Sub-Nanosecond Clock Synchronization and Precision Deep Space Tracking

摘要

Interferometric spacecraft tracking is accomplished by the NASA Deep Space Network (DSN) by computing the arrival time of electromagnetic spacecraft signals at ground antennas separated by timeliness on the order of 8000 km. Clock synchronization errors within and between DSN stations directly impact the attainable tracking accuracy, with a 0.3 ns error in clock sync resulting in an 11 nrad angular position error. This level of synchronization is currently achieved by observing a quasar which is angularly close to the spacecraft just after the spacecraft observations. By determining the differential arrival times of the random quasar signal at the stations, clock synchronization and propagation debs within the atmosphere and within the DSN stations are calibrated. Recent developments in time transfer techniques may allow medium accuracy (50-100 nrad) spacecraft observations without near- simultaneously quasar-based calibrations. Solutions are presented for the global network of GPS receivers in which the formal errors in clock offset parameters are less than 0.5 ns. Comparisons of clock rate offsets derived from GPS measurements and from very long baseline interferometry and the examination of cluck closure suggest that these formal errors are a realistic measure of GPS-based clock offset precision and accuracy.