Satgen 183   More Satellite Orbits  by GM4IHJ           26th September 92
BID is sgen183 . PLEASE DO NOT ALTER BID OR TITLE IF YOU RETRANSMIT
Continuing the orbit saga reported in satgen 182. January 1990 saw the
launch of a very ambitious series of Microsats . These included Uosat
Oscar 14, Pacsat Oscar 16, Dove Oscar 17 , Webersat Oscar 18 and Lusat
Oscar 19. All these satellites launched into low earth polar orbit via the
same Ariane launcher. Their mission ,to test several digital message
systems , a task which they have carried out most succesfully, such that
two further recent launches ( RM1 Oscar 21, Uosat Oscar 22) , have taken
roughly the same type of orbit - an orbit which has turned out to be far
less benign than expected .With hindsight the problems of this orbit might
have been appreciated earlier. There was a warning of what to expect when
the Russian Space station Mir tried to get X ray pictures of super nova
1987. When it became clear that the part of the sky in which Mir could get
useful data was much smaller that expected.
The reason for Mirs problem was that super nova 1987 was in the sky above
the Southern Hemisphere , and this part of the sky contained the South
Atlantic Anomaly - the zone in the sky above eastern Brazil where the Van
Allen Radiation belts come down nearly 600 kms closer to the earth than
they do over most of the rest of the world between 60N and 60S Lat.
Up to a few years ago there was no accepted explanation for this South
Atlantic dip in the radiation belts. But recent studies have confirmed
that it is simply the result of the fact that the centre of the earth's
magnetic field is not in the same place as the earth's centre of rotation.
In fact the magnetic centre is displaced about 600 kms off the rotation
centre along a line joining Singapore through the earth core to Brazil. So
the Van Allen belts being constrained by the magnetic field , line up with
that field taking no notice of the fact that this puts them out of
alignment with the earth's surface , bringing the belts down to a very low
altitude over Brazil. Hence every time a satellite above about 700 kms
transits the South Atlantic Anomaly it gets a dose of hard radiation which
is particularly dangerous for micro chips and onboard computer memory.
As a result of the above Uosat controllers noticed that their Uo14 memory
which was considered well protected , was suffering a lot of "soft"
interupts - radiation strikes which changed one bit of computer memory.
The cure in Uo14's case was to "refresh " the memory much more regularly
thereby correcting these radiation induced errors. This has worked well in
both Uo14 and its successor Uo22. What will be interesting in the long
term will be to see how the other two satellites launched since 1990 fare
( Fuji Oscar 20 and Kitsat 1). Both these satellites carry large memory
banks, but the big problem with them is their orbits . Oscar 20's orbit is
markedly elliptical and it goes to a much higher altitude than the other
microsats. So it will experience much more radiation damage than they do.
Kitsat by contrast has a much lower orbital inclination than the other
microsats and it too is at a higher altitude. It will therefore spend a
much longer period in the more intense part of the South Atlantic anomaly.
Life expectancy tests on all these Amsats should be very interesting.
Meanwhile what does these mean for future sats. Clearly all the new
commercial low earth orbit digicomms projects carrying large memory
modules (IRIDIUM ?) will need careful anti radiation design, and perhaps
it will be more economical to launch them as low as possible say no more
than 650 kms, then keep them up using long life ion drive thrusters to
offset the effects of atmospheric drag and maintain their height . Can we
get iondrives for next lot of larger memory Amsats? GM4IHJ @ GB7SAN