Effect of space vehicle temperature on the frequency of on-orbit Navstar clocks

Analysis of the frequency of on-orbit Navstar clocks is performed by the Naval Research Laboratory using both broadcast and post-processed precise ephemerides. The phase offset between the Navstar clock and the reference clock is computed from pseudo-range measurements obtained by the five GPS monitor sites and by the US Naval Observatory, Washington, DC, precise-time site using precise-positioning-service dual-frequency, receivers which correct for selective availability. The frequency offset of the early Block I rubidium clocks was found to be highly correlated with the temperature of the space vehicle and consequently with the earth eclipse seasons. By providing subsequent space vehicles with rubidium clocks having additional temperature control this correlation was suppressed. The Block I cesium clocks, on the other hand, showed no sensitivity to the space vehicle temperature. Recently, however, two Block II cesium clocks and a Block I rubidium clock with the additional temperature control have evidenced temperature sensitivity and have shown strong correlation with the onset of the eclipse seasons. Frequency-onset histories show this correlation for the cesium clocks for the first time. Scatter diagrams used to measure the degree of correlation of the frequency with temperature yield temperature coefficients larger than those measured in the laboratory prior to launch