Satgen292 Satellite Problem No5  Radiation Hazards    by GM4IHJ  29 Oct 94
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Satellites orbit in a very hostile environment. Early radio amateur
satellites such as Oscar 6 and 7, and RadioSports 1 to 8, were launched
into what was then regarded as very suitable "LOW" Earth Orbits, with
altitudes of about 1800 kms. None of these satellites carried complex on
board house keeping computers, but all had radiation sensitive solar power
panels. Most of these sats had long useful lives , eventually succumbing
to battery failure ( in those days automobile batteries rarely lasted more
than 3 years ). But , some of them also suffered severe radiation damage,
with RS1 and RS8 being possibly,the first examples of satellites killed by
cumulative radiation damage. In those days no one realised just how
intense the radiation was in the lower Van Allen radiation belt around the
earth. But by the death of RS8 , the message had got through that 1800 kms
was too hot for satellites and since then vitually all LEO low earth orbit
sats have gone no higher than 1000 kms. So few radiation failures have
occured with simple satellites since then. 
Unfortunately very few modern satellites are simple. Most now carry
complex house keeping computers and large memories for store and forward
or video functions. So while these modern birds orbit at about 1000 kms
their sheer electronic complexity makes them vulnerable to any remaining
radiation hot spots. Two hots spots exist, one major one minor. The major
one is called the South Atlantic Anomaly, a feature encountered by all
satellites crossing the Atlantic east of Brazil. The problem in this area
is one of quite simple physics - the Earths magnetic field is not centred
on the rotation centre of the Earth. The field centre is 600 kms out from
the rotation centre towards Singapore. Therefore the radiation bottled up
by the Earth magnetic field in the lower Van Allen Belt comes down much
closer to the earths surface over and east of Brazil, because Brazil is
antipodal to Singapore. All of which means that as the modern complex sats
swing over the South Atlantic they get a high dose of radiation.
Given the presence of this anomaly in the radiation zone it comes as no
surprise that modern packet satellites need tightly shielded electronics
and, frequent electronic error checking and correction, in order to
preserve their software contents. Recent evidence involving HF experiments
with satellites RS10 and RS12 has revealed a second previously unknown
source of radiation which is spasmodically injected into the Arctic and
Antarctic ionosphere on its sunward face, producing a second source of
potential radiation trouble for LEO sats.
Satellites far above the LEO orbit meet a quite different series of
radiation problem areas. Those in high apogee elliptical orbits like Oscar
10 and 13 , can dodge much of the radiation if their inclination is above
60 degrees , but AO10 at about 20 degrees has already lost its computer
control because of radiation damage caused by its low inclination orbit
being right through the densest parts of the Van Allen belts. AO13 was
fine at launch but solar gravity resonance has pulled its perigee very low
and altered its apogee latitude from near 50 down to 15 or so degrees
putting it into a much more difficult radiation environment. While last
but by no means least the 37000 kms apogees of AO10 and AO13 sometimes
take them outside the protection of the earths magnetic field out to where
they receive the full fury of solar storms.  All things considered
therefore, would be satellite designers need to be very careful where they
put their satellites. 73 de GM4IHJ @ GB7SAN