Relative time and frequency alignment between two low earth orbiters, GRACE

Abstract

The two GRACE (Gravity Recovery and Climate Experiment) spacecraft were launched into a near polar circular orbit around the earth in March of 2002. The two spacecraft serve as test masses to measure the Earth's gravitational field. Both spacecraft carry ultra-stable oscillators (USO) with an Allan Deviation of a few parts in 10 -13 for Tau = 1 to 1000 s. The USO's drive both the microwave links and GPS receivers. To cancel out long term errors on the USO's a linear combination of the 1-way microwave links is used (dual-one-way). In order to form the dual-one-way measurement and cancel our long term USO error, time must be synchronized between the two spacecraft to about 150 picoseconds. This synchronization is accomplished using the GPS data. For each spacecraft, the GPS data are used to solve for the orbital positions and the difference between the on-board clocks and a ground reference clock every 5 minutes. The relative clock is determined by the difference of these two solutions.Validation of the relative clock accuracy includes the solutions from overlapping data arcs which are typically less than the 150 picosecond goal and unique combination of the one-way microwave links that allows independent comparison of the GPS determine relative frequency of the USO's to a measurement made by the microwave link.

LEO satellite clock analysis and prediction for positioning applications

“…10 À 12 with an averaging time of 10 s are worse than the short-term stability of the USOs at 10 À 13 (Dunn et al 2003;Ko and Tapley 2010;Bertiger et al 2003;Weinbach and Schön 2012). This suggests that in the short-term phase, the GNSS estimation errors play a more important role than the USO clock stability itself.…”

“…According to (Dunn et al 2003;Ko and Tapley 2010;Bertiger et al 2003;Weinbach and Schön 2012), the USOs on-board the GRACE satellites should generally have an Allan deviation of 1 À 3 � 10 À 13 for an averaging time from 10 s to 1000 s. The frequency drift onboard the GRACE A and B satellites at 1000 days after launch amount to about À 2:8 � 10 À 11 /day and À 7:6 � 10 À 12 /day, respectively, and the USOs onboard the New Horizons (NH) spacecraft were also demonstrated to be around � 10 À 11 /day at 700 days after launch (Weaver, Reinhart, and Wallis 2008). As such, to simulate the clock behaviors of a typical spaceborne USO, flicker frequency modulation (FM) noise with an MDEV slope of zero at 1:5 � 10 À 13 is overlapped with a frequency drift of 10 À 11 /day, which has an MDEV slope of 1.…”

“…Examples are missions like the gravity recovery and climate experiment (GRACE) (Case, Kruizinga, and Wu 2010), the GRACE Follow-On (GRACE-FO) (Wen et al 2019), and the SENTINEL-3 (ESA 2012). The short-term stability of a typical USO can reach the level of 10 À 13 (Dunn et al 2003;Ko and Tapley 2010;Bertiger et al 2003;Weinbach and Schön 2012). Without a very high requirement on clock stability, most LEO satellites are equipped with oven-controlled crystal oscillators (OCXOs), or more practically, the temperaturecompensated crystal oscillators (TCXOs), which have a lower stability (Et Alia 2019).…”

Systematic reliability improvements on the GRAIL project

“…The fundamental method used for the ranging measurement has a long history of use over dozens of missions as a primary navigation tool. This method was further successfully extended on the GRACE mission [7], [8], [9] , which has been successfully mapping the Earth's gravity field since that launch in March, 2002.…”

Enhancing the kinematic precise orbit determination of low earth orbiters using GPS receiver clock modelling

“…A three-state model including clock time, frequency and frequency drift is commonly used to represent the stability Table 1 USO Stability on GRACE B Satellite (Bertiger et al, 2003).…”