In situ measurements of USO performance in space using the twin GRAIL spacecraft

Ultra-stable oscillators (USO) are flown on a variety of different science missions to provide stable timing and/or navigation. Typically, their performance is measured at the part in 10¹³ level before launch and can only be verified in space via measurements that must propagate through, and potentially be degraded by, the Earth´s atmosphere. To date, two missions are able to demonstrate USO performance in space, without atmospheric limitations, using twin spacecraft: Gravity Recovery and Climate Experiment (GRACE) and Gravity Recovery and Interior Laboratory (GRAIL). With the recent launch of GRAIL, a NASA mission to map out the gravity field of the Moon, the clock and timing community has access to microwave tracking data from a pair of satellites flying independent USOs. In addition, since GRAIL took a circuitous three-month journey to the Moon, there was a 30-minute opportunity to obtain USO versus USO data when the spacecraft were not in orbit around the Earth or the Moon. This paper presents GRAIL Allan deviation data obtained during this payload-checkout in September 2011 and during the Science Phase in March 2012, and also analyzes data from GRACE using these new methods.