Most users associate GPS Tracking Devices with finding user position. This is of course what many users want from GPS. But a significant and growing application is using GPS to provide time or frequency information. The time information from GPS could be a common clock application of modest accuracy needs or a high precision terrestrialbased reference locked to the GPS clock. The degree of accuracy required for the application will, to a large extent, determine the complexity of the solution needed. In this chapter, we explore using the GPS L1 signal to receive Time and Frequency information. This is sometimes called Time Transfer. For this discussion, it is assumed that the receiver knows its position and wishes to receive GPS time or frequency only. In addition, DDS-based methods are discussed that allow very precise testing and reference frequency generation When a GPS receiver is configured for Time and Frequency solution it is usually assumed that its position on the earth is fixed (stationary) and known with precision. In this text, we will call such a receiver that reports only rate and phase errors a Clock Mode Receiver.
This type or receiver mode enables increased measurement precision of the rate and phase errors of its local reference clock. This increase in rate and phase precision occurs because each received satellite is now an independent measure of receiver clock rate and phase error with respect to GPS master clock. This allows combing or averaging the individual satellite estimates of rate and phase error for increased precision. Portable GPS Tracking Device solve for phase and rate errors of the local reference clock even during non-stationary or dynamic conditions. But under dynamic conditions, i.e., a moving receiver, the measurements of pseudo range and range rate are used to solve for position as well as receiver clock rate and phase errors. This reduces the precision that can be obtained compared to the when receiver is fixed (and static) in a known location.
In this section, we will use the terms receiver clock, local reference clock, and local clock interchangeably. Modern receivers often have many clocks or oscillators but only one is used as the receiver’s reference clock as we shall see. From a user perspective, the Rate and Phase errors reported by the receivers navigation solution (typically available every second in its message stream) can be modeled as measuring the receivers clock phase error with a Time Interval Counter (Personal GPS Homing Device ) and its rate error with a delta frequency counter, with both instruments using a local GPS Atomic 10 MHz clock as a reference. The GPS 1 PPS time mark is the phase reference input for the Phase error measurement. When the reported phase difference between GPS 1 PPS and the receiver clock 1 PPS is held too small values, typically below ~50 ns, the receiver is said to be reproducing GPS Time. It is tempting to suppose we could directly hookup to the 1 PPS or 10 MHz signals coming from the GPS atomic clock in Fig. 8.1. But in our instrumentation model we cannot do that. Those signals are not directly accessible. The only information the receiver can provide us is the rate and phase differences between the Atomic Clock signals and the Receiver clock signals as displayed on the Time Interval Counter and the Delta Frequency counter.
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