Sub-Nanosecond Clock Synchronization and Precision Deep Space Tracking

Charles Dunn, Stephen Lichten, David Jefferson, Jarnes S. Border

Abstract:	Interferometric spacecraft tracking is accomplished by the NASA Deep Space Network (DSN) by comparing the arrival time of electromagnetic spacecraft signals at ground antennas separated by baselines 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 delays 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-simultaneous quasar-based calibrations. Solutions are presented for u 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 lung 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. Incorporating GPS-based clock synchronization measurements into a spacecraft differential ranging system would allow trucking without near-simultaneous quasar observations. The impact on individual spacecraft navigation error sources due to elimination of quasar-based calibrations is presented. System implementation, including calibration of station electronic delays, is discussed.
Published in:	Proceedings of the 23th Annual Precise Time and Time Interval Systems and Applications Meeting December 3 - 5, 1991 Pasadena, California
Pages:	89 - 101
Cite this article:	Dunn, Charles, Lichten, Stephen, Jefferson, David, Border, Jarnes S., "Sub-Nanosecond Clock Synchronization and Precision Deep Space Tracking," Proceedings of the 23th Annual Precise Time and Time Interval Systems and Applications Meeting, Pasadena, California, December 1991, pp. 89-101.
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