The Performance of Ultra-stable Oscillators for the Gravity Recovery and Interior Laboratory (grail)

In early 2010, JHU/APL delivered four ultra-stable oscillators (USOs) to the Jet Propulsion Laboratory (JPL) for the Gravity Recovery and Interior Laboratory (GRAIL) mission. GRAIL is a NASA Discovery lunar mission with the goals of mapping the entire lunar gravity field and, through the geodesy determined from this result, attempt to define the most likely model for the moon’s deep interior. Similar to its 2002 predecessor, the Gravity Recovery and Climate Experiment (GRACE), GRAIL will use two formation flying spacecraft with a dual one-way ranging instrument to measure the relative line-of-sight distance between the spacecraft. From these measurements, the relative velocity between the spacecraft can be determined and gravitational force computed. Unlike GRACE, GRAIL has no continuous GPS viewpoint. To compensate for this lack of a continuous positioning reference system, the two spacecraft of GRAIL will form a time transfer link, periodically assisted by ground mission operations. The GRAIL time transfer link places an additional demand on the USO frequency stability requirements with a drift rate requirement of < 7E-11/day, nearly ten times better than the predecessor GRACE requirement. Along with achieving the drift rate, the delivered USOs showed Allan deviation of < 2 E-13 at 10 and 100 second time intervals, with two units measuring in the 7 to 9 E-14 region of stability. Our paper will present these performance details for each of the delivered USO’s and discuss the GRAIL USO requirements in the context of the emerging mission needs for both excellent short-term stability and low noise for metrology and long-term stability for assisting spacecraft navigation.