Satgen254 Satellite Spotter Part 3 by GM4IHJ 5 Jan94 BID of this msg is SGEN254 Please use this BID if you retransmit the msg 3. DOPPLER Probably the best way to identify a signal source as a satellite, is Doppler shift. The gradual change of the frequency of the signal you hear as the satellite changes its aspect with respect to your station. When heard initially the satellite will probably be approaching your station. So Doppler shift will be positive. The signal you hear being higher in frequency than the one actually transmitted by the satellite . Then as the satellite goes past your station the frequency you hear will get lower , so that as the satellite pulls away from your station the doppler shift has become negative and you hear a signal frequency which is less than that actually being transmitted by the satellite. For RS12 on 29.408 MHz this can mean a start 600Hz high =29.4086, becoming the sat frequency 29.4080 as it goes past you , then continuing to drop in frequency to 600 Hz low = 29.4074 MHz as it goes out to your station horizon ie a total shift of about 1200 Hz for a near overhead pass. Five times higher in frequency at 145.980 MHz RM1 can shift from 5 times higher to 5 times lower ie 3KHz high to 3 KHz low 145.983 to 145.977 MHz. Which will require the operator to retune several times particularly in the middle of the pass where doppler shift changes very quickly. Please note that if your strange signal is from a LEO Low Earth Orbit sat its Doppler shift will always go downwards in frequency. High orbiters such as Oscar 13 change frequency slowly upwards for most of their orbit , then the frequency drops back very quickly as the satellite comes nearest to the Earth. As high orbiters stay in range of a station for several hours,there should be no difficulty in separating them from LEO sats which never stay in range for more than about 20 minutes. Geostationary satellites have almost no doppler shift on their signals but they reveal what they are by staying in exactly the same place in your sky on the geostationary arc equator wards of your station, all day and night. Only when a LEO sat such as RS12 comes nealy overhead your station , do you hear its full 1200 Hz of doppler shift during its 17 minutes or so maximum pass time. An RS12 pass halfway out to your horizon lasting say 10 minutes max will only doppler 300 high to 300 Hz low ie half the overhead shift, and a pass right out on your horizon will have very little doppler shift and be heard for only one or two minutes. So when you hear a strange signal , keep measuring it frequency . If it goes down in frequency and disappears in about 17 to 20 minutes maximum, it is probably a LEO sat. A high orbiter will still be there two hours latter and will probably have positive increasing doppler frequency, Whereas a signal with no frequency shift may be a geosat but only if its bearing and elevation are exacly right for the geostationary arc. Doppler is therefore the best tool of all for recognising satellites. Indeed doppler shift can tell you lots more particularly if you are interested in propagation of radio waves , but that must wait for another satgen as it does not really concern the satellite spotter. The RSGB Space Radio Handbook pages 13 to 18 contains more detailed explanation and examples of doppler shift, and pages 133 to 143 report several kinds of doppler experiments which can be performed by the average amateur radio station operator ( no fancy maths Hi ). 73 de GM4IHJ@GB7SAN