Estimation of Clock Stability Using GPS

The recent development of low-cost, high precision oscillators has allowed many applications in various fields to become financially feasible. The stability of an oscillator is ultimately what determines its usefulness for a certain application, and is therefore desirable to quantify. Current methods of evaluating stability require a direct comparison of the oscillator under test (OUT) with a more stable reference oscillator, the cost of which often offsets the initial benefit of a low-cost device. However, a relatively inexpensive GPS receiver is capable of exploiting the highly stable GPS time scale, thus obviating the need for an expensive reference oscillator. By allowing the OUT to drive a GPS receiver, and processing the data with precise GPS orbits and clock corrections to eliminate the effects of Selective Availability (SA), the time series of computed clock offsets provides a measure of the oscillator’s stability relative to GPS time. The use of GPS for assessing clock stability in the time domain is evaluated herein via the computation of Allan variance values. Performance of one rubidium and three ovenized crystal oscillators are investigated. Results show the method is limited to time intervals less than about two seconds or longer than about 300 seconds, where the effects of measurement noise an