Rapid Re-Acquisition of Coherent Doppler following a Planetary Occultation in the MESSENGER Mission to Mercury
- Paper number
IAC-08.B2.3.5
- Author
Mr. Karl Fielhauer, Johns Hopkins University Applied Physics Laboratory, United States
- Coauthor
Mr. Dipak Srinivasan, Johns Hopkins University Applied Physics Laboratory, United States
- Year
2008
- Abstract
The MESSENGER spacecraft, launched 3 August 2004, is currently en route to its 18 March 2011 insertion into Mercury orbit. During the cruise phase, there are three Mercury flybys for trajectory optimization , during which science data, including coherent RF Doppler measurements required for the gravity science investigation, are collected. The first flyby, which occurred on 14 January 2008, included a 45-minute occultation that completed less than 4 minutes prior to the closest approach (CA) of the planet. The Doppler data around CA are the most important data for the gravity field determination, which in turn provides information about the planet’s interior. The spacecraft occultation prior to CA presented a significant challenge to the RF operation, in that the uplink and downlink signals had to be re-locked with high-precision timing and pointing with the NASA Deep Space Network’s 70-m dish at the Goldstone complex. An optimized uplink acquisition sweep, which minimized the total acquisition time by minimizing the sweep range and maximizing the sweep rate based on precise predictions of the spacecraft’s transponder best-lock-frequency and RF link margins, was developed, tested, and implemented for this event. The ground station antenna pointing also had to be off-pointed from nominal (optimized downlink) to an optimized uplink/downlink pointing due to the high tangential velocity of the spacecraft; the uplink position of the spacecraft differed from the downlink position sufficiently such that the required uplink power margins would have been insufficient for the rapid acquisition under the nominal pointing configuration. These new pointing requirements were developed, tested, and implemented during the first Mercury flyby. The overall plan developed for this operation was successfully implemented and provided the required data for the gravity science investigation. A similar operation will be necessary for later MESSENGER operations, and may be generalized for other spacecraft missions in which coherent Doppler data is required or desired quickly after a planetary occultation egress.
- Abstract document
- Manuscript document
(absent)