Satgen230 Satellite Propagation Worldwide Part 3 by GM4IHJ 21st Aug 93 BID of this msg is SGEN230 Please use that BID if you retransmit this msg Part 1 discussed asymetric HF conditions on satellite communications , and the effects of Aurora on HF, VHF and UHF satellites. Part 2 discussed the so called "Auroral Es " propagation through the Aurora. Part 3 will discuss how the E layer of the ionosphere affects satellites. The E layer is located at a height of roughly 90 kms. It is not normally as intensely ionised as the higher F layer because the E layer atmosphere is thicker and any atom which is ionized quickly finds an adjacent electron to neutralise it. In its normal moderately ionised condition the E layer acts as a minor absorber of satellite signal, reducing their signal strength slightly. The E layer is however capable of being heavily ionised in discrete spots. This so called "SPORADIC E" ( SpE) is caused when relatively dense collections of meteoric dust form at eddies in the Jet stream upper atmosphere winds. These clumps can collect ionized particles and hence, form local patches of very dense ionisation capable of scatter propagating HF, VHF, and very rarely UHF signals . This is the foundation of the SpE mode of propagation favoured by terrestrial VHF stations for communications of up to 2300 kms or so. Sporadic E occurs seasonally between latitudes 30 and 75 in both hemispheres. There is a strong 3 month season in the local summer, and a weaker 1 month season in the middle of the local winter. Spe is a daytime phenomena, only present when the Sun is above the horizon bombarding the Elayer and producing ionised particles which stick to the meteoric dust. At night recombination/neutralisation is rapid and the SpE rapidly disappears. In contrast to the above stations near the Equator seem to have SpE almost the whole year around. GM4IHJ has only personal evidence for this from military VHF operating in the 1940's. There is as far as is known , no literature on this aspect of SpE ( please tell GM4IHJ if you have any details ). So tropical operators should get terrestrial SpE Dx all year. There is however a price to pay for SpE , where ever you are , if you operate satellites. A satellite operator trying to uplink 145 MHz to a satellite when SpE lies along the line of sight may well find that the satellite cannot hear him, and a check on the satellite downlink beacon may reveal that it to is attenuated. Fortunately SpE patches rarely block access to the whole of a satellite pass. In practise at GM4IHJ Fuji Oscar 20 can be blocked for most of the first half of a pass but rarely for the second half. With most Spe patches occurring south of UK , it is generally the part of the pass over southern Europe which suffers. Equally obvious from Scotland and also reported from Poland , Pacsats using 145 MHz uplinks are often affected by summer or winter daylight SpE. Access to Pacsat Oscar 16, Uosat 22 and Korean sat 23 are all affected at times, and GM4IHJ is forced to use the higher polar passes clear of SpE or the night time passes when Spe has faded. As with Auroral Es propagation, GM4IHJ has no reports from outside Europe. Reports from outside Europe would be welcomed , particularly from the southern hemisphere and the Equatorial regions. To be continued 73 de GM4IHJ @ GB7SAN