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Precision and Accuracy of USNO GPS Carrier Phase Time Transfer: 2012 Update
NAVAL OBSERVATORY WASHINGTON DC
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The United States Naval Observatory USNO produces GPS carrier-phase time-transfer GPSCPTT estimates for approximately 100 receiver clocks daily. All estimates are available with 16-h latency a subset of approximately 34 are available every 6 h with 3-h latency plus 24 h of predictions. The once-per-day post-processed estimates are referred to as rapids USR the four-times-perday estimatespredictions are referred to as ultra-rapids USU. The ultra-rapids are suitable for real-time applications. We investigate the uncertainty of USR and the first 6 h of USU predictions by comparing them to four weeks of IGS Final Clock solutions along 4-6 timing links. No day-boundary discontinuities DBDs were removed. USR exhibited a 60-100 ps RMS difference and a few to tens of picoseconds time stability with respect to the IGS Finals, supporting an estimate of 125 ps USR standalone time stability. White FM characterized the USR-IGS difference for most averaging times observed, with USR-IGS frequency stabilities of 0.9-2.1x10 -15 at averaging time tau 1 d. DBD removal would likely improve that value. USU clock predictions exhibited bi-modal performance with respect to IGS Finals, with links to site WES2 exhibiting noticeably worse behavior than links not utilizing it. USU-IGS differences were 139-220 and 470-584 ps RMS. USU-IGS time stabilities of 133 ps or better were observed for non-WES2 links, supporting an estimate of 153 ps time stability for USU clock predictions. Non-WES2 USU-IGS frequency stabilities exhibited 8x10 -14 8x10 -15 frequency stabilities for tau 6 h the prediction length.
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