Satgen368 Mars Relay Preparation by GM4IHJ 13 April 96 As announced in QST and Amsat Journal, the Mars Global Surveyor space probe is scheduled for launch in November 1996. 20 to 30 days after launch when the probe will be between 5.6 and 8.4 million Kms from Earth, test transmissions from the probes Relay beacon on 437.1 Mhz may be receivable by radio amateurs. The signal will be extremely weak, and adding to this difficulty will be various factors whereby tracking an interplanetary probe differs markedly from tracking a satellite orbiting the earth. It may therefore be useful to consider ,some of the problems involved , and, planning and rehearsing ways to deal with them. The probes test schedule - the beacon operation will commence with 24 hours approx of transmission of a 1.3 watt unmodulated CW signal. This will be followed by tests of the probes repeater functions using modulated signals , operating for an as yet unspecified length of time. Finally in its third period of operation the probe will send another 24 hours of unmodulated CW. Please note that we live on a planet spinning on its axis, So individual stations are unlikely to have the probe above their station horizon for more than 12 hours per day. Tracking - the orbit can be considered as half of an ellipse. The Sun is at one focus. The orbit perihelion ( nearest the Sun) is, near the Earth shortly after launch. The orbit aphelion ( furthest from the Sun ) will occur as the probe reaches Mars. This probe orbit round the Sun is quite different from either a satellites orbit around the Earth, or the orbit of Mars. But as radio amateurs will only be concerned during the first 30 days it should be possible to publish predictions of the probes right ascension and declination at regular intervals , so that individual stations can then calculate the probes Azimuth and Elevation from their particular station. Please note that when the probe is launched it will be aimed well ahead of Mars. It will not be possible to point at Mars and assume that is the position of the probe until long after this test is over and the probe gets near Mars. Reception - the signal will be way below the levels encountered in earth satellite work. So would be receivers should have thoroughly tested and proved their digital signal processing capability. Equally important as tests at GM4IHJ have confirmed already , the reception frequency band should be checked to establish where and when local signal artifacts could confuse reception. Noting that the unmodulated probe signal will sound no different from the many sprogies, blips and bleeps of existing radio pollution around 437.1 Mhz. Less easy to rehearse will be the tracking and reception. Narrow beam antennas with accurate Az and El pointing will be a must. But perhaps the biggest difficulty will be doppler shift. The space craft itself will be receding at about 3.6 Kms/sec. but additional doppler correction will be necessary to allow for station motion as the Earth orbits the Sun, and station motion as the Earth spins around its axis.