Satgen 182  Satellite Orbits 1    by GM4IHJ                 19th Sept 92
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Most amateur satellites go into orbits determined by the main payload. The
people paying for the launch get the orbit they want and, we , hitchhikers
accept the orbit as part of our free/cheap ride concession.
Early AMSATs were mostly sent aloft with US military payloads going into
low earth near circular orbits in the altitude zone 1300 to 1800 kms
height, and inclined at just over 90 degs to the Equator. This gave
excellent orbits high over both polar regions,  able to see stations as
far apart as the UK and California , at the same time.
Early Russian amateur sats  had similar orbits excepting that they orbited
just east of the pole with inclinations of about 82 degs. 
Then the Russians introduced the first important change by launching six
satellites at the same time, so that eventually they came past your
station one after the other providing almost continuous communications
coverage.
Unfortunately this situation could not last. Almost half the Amsats and RS
Russian sats suffered radiation damage as their high orbits took them into
the lower edges of the earths radiation belts - thereby gradually
destroying their onboard controls and computers. So since 1982 almost all
Amsats and RS sats, have orbited below 1200 kms . This avoids serious
radiation damage but it results in the satellite no longer being able to
connect UK to California, being limited to UK to US Eastern states for
simultaneous coverage.
Breaking this pattern, Amsat Oscars 10 and 13 used Ariane civilian
launches to geostationary transfer orbits. Then once safely there, their
amateur controllers ordered the sats to fire their auxiliary kick motors -
small rockets designed to raise the orbit low point perigee well above the
atmosphere and, tilt the orbit to an inclination of about 60 degs with
respect to the Equator. This did not work for Oscar 10 which was marooned
in a low inclination near equatorial orbit where radiation damage rapidly
destroyed its onboard computer. So Oscar 10 is now uncontrollable but
still works spasmodically on 435 up 145 down Mode B. Oscar 13 fared better
and got to 57 degs inclination almost clear of radiation problems, but
like the Russians before them, the controllers found that at Oscar 13's
36000 km high point it was being pulled away from the Earth by the Sun.
Worse still, this meant that Oscar 13's perigee low point was being pulled
down to a very low altitude where the satellite could be lost due to
atmospheric drag. At the moment the Solar pull is no longer in resonance,
so Oscar 13 is relatively safe at about 600 kms perigee, but in the next 2
years or so the Solar pull is likely to become effective again pulling
Oscar 13 out of orbit as perigee eventually drops into the thicker part of
the atmosphere, where Oscar 13 will burn up.
When Amsat sent up high elliptical orbiters, the Russians adopted a
different strategy - sending their latest RS sats up, permanently fixed
inside geodetic sats and navigation sats. There the new RS's have the
benefit of the big power supply of the parent sat, allowing the RS
transmitters to be much more powerful and capable of new techniques such
as digital FM repeating. Meanwhile Amsat has launched a fleet of small
micro sats. About which I will write more , later. 73 de GM4IHJ @ GB7SAN