Precision and accuracy of USNO GPS carrier phase time transfer: Further studies

The United States Naval Observatory (USNO) produces GPS carrier-phase time-transfer (GPSCPTT) estimates for approximately 100 receiver clocks with 16-hr latency daily in its so-called "rapid" processing. We investigate the uncertainty of these estimates by comparing one year\´s worth of USNO rapid GPSCPTT estimates to those obtained from two-way satellite time transfer (TWSTT) and from the 1GS rapid combined solutions. The time stability of USNO rapid GPSCPTT estimates appears to lie between tens and hundreds of ps for averaging times of 300 s and larger, remaining approximately constant as averaging time increases. The frequency stability of L\´SINO GPSCPTT estimates lies between 10^-15 and 10^-14 at an averaging time of one day, decreases as averaging time increases, and could potentially achieve 10^-16 in 10 d. More work is needed to establish credible frequency-uncertainty estimates at the averaging times (5-20 d) characteristic of the overlap between primary frequency standard evaluations.