Performance Evaluation of the GPS Block IIR Time Keeping System

The Time Keeping System (TKS) is essential to the GPS Block IIR total navigation payload and must provide an accurate time base for each GPS space vehicle. Performance analysis of the TKS has been reported previously for the case in which a rubidium (Rb) atomic frequency standard is used as a reference. In this paper the system error model of the TKS is developed and the performance of the TKS output using either a cesium (Cs)or a rubidium (Rb) atomic frequency standards as a reference is evaluated. The contributions to the TKS output Allan deviation was examined from each of the three noise sources: atomic frequency standard (either Cs or Rb), voltage control crystal oscillator, and phase meter (PM). In addition, since the TKS does not have a baseplate temperature controller and the VCXO of the TKS is temperature-sensitive, a TKS error is induced when the VCXO is subject to the orbital temperature variations or the space vehicle umbra during solar eclipse. These two temperature-induced TKS output errors are also examined. Sensitivities of the Allan variance at the TKS output for each of its independent input noises are provided and they are valuable for design trade-off and troubleshooting. The results of the paper compare favorably with those obtained by the TKS system testing. The paper will show that when the TKS components meet or exceed their specifications, the RAFS TKS and the CAFS TKS should meet their specifications. See also ADA419480. Published in Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting (28th), p441-453, 1996. Presented at Proceedings of Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting (28th) held at the Hyatt Regency Hotel, Reston Town Center, Reston, VA, 3-5 December 1996.