Satgen304 Satellite Hindsight No3 by GM4IHJ 21 Jan 95 BID of this msg is SGEN304 Please use this BID if you retransmit this msg The original Viking sea rovers were superb explorers, but they had no luck in Greenland , where the climate changed and froze them out. Like them, the modern day NASA Viking Mars probes were initially very successful. They established automatic reporting stations on the planets surface and sent back a wealth of scientific data. Indeed so successful was the Viking Mars project that extra funding was granted to extend its useful life far beyond the 6 months originally specified and if things had gone as planned at least one Viking probe might have continued to send data for several more years. Unfortunately , things did not go exactly as planned. The problem began as a sensible engineering suggestion , that in view of the extended life of the remaining probe, a change should be made to its battery charging routine, to better meet the needs of its aging batteries. So new battery routine orders were compiled and transmitted from earth to Viking. These order were received by Viking and stored in temporary memory, where they were error checked and proved correct. Then having confirmed correct receipt of the new orders, the controllers next ordered Viking to put the orders into operational memory. At that point all communications with Viking ceased and nothing has been heard from Mars since. Distraught programmers soon discovered that in the earlier part of the mission there had been several software change orders from Earth, which did not accord with their software location chart. In fact they had loaded the new battery routine into a software location already occupied by the all important instructions which kept Vikings communications antenna aimed at the Earth. In short the new battery routine may still be working, but Viking has no antenna control data and cannot find the earth unaided. Radio Amateur satellites have often had uploads of new or revised software, but so far no tragedy of the Viking type has been reported. However we should be prepared. For as Murphy reminds us. If something can go wrong. It eventually will, and it could be costly as in this next case. Viking had given good service but this was not the situation in regard to a brand new geostationary comsat launched a few years before Viking. This expensive bird was delivered safely to its alloted station and successfully set to work by the building contractor. At which point it was handed over to it user/controllers. It did not last long. Receiver A was noisy so the duty controller sent up a simple order telling the sat to switch to receiver B. Unfortunately receiver B had gone completely dead and there was no way to get the satellite to receive an order to switch back to receiver A. At which point other user crews stationed around the satellite footprint began to notice that it was drifting off station. Indeed one station in a sub arctic test site soon lost the satellite below his horizon, as he made frantic calls via another satellite to try to get action from the main ground station controllers. Unfortunately they already knew there was something amiss but could not think what to do about it, and the satellite eventually drifted out of the geostationary arc and was lost completely. All because the controller failed to send a sensible safety first message Eg "Switch to receiver B. In the event that you hear nothing from us in the next ten minutes switch back to receiver A". 73 de John GM4IHJ @ GB7SAN