GPS Time Tag Using Software GPS Receiver Approach

摘要

In addition to global positioning applications, GPS has been widely used for time and frequency transfer. A GPS software receiver developed at the Air Force Research Laboratory at the Wright Patterson Air Force Base only has positioning capability, although information needed for the timing and frequency applications are already embedded in the software routines for position applications. This paper presents approaches and methods developed to extract the embedded information so that time and frequency can be accessed explicitly. Specifically, we implement GPS time tag at each output data point generated by the software tracking routine. One possible application of this work is to synchronize different GPS receivers using a common GPS reference time. The paper will discuss the relationship among several important timing quantities including the receiver bias time, GPS signal propagation time from a satellite to a from a receiver antenna to the A/D converter of the receiver front end. An analysis of the positioning solution error due to the A/D sampling frequency deviation from its nominal value is also presented, followed by the description of a software approach to calibrate the sampling frequency deviation. Two different GPS receiver front ends are used in this study to experimentally demonstrate the feasibility and validity of the time and frequency transfer functions. The nominal sampling frequencies for the two receiver front ends are 5 MHz and 5.456 MHz respectively. Using the software approach described in this paper, the sampling frequencies are calibrated to have deviations of -12 and -353 Hz, respectively, from their nominal values. For both receivers, the self-consistency tests of a single receiver using different satellites as the reference satellite were performed to evaluate the time tags precision. The selfconsistent tests show that there is less than one nanosecond of time difference between the corresponding GPS time tags obtained for different reference satellites. Preliminary cross-consistent tests between two receivers were also performed to demonstrate that the timing differences between the receivers are in the tens of nanoseconds range.