Satgen 187 Satellite Stabilization Pt2 by GM4IHJ 24th October 92 The BID of this msg is sgen187 Always use this BID if you retransmit The Phase 3 satellites Oscar 10 and 13 go out to a distance of 36000kms from the Earth in elliptical orbits where the sat moves quickly when near Earth at perigee orbit low point, but moves very slowly , loitering for up to 6 hours near the apogee high point of the ellipse. So when near apogee the sat is roughly 36 times further from Earth than say a typical low altitude near circular orbiter. To cover this long signal path requires bigger antennas at the ground station and, on the sat. But this means a narrow beam antenna at the sat. So the sat must be stabilized to point accurately at the earth , to keep the earth inside the sats antenna beam. This kind of stability can be achieved by spinning the satellite at quite high speed Eg 20 to 30 rpm. So the Phase 3 sats use magnetic torqueing coils to build up the spin. This requires careful energising of the sats internal coils and is not a situation which can be changed quickly. Oscar 13 launched into a situation were it was spinning the wrong way round. But it proved safer to spin it up the wrong way and leave it that way, than to de spin it through the point where it had no spin then hopefully try to start it spinning the other way. Because at the point of no spin it would probably have been uncontrollable.- noting that you need it to point accurately earthwards to order it what to do next, and this is impossible if it is wobbling all over the place and not looking at earth. So Oscar 13 spins the wrong way and it also suffers from spin modulation of its signals, because the sat is too small a platform to provide a regular circular 2m antenna beam . So as it spins you get 3 signal peaks and 3 signal lows per rev. Geosats also sit out at 36000km, but they carry fuel tanks and thrusters which can be used to keep the satellite spinning the correct way for the desired stability. But spinning the geosat introduces an interesting problem. Where do you put the sat antenna so that it points steadily at earth and does not spin ? The answer is to mount the antenna above a shaft through the satellite spin axis, and to de spin this shaft and antenna using an electric motor turning the shaft at a rate and direction which exactly opposes and cancels the sat spin at the antenna. This is possible because the heavy satellite spinning one way is not upset by the much lighter antenna spinning the opposite way. This is not the whole story. Both Phase 3 and Geosats have additional stability problems . Part 3 in the next satgen reports what we will have to do , if we wish to keep long range satellites stable with respect to the earth , for periods of ten years or more,so that we can recoup their very high building/launch costs. DUPLICATION OF BULLETINS is being caused when a station retransmits a bulletin with a new BID, thereby producing two copies each with a unique BID Bulletin Identification. Readers of Space bulletins will notice that in the past 3 months most spac/amsat/arrl bulls are using unique BIDs Eg satgen bulls are now always sent with a BID starting sgen followed by a sequence number. Please see that if you retransmit any space/amsat/arrl bull , that you leave it with its original BID, EG to retransmit this bull use SB AMSAT @ AMSAT $.sgen187 73 de GM4IHJ @ GB7SAN