Moon-to-Earth: Eavesdropping on the GRAIL Inter-spacecraft Time-transfer Link Using a Large Antenna and a Software Receiver

Stephan Esterhuizen

Abstract:	NASA’s twin GRAIL [1] spacecraft (Ebb and Flow) arrived at Earth’s Moon on New Year’s Day, 2012. GRAIL’s primary mission is to create a high-resolution map of the Moon’s gravitational field by measuring very precisely the change in distance between the two spacecraft [2]. Each spacecraft transmits two signals to the other spacecraft, a PRN code modulated on a 2 GHz carrier (S-band), as well as an unmodulated carrier at roughly 33 GHz (Ka-band). Since it’s not feasible to synchronize the two GRAIL spacecraft’s clocks via GPS (as was done with GRACE [3] [4], a Earth-gravity mission launched in 2002), the S-band signals are used as a timetransfer link to synchronize either Ebb’s clock to Flow or vice versa. As an independent measure to determine the clock offset of the GRAIL ultra-stable oscillators to UTC(NIST), an experiment was conducted where our JPL team used a large antenna on Earth to eavesdrop on the inter-spacecraft time-transfer link.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2351 - 2356
Cite this article:	Esterhuizen, Stephan, "Moon-to-Earth: Eavesdropping on the GRAIL Inter-spacecraft Time-transfer Link Using a Large Antenna and a Software Receiver," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2351-2356.
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