Enhancements to the GPS Block Iir Timekeeping System

John Petzinger, Randall Reith, Todd Dass

Abstract:	The ITT Industries-developed GPS IIR satellite payloads have been on-orbit since 1997, providing outstanding signal-in-space performance. Much of credit for this outstanding performance can be given to the GPS IIR Time Keeping System (TKS), and the GPS-IIR spacecraft bus, which keeps the payload in a mechanically and thermally stable condition. The TKS consists of a VCXO phase-locked to a rubidium atomic frequency standard (RAFS) via a control loop implemented in software. Because the TKS is constantly comparing the phase of a VCXO to the phase of the RAFS, it can autonomously protect GPS users from random frequency variations that might occur in either component, and also from potential system failures in the TKS. In addition, because the TKS control loop is implemented in software, it can be modified or tuned to handle these random events in an optimal fashion. Although SVN 43’s VCXO is atypical, we characterize the frequency of occurrence, magnitude, and seasonality of its frequency variations. We then describe how the addition of a nonlinear error processor to the TKS control loop, called an Adaptive Time Constant (ATC), was used to meet the challenge of the frequency variations. The rationale for ATC, an approximate maximum likelihood error detector, is presented. By analyzing on-orbit telemetry, we show how the TKS, after activation of ATC, has robustly shielded GPS users from VCXO disturbances that would have caused outages. Data from both SVN 43 and SVN 46 show that even during stable periods, ATC reduces undesirable noise from the TKS output. Using a detailed TKS simulation, we show that the TKS with ATC still protects users from potential anomalies in the RAFS. The conclusion is that ATC has improved TKS timing stability and TKS availability on the SVs that are using it.
Published in:	Proceedings of the 34th Annual Precise Time and Time Interval Systems and Applications Meeting December 3 - 5, 2002 Hyatt Regency Reston Town Center Reston, Virginia
Pages:	89 - 107
Cite this article:	Petzinger, John, Reith, Randall, Dass, Todd, "Enhancements to the GPS Block Iir Timekeeping System," Proceedings of the 34th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, December 2002, pp. 89-107.
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