Satgen228 Satellite Propagation Worldwide Part 1 by GM4IHJ 7th Aug 93 BID of this msg is SGEN228 Please use this BID if you retransmit the msg Signals going to and from satellites travel through the atmosphere and, the ionosphere ( the upper portion of the atmosphere above 40km altitude which is non uniformly electrified by bombardment from the Sun and other agencies ). Signals going up or down can be seriously attenuated, and, bent away from a straight line of travel. The first part of this report discusses some of the Ionospheric effects. The ionosphere has layers of ionised gas at various heights, having been electrically charged by bombardment from Solar radiation and, to lesser degree, Galactic Cosmic radiation. In general the lightest atoms form the top most layers ( hydrogen ) while succeeding lower layers are composed of heavier atoms ( oxygen, nitrogen ) and still further down the yet heavier molecules of these gases. The ionised zones are not fixed in height. They move up or down , concentrate or disperse with the changing bombardment which varies during the day and night, still further varies from season to season , and last but by no means least varies over a roughly 11 year cycle of max through min to max again in step with the varying level of Solar Flux , more commonly known as the Sunspot Cycle. The ionosphere has it greatest effect on the low frequency signals. HF satellites in the range 3 to 50 MHz, Eg 29 MHz Radiosport satellites being where most radio amateur encounter its effects .Because the Sun is absent at night, ionisation is greatly reduced, and so HF signals are less affected at night. But the worst case propagation occurs when typically a station in daylight in say Africa uses RS12 HF 2way to talk to a station in Brazil still in pre dawn darkness. The Brazilian gets excellent night time up and down links through his ionosphere but the Africans signal goes up and down through the daylight ionosphere and his results will be much poorer than those of his contact partner. In the African evening= Brazilian afternoon the situation is reversed, Brazil gets the poor links and Africa gets the good ones. This typical case of asymetrical conditions on a satellite link can occur anywhere when operating across the Solar Terminator Dawn/Dusk line, it is regularly encountered USA <-> Europe ; Europe <-> Asia and S America <-> Africa. Very few operators appear to understand or allow for this problem. AURORA . Less regular but still a nuisance on both HF and VHF satellites is Aurora. Stations above latitude 55N and 55S ( GM LA SM KL etc ) get auroral problems on satellte links on 30 to 50 days of the year. At best the presence of the Auroral curtain above their polar horizon gives their signals a rough note. At worst it destroys the signal. At HF RS10 and 12 can produce multiple signals. At VHF and UHF Auroral scintillation can break up the signal completely at times. This break up effect is particularly noticable on Fuji Oscar 20 when it is accessed from Scotland whilst the satellite is seen through the edge of the auroral curtain as the sat transits down between Greenland and Labrador. Contacts G <-> W can be ruined at times particularly when an auroral storm moves the auroral curtain further south. Stations as low as latitude 35N or 35S can be affected by once every 5 year approx monster auroras. But stations nearer the Equator are not affected by polar aurora when operating satellites. The auroral curtain is no use to them because it never comes above their horizon. By contrast high latitude sat ops are plagued by aurora almost weekly , but at least have the satisfaction of being able to use it for terrestrial DX out to about 2000 kms. 73 de GM4IHJ @ GB7SAN