Satgen270 Redundancy in Space by GM4IHJ 28th May 94 BID of this msg is SGEN270 Please use this BID if you retransmit this msg Redundancy in an employment sense, usually means someone has lost his job and is unemployed. There is however another use of the word which has pleasant connotations. This second meaning is used to describe the satellite which needs one, but has two transmitters, or, the important resistor in a circuit , which instead of being a single resistor, is a collection of resistors in series and parallel, amounting to roughly the same value of resistance . This type of redundancy is a safety device. One resistor of 100 ohms fails, and thereby causes loss of a complete transmitter. By contrast if two pairs each of 100 ohms in parallel , are then connected in series . Failure of one resistor is not catastrophic , and failure of two ( one in each pair ) only reduces the performance slightly. In software as well as hardware we can also introduce redundancy by adding parity checks and bit counts , or repeats of important orders. Indeed some satellites carrying sensitive digital equipment through the South Atlantic Radiation anomaly would soon be useless if software redundancy and constant repeating or refreshing of important messages was not fitted as standard. So despite the expense, redundancy in a particular satellite can be vital to its life expectancy. WE do not need to stop there. Multiple separate satellites such as Amsats microsats provide another form of redundancy - with safety in numbers, being the theme . A case of not putting all ones eggs into the same basket an old fashioned but still valid piece of advice. All of which brings into focus several important points as we approach Amsats 25th anniversary. Satellites such as Uosat 1 and 2 suffered component failure early in their lives. But few people remember that, because the redundancy which was carefully included in their designs allowed them to carry on for many years, indeed Uosat 2 with its 11 years of operation is amongst the all time longest lived of satellites, and lest we forget, also demonstrates another form of redundancy whereby microsats of that type cost less than 1/100 th of the price of the monster comsats but are far more than 1/100 th as useful and generally longer lived. Naturally the big commercial communicators are begining to get the message. Teledesics proposed 840 Low Earth Orbit international phone sats are a case in point, as are the 60 odd LEO sats proposed by Motorola for their soon to be tested Iridium chain. So where microsats have led the way it begins to look as if low earth space will soon be as crowded as a certain London ring road. Where does this leave amateur radio? However crowded it might appear with upwards of a thousand commercial comsats there will still be space for LEO Amsats. Surely this is still the way for Amateur radio to go . We can get ten or a dozen microsats some with SSB/CW and some with digital communications for the price of a Phase III bird. Given the problems with existing Phase III birds , perhaps it would be good sense after Phase IIID to concentrate on cheap LEO sats with lots of redundancy , giving something on every band.73 de GM4IHJ @ GB7SAN