Satgen310  Satellite Hindsight No.8        by GM4IHJ           4th Mar 95
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One of the biggest shocks in the last 30 years has been the discovery and
gradual understanding of the South Atlantic Radiation Anomaly. James Van
Allen and several other satellite pioneers had roughly plotted out the
extent of the Earths radiation Belts, and it was assumed in the early
1980s that provided a satellite stayed below 1200 or above 30,000 kms , it
was safe from major radiation damage. 

Uosats were the first Amateur radio satellites to begin to employ large
amounts of onboard computer hardware. So it was these satellites which
first encountered the low altitude extension of the radiation belts which
gets down to 600 kms altitude over Eastern Brazil and the adjacent South
Atlantic. We now know that this downwards extension is caused by the
asymetry of the Earths Magnetism, whereby the centre of the Magnetic Field
is displaced about 600 kms beyond the Earths Rotation centre in the
direction of Singapore, causing Brazil and the adjacent Atlantic which are
roughly antipodeal to Singapore to be 600 kms closer to the bottom edges
of the radiation belts. Fortunately the Uosats had flexible onboard
software which could be error checked, washed and refreshed frequently ,
in order to avoid a build up of onboard program errors due to SEUs single
event radiation upsets. But even this has not always protected them.

Even more recently the Radiation Belts have added another surprise with
the discovery that in places the radiation is not just simple lightweight
hydrogen protons and electrons collected from the Sun, but, heavyweight
nucleii of atoms such as iron , of the sort found in Cosmic Rays - A form
of radiation which produces far greater damage to satellite software than
the products of lightweight atoms.

Experience in Amateur Radio has shown that all the digital satellites are
prone to rather low Mean Time Between Failure rates, with most of the
failures being due , either to radiation damage, or, the use of incorrect
uplink commands. It would seem therefore that while Amateur satellite
builders must be careful to protect digital satellites in every way , in
view of what has been happening to Amateur Radio digital satellites,
perhaps an even bigger question mark hangs over the 1000 or so digital
satellites planned for commercial launch, into this same low altitude
environment, over the next 6 or 7 years. First in the field should be
OrbCom with 26 satellites rather similar to Uosat 22, but perhaps
requiring much more extensive software protocols to make their operations
idiot proof. Next up should be the 60 odd satellites of the Iridium
constellation. These will have satellite to satellite linking of calls,
from mobile phones, a service which would appear to be much more sat
onboard software intensive than the store and forward digisats. What will
be the fate of these satellites in the South Atlantic Anomaly ? Clearly an
MTBF of only 2 months or so per satellite, as experienced by some of the
Amateur Digisats will surely be quite unacceptable in a commercial
situation featuring 60 satellites EG one satellite failure per day. 
Future successes or failures around 600 kms altitude should be of great
interest to both the Amsat community and the commercial companies.
73 de GM4IHJ @ GB7SAN or gm4ihj@delphi.com