On-Orbit Frequency Stability Analysis of the GPS NAVSTAR-1 Quartz Clock and the NAVSTARs-6 and -8 Rubidium Clocks

摘要

This paper presents an on-orbit frequency stability performance analysis of the GPS NAVSTAR-1 quartz clock and the NAVSTARs-6 and -8 rubidium clocks. The clock offsets were obtained from measurements taken at the GPS monitor stations which use high performance cesium standards as a reference. Clock performance is characterized through the use of the Allan variance, which is evaluate for sample times of 15 minutes to two hours, and from one day to 10 days. The quartz and rubidium clocks' offsets were corrected for aging rate before computing the frequency stability. The effect of small errors in aging rate is presented for the NAVSTAR-8 rubidium clock's stability analysis. The analysis includes presentation of time and frequency residuals with respect to linear and quadratic models, which aid in obtaining systematic and random effects. The frequency stability values were further processed with a time domain noise process analysis, which is used to classify random noise process and modulation type. NAVSTAR-1 results indicate good performance for a quartz clock. Comparison of the quartz clock's stability with the best on-orbit cesium clock results indicates that the cesium standard is more stable by at least a factor of two for a 900 second sample, and increases to two orders of magnitude for a one day sample time. The NAVSTAR-8 rubidium clock differed from the NAVSTAR-6 rubidium clock in its improved thermal environment. This rubidium clock exhibited an effect that lasted for nearly five months. Following this transient, the rubidium clock performance with better-than-expected stability. A final discussion of quartz, rubidium, and cesium on-orbit will be presented.