Computing the USO frequency instability of GRACE satellites

The GRACE (Gravity Recovery and Climate Experiment) mission was launched in March 2002 to determine the Earth´s gravity model by using accurate measurements of the inter-satellite distance between two identical satellites. A strategy to combine two one-way ranging measurements, called dual one-way ranging (DOWR), is used to cancel out the common frequency noises of the oscillators embedded in the two satellites. The ultra-stable oscillator (USO) is the source for the reference frequency for each satellite. The time, generated by the USO, drifts due to the frequency instability of the USO. The time drift can be computed with accurate GPS phase measurement during the precision orbit determination process (POD) and leads to a time-tag correction. This study analyzes three methods to compute the upper boundary of the frequency instability level of the USOs. The first method is to use the DOWR measurement. The second method is to use the time-tag correction for each satellite. The third method is to use the difference of the time-tag corrections of the two satellites. This paper will show which method is most reliable for monitoring the level of frequency instability of the USOs.