First Phase Clocks and Frequency Transfer

Abstract

In Švehla and Rothacher (2004a, b, 2005a, 2006b), it was demonstrated for the first time that clock parameters for GPS satellites and ground stations can be estimated solely from the carrier-phase GPS measurements. These also allow frequency transfer with a very high level of accuracy of a few parts in 10^{- 16} (≈25 ps/day in terms of linear time rate). The main motivation for the development of the phase clock approach is to avoid the colored systematic noise that is introduced by using code, or smoothed code GPS measurements and other possible biases in the official GNSS clock parameters provided by IGS. On the other hand, phase clocks completely absorb the GPS radial orbit error and are fully consistent with the LEO carrier-phase measurements when determining kinematic or reduced-dynamic LEO orbits, since in both cases carrier-phase ambiguities are estimated. Phase measurements from a GPS ground network of about 40–50 stations tracking about 30 GPS satellites in MEO orbit form a closed, internally connected system, in which the phase information of one clock can be related to that of any other GPS satellite or a ground station clock in the network, even on the antipodal side of the world. This opens up the possibility of high-precision positioning and especially intercontinental non-common view frequency transfer of utmost accuracy. We may say, phase clocks are the optimal way to compare phase information between ground station clocks and/or LEO/GNSS satellites. Later on in this thesis, we introduce the concept of track-to-track ambiguities to optimally fix carrier-phase ambiguities to their integer values.

Later, phase clocks were also studied in Dach et al. (2005, 2006); Bauch et al. (2006) and in Matsakis et al. (2006) over longer periods of time and have been compared to other time/frequency comparison techniques. Ambiguity resolution with phase clocks was demonstrated for the first time in Švehla and Rothacher (2006a) and later on in Mercier and Laurichesse (2007), Delporte et al. (2007, 2008). Starting with GPS Week 1449, JPL started providing additional information on clock time bias and drift relative to the reference clock in the IGS network in their IGS reports, see Desai (2007). In their IGS reports, as a reference clock JPL uses exclusively IGS station USN3 (US Naval Observatory), or in some cases AMC2 (Colorado Springs). Besides CNES, all IGS Analysis Centers provide satellite clock parameters calculated using carrier-phase and pseudo-range measurements in order to support both time and frequency transfer at the same time. Thus, IGS clock parameters are more applicable to PPP (Precise Point Positioning) than to frequency transfer. This section describes the estimation of phase clocks and their application in frequency transfer and precise point positioning.