Mars Express and the NASA landers and rovers on Mars - Sustaining a backup relay in an interplanetary network

摘要

The Mars Express Lander Communications subsystem MELACOM was initially designed to relay data from/to BEAGLE-2 once landed on Mars. But its receiver did not pick-up a signal during all the attempts of communications with the European lander in the two first months of 2004. Fortunately, intrinsic to the design was the capability for cross-support with other landed assets via the implementation of version 2 of the draft Proximity-1 protocol, CCSDS 211.0-R-2, designed to ensure reliable data transfer between remote autonomous communication nodes operating in close proximity. Since then, MELACOM has been successfully used to test the Forward and Return link with the NASA-JPL Mars Exploration Rovers Spirit and Opportunity and with the lander Phoenix, perform Doppler measurements with Opportunity, and record in open loop the Entry, Descent and Landing of Phoenix at Mars (25 May 2008). The purpose was to provide JPL with carrier and tones detection and possible contingency support via full spectrum analysis. In July 2010, in order to complement its own orbiters coverage, NASA requested ESA to provide a backup relay support at Mars with Mars-Express, for the MSL-Curiosity mission, in cruise to Mars since its launch on the 26 November 2011. This support covering the MSL EDL and on-surface operations, is made possible by the third mission extension granted to Mars Express until end of 2014, and by a rephasing of the spacecraft orbit early April 2012. This paper presents why this support is a new challenge for Mars-Express : 1. In October 2011, the Mars-Express operations concept has been changed and made more robust to mass-memory anomalies experienced since August 2011. This led in particular to prepare for the MSL support, the backup MELACOM radio unit of Mars-Express, not switched on for 8 years in orbit around Mars. 2. In order to improve the MELACOM capability on the forward and return link, ESOC granted to its UK manufacturer QinetiQ, a contract to develop new software, loaded and tested on Mars-Express with Opportunity before MSL arrives at Mars. 3. Curiosity's UHF radio is an ElectraLite model, the design of which is different from the CE-505 carried by both MERs and Phoenix. Ground compatibility tests took place at JPL in March 2012 to verify the radio interfaces. 4. This support has to interface with the new Mars Relay Operations Service (MaROS) infrastructure at JPL which did not exist during the Phoenix mission. 5. Wherever possible, it is intended to integrate MSL activities with Mars-Express mission planning and science observations of its payload instruments, in order to prove the concept of MSL operations via the backup relay channel and improve MSL data return. Mars-Express has planning cycles of several months, to be compared to a few Martian days planning cycle for rover operations. Initial test passes from August 2012 after MSL landing should take place to prove link and contingency support capability. Further relay operations between MEX, Curiosity and Opportunity are also considered in the future. In conclusion, the paper draws a perspective for these cross-support relay activities and steps towards further building interplanetary network infrastructure.