Satgen 485 Mobile Phone Topics by GM4IHJ (BID SGEN485) 1998-07-11 The arrival in the B orbit plane of the last instalment of the Iridium mobile phone satellite constellation, completes a very successful operation by the Motorola consortium. So far there have been 4 satellite failures out of 68. Roughly in line with the anticipated rate of attrition with satellite launches of this type. The Iridium satellites should soon be operational, and a year from now it will perhaps be becoming clear as to whether this most elaborate of the mobile phone systems is likely to be a commercial success. Following someway behind Iridium, is Globalstar, which now has 8 satellites in orbit. These satellites are higher than the Iridium ones at 1250 kms v 775 kms, so they have a bigger individual ground footprint at 33 degs v 27 degs. This bigger footprint allows their orbit plane separations to be greater than Iridiums at 49 degs v 31.7 degs. But unlike Iridium there appears to be no intention to daisy chain direct from one satellite to another. Globstar traffic intended for mobile or fixed reception outside the original uplink satellites footprint will be routed down to a ground terminal in that original satellites footprint and thence via land line to the exchange of the distant intended fixed location recipient or , to an appropriately located distant Globalstar uplink station , thence up to a satellite for downlinking to the mobile recipient. Globalstars coverage does not quite cover all the earth , whereas Iridium satellites with their higher orbit inclination do. Not that there will be many telephone users in the arctic and antarctic, but there will be remote unmanned stations such as oil pipe lines and wells, which can be called up via the mobile phone satellites and asked to download operational data to an office located in a temperate climatic zone. Some recent professional discussion has considered whether it might be possible to monitor other satellites, when they are over distant parts of the world by interogating them via Iridium. The snag is that Iridium uses downward looking antennas, so it does not see other satellites clearly , particular when these satellites are at greater altitude than Iridium. The idea may however be applicable to mobile phone sats in higher orbits, and anyway one looks at it, the possibilities then become fascinating. " Call up the International Space Station via Iridium" ? or better still "Discard expensive TDRS and substitute Iridium or similar mobile phone network satellites"? But lest anyone be carried away by the above dreams of long chats with the cosmonauts, there are problems , beside those caused by the narrow beam aspects of Iridiums antennas. Satellites move fast , and satellites in different orbit planes, at different orbital inclinations and altitudes will have fierce relative velocities and hence large doppler rates and , short "in contact times" before a switch to another link satellite will be necessary. Even worse the data handling rates of the mobile phone circuits are far too low for NASA requirements