Numbered panels in Figure 1 correspond to changes in the GBT signal shown in Figure 2:

(1) Phoenix separates from its cruise vehicle and turns on its transmitter.

(2) The frequency, which had been slowly decreasing due to the Doppler effect of Phoenix’s free fall towards Mars, now increases rapidly (from 2 to 3) as atmospheric friction slows the spacecraft from 20,000 to 900 mph.

(3) The parachute deploys, slowing Phoenix to ~ 200 mph.

(4) The parachute is jettisoned, and the retrorockets fire, bringing Phoenix down to a soft landing on Mars.

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The Green Bank Telescope (GBT) supported the successful landing of the NASA Phoenix Mars Lander on May 25, directly observing radio frequency transmissions from the lander. Phoenix is the first spacecraft to land in Mars’ polar region and study its landscape, subsurface ice, and minerals.

NASA’s Mars Polar Lander failed in 1999 due to a software glitch that caused the landing rockets to shut off prematurely. Lessons learned from that mission helped make Phoenix a success. NASA placed great importance on monitoring Phoenix via multiple channels throughout the critical atmospheric entry, descent, and landing (EDL) sequence.

Phoenix was designed to transmit data to one or more of the three satellites orbiting Mars: Renaissance, Odyssey, and Express. These orbiters relay data at X-band to one or more of the Deep Space Network (DSN) 70-meter antennas in California, Australia, and Spain.

For the critical EDL sequence, NASA wanted to also monitor the signals directly from the spacecraft. Only the 100-meter Robert C. Byrd Green Bank Telescope is sensitive enough at the frequency (~ 401.6 MHz) of Phoenix's faint signals from Mars to accomplish this vital direct monitoring task.

If a malfunction occurred during EDL, GBT data would be vital to the subsequent analysis and diagnosis. Fortunately, everything worked correctly. The GBT data are now being integrated with orbiter data to improve our knowledge of the spacecraft’s trajectory, an important input to future mission planning.

GBT staff, including Gary Anderson, Frank Ghigo, Toney Minter, Roger Norrod, and Galen Watts, began preparing for the Phoenix mission in December 2007. They have collaborated with radio science experts Peter Ilott, Sami Asmar, and Sue Finley from NASA's Jet Propulsion Laboratory (JPL), veterans of planetary exploration missions, including the Mars Rovers, Spirit and Opportunity.

The JPL scientists integrated their own sensitive sampling and data recording equipment with the GBT. NRAO Electronics staff Dave Woody and Nathan Sharp helped set up and connect the JPL equipment. Computer staff Chris Clark, Wolfgang Baudler, and Gene Runion were essential in connecting the JPL equipment to the Internet such that GB data displays could be seen at JPL.

During tests in December 2007 and March 2008, the GBT observed signals from the Mars rovers, which transmit at the same frequency and at a similar power level as the Phoenix lander. Shortly before the March tests, problems with the cryogenics of the GBT prime focus receiver were discovered and repaired, with the expert help of electronics and cryogenic technicians Jonah Bauserman, Bob Simmons, and Kenny Lehman. The March tests were successful, though some interfering signals were detected which, if they occurred during the Phoenix EDL, would obliterate the data being collected.

To identify the radio frequency interference (RFI) source, the GB Interference Protection Group (Carla Beaudet, Wes Sizemore, and Paulette Woody) swung into action. They researched and analyzed Earth-orbiting satellites that might interfere and monitored the local area for possible local RFI sources. They also contacted spectrum managers at NSF, NASA, and other agencies. Two Earth-orbiting satellites that would have caused serious interference were identified. The managers of these satellites agreed to turn them off during the critical Phoenix observing period.

Education Officer Sue Ann Heatherly organized public education and outreach events for the Phoenix EDL. A video link was established from GB to the Clay Center for Arts and Sciences in Charleston, WV. Peter Ilott gave a GB colloquium on "Listening to Mars" that was transmitted to the Clay Center. The video link from the GBT Control Room to the Clay Center was available throughout the EDL sequence, as was a streaming video feed to the Internet. Computing staff Chris Clark, Carolyn White, Charlie Myers, and Steve Tritapoe established the video/audio links; Taylor Johnson and Mark Adams assisted with the webcast.

The GBT Control Room was crowded as Phoenix’s entry into the Martian atmosphere, descent, and landing took place between 07:00 and 08:00 p.m. EDT on Sunday, May 25. The JPL scientists operated their recording equipment; GBT telescope operators Barry Sharp and Eric Knapp were on duty; and Frank Ghigo made sure that the GBT accurately tracked Mars. Software, receiver, microwave, and mechanical engineers were on hand and ready to resolve any last-minute technical problems: Bob Anderson, Gary Anderson, Roger Norrod, Steve White, Joe Brandt, Mark Whitehead, and Mike McCarty. Interference sleuths Carla Beaudet and Wes Sizemore checked for RFI. Bill Saxton operated the video camera, and Chris Clark monitored the Internet feeds.

Green Bank summer students Anthony Woody, Colin Slater, Stephanie Moats, and Marc Eimers shared in the excitement. An electronics engineer from Ohio, Rick Hunter, who had designed communication equipment on Phoenix, was visiting. Heather Niday reported for the local radio station, WVMR, and newspaper, The Pocahontas Times. Members of the Central Appalachian Astronomy Club had "front row" seats.

The Control Room videocast was viewed and enjoyed by visitors to the GB Science Center and the Clay Center. Both auditoriums were filled to capacity. Sue Ann Heatherly and Toney Minter provided running commentary for the remote audiences, interviewing scientists and engineers, and answering questions. Peter Ilott was continuously on the telephone with the JPL control center; he called out the EDL events as they happened. Sue Ann repeated the information for the science center and webcast audiences.

The signal from Phoenix as received by the GBT appeared at the expected time. The signal displays from two JPL data recording devices were also transmitted to Pasadena in real time for Phoenix mission controllers.

Changes in the GBT signal strength and Doppler shift demonstrated that Phoenix was moving through the fast and exciting EDL sequence, entering the atmosphere, decelerating, deploying its parachute, firing its retrorockets, and finally landing on the Red Planet. When a safe landing was confirmed, sustained applause broke out!

Everyone at the NRAO, whether in the GBT control room or elsewhere, was pleased and proud that we had participated in this new phase in the exploration of Mars.

The GBT Supports the Phoenix Mars Lander Mission