlled crystal oscillator (VCXO)

摘要

Since the launch of the first GPS Block IIR (SNV43) on 7/22/97, its TKS performance has been outstanding. SVN43 TKS has the lowest Hadamard deviation of all the GPS constellation satellite clocks. Nevertheless there have been five reported Block IIR TKS anomalies in which non-standard codes were transmitted by the SV to protect GPS users. ITT analyzed the first two events, which occurred on 12/10/97 and 2/9/98, and conjectured with limited data that large frequency jumps in the Rb clock output were the cause of both problems. These two anomalies occurred outside of the normal daily contact windows by the ground control, so no recorded telemetry is available to accurately identify or simulate the cause of these anomalies. To remedy this shortcoming, the satellite software was modified in May 1998 to store key data in several circular buffers in a +-150-second window around a TKS transient. these buffers in the SV computer memory can be dumped at subsequent SV contact with the ground for analysis. Since the upload of the modified software, three TKS anomalies were reported on 8/16/98. 1/29/99, and 7/22/99. This paper presents the TKS simulations performed to reproduce these three events, and will show that they were most likely caused by VCXO transients. Recorded data indicate VCXO transients are highly correlated with eclipses. The paper will also suggest ways to prevent or curtail anomalies caused by VCXO transients.