Rubidium Clock on SVN62: A Bit Disappointing

August 19, 2010 By: Richard B. Langley

The rubidium atomic clock in use on SVN62, the first block IIF satellite that was launched on May 28, 2010, is not living up to expectations, according to a report from the United States Naval Research Laboratory. The clock is used to control all of the signals transmitted by the satellite, and its performance ultimately affects achievable GPS positioning accuracy.

Using data from the Center for Orbit Determination in Europe, the NRL report documents anomalous fluctuations in the clock signal with distinct 12-hour and 6-hour periodicities (see figure). The exact cause of the fluctuations has not yet been identified, although it is speculated that the fluctuations are of thermal origin like SVN62’s L5 phase variance detected earlier. But note that the clock signal analysis relies only on L1 and L2 measurements.

These fluctuations detract from an otherwise acceptable performance of the clock, which at averaging intervals shorter than about 1,000 seconds is better than any existing on-orbit GPS clock. But over longer periods, it performs only like an average Block-IIR rubidium clock. This is disappointing, since similar IIF rubidium clocks tested on the ground show performance approaching that of hydrogen masers, the highest stability clocks in common use. Hydrogen masers will be used on all Galileo satellites and one is in use on the GIOVE-B test satellite.

Phase (time) error residuals of SVN62 with respect to UTC(NRL) for the period July 17 through July 26 after the removal of a conventional frequency model to remove a constant offset and long-term drift.