ACCURACY AND PRECISION OF USNO GPS CARRIER-PHASE TIME TRANSFER

The United States Naval Observatory (USNO) produces GPS carrier-phase time-transfer (GPSCPTT) estimates for approximately 100 GPS-receiver clocks daily. All estimates are available with 16-hr latency; a subset of approximately 34 are available every 6 hours with 3-hour latency plus 24 hours of predictions. All can be downloaded immediately after completion from a USNO Web site. However, despite (or perhaps because of) the continuous nature of this operation, little is known about the precision and accuracy of these estimates. The goal of this work is to assess the uncertainty of these time transfer values. Comparison measures used include estimates obtained from two-way satellite time/frequency transfer (TWSTFT), and GPS-based estimates obtained from the International GNSS Service (IGS). USNO GPSCPTT values are estimated to have an uncertainty of several hundred picoseconds when values from the IGS are used as a benchmark in the computation. Frequency values have a few times 10-15 fractional frequency uncertainty. TWSTFT values confirm that USNO GPSCPTT estimates are accurate to at least a few nanoseconds