Satgen381 MobilePhone Am+Comsats by GM4IHJ 13 July 1996 The following answers questions posed by readers of Satgen 378 (Picosat) and Satgen 370 (Commercial LEO sats). KC1QF will be talking about Picosat at this years Amsat NA symposium at Tucson Arizona. Design information is available at http://www.cordis.lu/esprit/src/picosat2.htm . As Peter stresses , Picosat is designed for those who wish to communicate. A feature which has top priority with most correspondents to IHJ. So lets hope Amsat can give the project their backing. Meanwhile for those who wish to study the parallel commercial ventures in the the Mobile Phone / Message satellite field, things are about to get interesting. Orbcomm launched their first two Message satellites in April 95. Unfortunately the joint launch of the next pair failed, and there has been a break in the launch sequence whilst the Pegasus winged launch platform is checked out thoroughly. According to US reports Orbcomm is operational with just these 2 of the eventual fleet of 36 satellites in orbit. Certainly the first two sats appear to have proved the design concept. The only question mark remaining is the Pegasus winged launcher, which had "not unexpected teething troubles ", but now appears to be working well. At least two more sats are expected to launch this year. IHJ does not know their operating frequency. Iridium the proposed 60 Leosat constellation from Motorola should start testing soon. Both Uplink and Downlink frequencies are proposed for the band 1613.8 to 1626.5 MHz. There has already been severe criticism of this proposal from Radio Astronomers who use the adjacent band 1610.6 to 1613.8 MHz to monitor the important Galactic and Inter Galactic radio frequencies transmitted naturally by the OH hydroxyl radical . The problem according to Radio Astronomers lies in Iridiums proposed use of multiple transmission units feeding phased array antennas on the satellites. Given the nature of this use of multiple radiators , cross modulation products will be very difficult to avoid, and these products will they say destroy or seriously limit radio astronomy observations in the adjacent frequency band. No one should under estimate the problems inherent in this field. Doppler shift is high at the frequencies proposed so signals will swing over a wide band. But use of frequencies above 500 MHz will at least avoid the problems of Faraday rotation of polarisation, Sporadic E blanketing, Aurora, and Scintillation effects which plague satellite work below 500 MHz. What still looks like a major problem however will come from radiation hits, particularly as the satellites transit the South Atlantic Radiation Anomaly. These mobile phone sats , need idiot proof access and inter satellite linking requiring a lot of memory and software, which will be difficult to keep intact. Several correspondents ask about Geosat mobile phones , but these are a different proposition requiring as they do , high Tx power and high gain antennas.