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Detection of the Mars Global Surveyor Satellite Beacon

by Mike Cook, AF9Y

In late November of 1996, many hams and amateur SETI stations around the world participated in a NASA experiment to detect a beacon signal from the Mars Global Surveyor (MGS) satellite. The beacon was a 1 watt carrier which was activated when MGS was 5+ Million km from the Earth. This experiment was a unique opportunity to test the limit of an amateur receiving system. Key factors were:

  1. A very stable, continuous carrier signal similar to the type of signal expected from SETI.
  2. A known receive signal level which spanned the range of -187 dBm to -217 dBm.
  3. Signal Doppler from Earth and the Spacecraft motion.
  4. Requirement for Low Noise Preamp
  5. Requirement for high gain antenna system with accurate Az/El tracking.
  6. Requirement for accurate frequency measurement and stability at 437.1 Mhz
  7. Requirement for high performance signal processing software.
My attempt to detected the signal was with an antenna system much smaller than the average SETI station. The receiving system consisted of:
  1. Two low cost ($30) 5 foot Helix antennas with a total gain of 16.4 dBic.
  2. < 0.5 dB Noise Figure Receive System
  3. FFTDSP signal processing program (available at http://www.webcom.com/af9y)
The FFTDSP program was operated in a 16x integration mode which provided a lower receiver detection limit of approximately -176 dBm. This integration level allowed detection of doppler rate change for verification of the extra terrestrial signal. With the antenna gain of 16 dB, the MGS signal was expected to be -171 dBm at the peak level.

thumbnail The signal was detected immediately at beacon turn-on and was tracked for over an hour. The expected 6 dB of margin was verified as the signal dropped from -187 dBm to -193 dBm before the trace disappeared from the FFTDSP display. More spectrograms and a correlation to NASA's antenna gain plot can be seen on my webpage at http://www.webcom.com/af9y. Pictures of the helix antennas and testing techniques are also available.

Detection of the MGS carrier was a great thrill and ranks up there with my first Moonbounce (EME) ham contact. An even greater challenge lies ahead for detection of MGS while in orbit around Mars. At that time the signal will be 25 dB weaker. In addition, there will be three forms of doppler to deal with in the signal processing. While this would be a very difficult challenge, I believe it may be possible for the larger amateur SETI stations. As a group, we need to participate in more tests like this to assure that the receiving capability is at the level we expect.

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