Satgen 165     Geostationary Questions    by GM4IHJ              24 May 92

In the last few months, I have received a number of questions about
Geostationary satellites - the satellites which orbit approx 36000 kms
above the Equator and , by virtue of their 24 hour orbit period, appear to
stay in the same place in the sky relative to a terrestrial user.
Q1.  What geostationary satellites can I see. You can if you have a good
telescope and a clear night sky, see any geosat whose equatorial longitude
is within about 72 degrees east or west of the longitude of your location.
Eg at 4 degs west longitude GM4IHJ can see geosats stationed between 68
east and 76 west.
Q2. What geosats can I receive ? This question provokes a different answer
to Q1 above, because you can only receive geosats above your horizon IF
THEY ARE POINTING THEIR NARROW SPOT BEAM ANTENNA AT YOU.  The footprint or
spot covered by the geosats antenna is quite precise , so that no power is
wasted outside the wanted service area. A TV sat like Astra covers UK and
Western Europe, but though Arabsat may be above your horizon , its antenna
is not pointing at you. Some geosats use a spot beam and a wide angle
hemisphere beam but the power level in the hemisphere beam is much reduced
and you need a big dish to receive it ( Eg Russian Gorizont ). In other
cases the spot beam is manoeuverable as in NASAs TDRSS Tactical Data Relay
which is used to communicate to and from the Space Shuttle. So you only
hear TDRSS when it is pointing at a Shuttle or a Landsat near you.
Q3. Are geosats really fixed ? POSITIVELY NOT . There are three main
reasons why not. Firstly the gravitational field above the Earth is not
the same everywhere around the Equator. Over the Atlantic the geosat
behaves as if it were on a hill. It is constantly rolling slowly down the
hill west or east and its controllers must use thrusters to keep pushing
it back on station. By contrast geosats over the Indian Ocean behave as if
they were in a hole. Once there they need no fuel to stay there. So the
Indian ocean slot is where most old satellites are sent to die. 
The second factor disturbing geosats is Solar gravity. In our summer the
Sun is above Latitude 23 pulling the geosat north. In our winter the Sun
goes down to latitude 23 South pulling the geosat southward. Again this
has to be corrected by use of thrusters and fuel. The Sun's effect is
relatively easy to predict but that of the third body in the system - the
MOON , is not so easy. Being much closer to Earth and moving over a wider
arc than the Sun, it has considerable effect on each geosat's position.
All geosats start with at least 80 Kg of thruster fuel. A good satellite
controller can keep a geosat within half a degree of station for ten
years, but in some awkward cases the fuel runs out long before this. At
which point if no fuel is left the " no longer geosat" drifts slowly
around the Equator sometimes coming to a halt in the Indian Ocean or the
Pacific "gravity hole", But modern practise is to use the very last bit
of fuel to raise the sats orbit slightly , so that it gets well clear of
the crowded geostationary belt.

More on geosats next week. 73 de GM4IHJ @ GB7SAN