Development and Evaluation of GPS Space Clocks for GPS III and Beyond; Journal article

摘要

The current GPS has exceeded its globally averaged position and timing accuracy of 16 m (50 YO spherical error) and 100 ns (1 U) as stated in the 1990 GPS System Operation Requirements Document (SORD). The 1999 GPS Operational Requirements Documents (ORD) set a new goalfor the GPS ZZZ and beyond. The 1999 ORD specifies the ranging accuracy 1.5 m threshold and 0.5 m objective. The 95% time transfer accuracy threshold and objective are 20 ns and 10 ns, respectively. This paper will evaluate how the current clocks and the clocks being developed can support the ORD threshold and objective. The paper will include the following topics: (1) atomic clocks on the GPS Block ZZ space vehicles, ( 2 ) estimated accuracy of the ZZF Rb clock by Perkin Elmer and digital Cs clock by Datum-Beverly and assessment of their performance against the ORD threshold range requirements, (3) description of the new space clocks being developed jointly by the GPS JPO, Aerospace, and NRL, and evaluation of their predicted pe6ormunce to see if they can support the ORD objective of 0.5 m (rms), and (4) Prediction of the GPS signul-in-space accuracy, including all the space and control segments errors, using ZZF Rb and Cs clocks. The predictions are based on replacing the NZMA estimated GPS ZZ/IZA/ZZR clock data, contained in the actual tracking data of the GPS monitor stations and the NlMA tracking stations, by simulated IZF Rb and Cs clock data. A Kalman filter similar to that of the OCS then processes the resulting tracking data and the estimated results are compared with NIMA estimates treated as truth. Evaluations of the various options to see whether the ORD objective can be achieved based on the predicted signal-in-space accuracy are included.