Satgen 570 SatSignal Reception 6 by GM4IHJ (BID SGEN570) 2000-02-26 Going higher in frequency up into the EHF (Extremely High Frequencies) allows wideband signal modulation types to be used. Offering protection from qrm, higher communications speeds, higher antenna gain and absence of propagation problems such as Faraday Rotation of signal polarisation , and Sporadic E layer ionisation. But it does introduce a new set of propagation problems whereby communications paths at EHF to and from space can be seriously affected by attenuation in the atmosphere. Foremost among these atmospheric effects are those due to water vapour or ice. Even on high elevation angle paths to and from satellites , rain and clouds which produce only 0.1dB of attenuation at 1 GHz, and 1dB at 10 GHz , can produce 20 dBs attenuation or more if you operate from a ground site near an ocean or a polar site, or , from a humid equatorial region. By contrast desert regions are almost as attenuation free as operating from an aircraft 5 miles above the earth. So Arizona is definitely favoured over Aberdeen Scotland. Equally important as signal frequencies go higher are the specific attenuation peaks at frequencies where the size of a particular component of the atmosphere corresponds to the wavelength being used. Specific peaks are 23 GHz (water vapour), 60 GHz and 118 GHz ( oxygen), and 183 GHz (water vapour). So terrestrial frequencies up or down are not advised in these sections of the spectrum. Although these frequencies by their very nature are tailor made for inter satellite links above the atmosphere, such as Iridium 23 GHz, and military comsats 60 GHz. At present while the military are interested in trials using L band 1.6 GHz Iridium sats. Most of their traffic will be carried on 21 GHz downlinks and 44 GHz uplinks , on geostationary satellites which have a reasonably high elevation angle as see from most temperate and equatorial operating areas. In the commercial arena the situation is far less clear. There is presently a wait and see attitude amongst most users of the satellite mobile phone systems. Between them Globalstar and Iridium provide an interesting contrast in styles. Globalstar with a 2.5 GHz downlink to mobiles are not in trouble with the radio astronomers , but Iridium on 1.6 GHz down have had some problems, particularly in getting ground station licenses. The real test between these two is however in respect of Iridiums inter satellite linking and Globalstars simpler bent pipe system . The question is , are we ready for inter satellite linking between low earth orbit satellites . Radio amateurs who have participated in the various amsat experiments requiring intersat linking, know only too well how complex it can be. Yes geosats do it regularly but they do not change position with respect to one another. The jury is definitely out on this one. Though it should not be too long before the commercial groups proposing high speed internet linkage for all, via low earth orbit sats, start to do some flight testing. Meanwhile it is noteworthy that these planners suggest they will need several hundred LEOsats so that you see one at high elevation avoiding atmospheric absorption, at all times.