Satgen255 Satellite Spotter Part 4 by GM4IHJ 12 Feb 94 BID of this msg is SGEN255 Please use this BID if you retransmit the msg How long do you hear the signal for ? This feature , coupled with details of doppler shift on the signal frequency, is the best guide to whether your unknown signal is from a satellite, or , not. LEO Low Earth Orbiters under 1800 kms altitude are above your station horizon for a time in minutes depending on how high they are. Fuji Oscar 20 ( max altitude 1750 kms) can be heard for up to 24 minutes maximum on any near orbit pass. By contrast the lower altitude RS10, RS12 , NOAAs and Meteor weather sats around 900 to 1000 kms in altitude can only be heard for a maximum of 17 minutes or so each near orbit pass. While the Space Shuttle orbiting much closer to the earth at say 280kms altitude on a 57 degree inclination orbit over UK , will be heard for only 9 minutes or less on the closest orbit to your station of the day. Please note that these maximum duration passes occur only when the satellite comes nearly overhead your station. If the satellite passes near your horizon at long range from you, you will only hear it very briefly however high it is. So, if you now combine Timing data and doppler data, you will find :- a. Very short duration plus minimal doppler = possible sat on your horizon b. 5 to 10 min duration + 600 Hz overall doppler shift on 29MHz, (or 3 KHz 145MHz, or, 9KHz on 435MHz) may be LEO passing halfway to horizon. c. 15 to 24 minutes duration with twice the above doppler shifts , could be a LEO sat going nearly overhead. Also remember that LEO sat reappear at intervals of 85 to 120 minutes . Non LEO orbits. A signal heard for more than 20 minutes can sometimes be a sub horizon LEO 29MHz sat such as RS10, but this sub horizon propagation is very rare, almost never happening with satellite signals at higher frequencies. So a long duration signal is unlikely to be a satellite unless it is one which operates at least 20000 kms above the Earth ( sats avoid the Van Allen radiation belts 1800 to 20000 + kms ).Satellites orbiting at great height ( Amateur Phase 3 elliptical orbiters such as Oscar 13, Russian Molniyas, and GPS and GLOSNASS navigation sats) have several good recognition features. The amateur radio sats send their callsigns regularly. The Molniyas occupy frequencies around 3.5 GHz and broadcast TV and Data on several adjacent channels, and the Navsats use a readily identifiable pseudo random noise spread Spectrum signal. Apart from these there used to be some experimental sats such as SRET2 but these old sats have ceased transmitting, and the only experimental sats now in high altitude orbits are mostly in orbits following the Earths magneto tail out as far as the MOON. These magneto tail chasers are easily identified because their orbit keeps them ANTISOLAR ie always directly down the midnight line out from the earth. Geosats can be a special case for the sat spotter. They have no doppler, and they do not move in the sky. But they must be on the geostationary arc a special curve across the sky equator wards of your station and no more than 75 degrees east or west of your station meridian of longitude. They also have regular signal patterns , spread across up to 50 or more channels evenly separated in frequency. They often relay military and commercial traffic from ships and aircraft. So the channels are used in short bursts as one mobile signs off and then another signs on. 73 de GM4IHJ @ GB7SAN