A navigation-satellite receiver comprises a crystal oscillator that is affected by local ambient temperature in a repeatable way. After locking onto a GPS satellite, the receiver is used to calculate the true frequency bias of the local crystal oscillator. GPS-system lock provides an atomic-clock basis for such measurements of true frequency. The current temperature of the crystal is measured and recorded in association with the true frequency bias measurement. The data is then used to generate a ninth-order polynomial that describes the frequency drift of the crystal over temperature. Then during receiver initialization when the local reference oscillator is not in lock, the ambient temperature is measured and used to index the ninth-order polynomial to estimate the actual crystal frequency. Such frequency estimate is then used as a basis to find signal from visible SV's in an overhead constellation.