Satgen 166    More Geosat Questions        by GM4IHJ           31st May 92

One area of misunderstanding about geosats is signal polarisation. Astra
for example uses alternate vertical and horizontal linear polarisation on
adjacent channels. This allows closer channel spacing without mutual
interference. BUT - signals leaving Astra are not H or V with respect to
the Equator. Astra's service area is around 50 degs latitude north. So
vertical transmissions leaving Astra tilt 50 degs from Astra's vertical.
If you go for DX on the fringe of a geosats service area , you must expect
to receive polarisation which is not vertical or horizontal to you. Each
geosat will be different because of its displacement relative to you.. For
this reason constantly variable motor driven polarisers which can be
turned to any angle are preferred to magneticallly switched two position
90 deg only shift polarisers. Equally noteworthy if you , receive a
transmission from a geosat using a hemisphere broad beam antenna, you
again must expect to adjust your reception polarisation well away from
vertical and horizontal..
Another problem geosat DXers face is that geosats near their horizon have
a lengthy signal path through the lower atmosphere. This results in
atmospheric refraction bending the signal path . Sometimes you get it
sometimes you do not. A similar condition limits geosat usage to non polar
latitudes. Surface ducting common in cold climates can seriously affect
stations anywhere above latitude 66 degs.
Anyone who monitors a TV geosat soon discovers that not all its signals
produce pictures. Using a receiver such as an ICOM R7000 in place of or in
parallel with your 900 to 1750 Mhz first IF receiver, you can easily pick
out the broadband TV sigs even when coded, but you may also find digital
telemetry signals usually at least one at each end of the geosats overall
bandwidth ie lower edge channel one , upper edge channel 16. These are
legitimate beacons and or two way control channels up and down through
which the ground controllers monitor and change as necessary , the sats
functions and housekeeping. 
Geosats exist in a very harsh environment. The Earth's magnetic field is
crushed inwards on the sunward side. So over 24 hours the geosat can go
from reasonable protection, from the solar wind at night, when it is in
earth shadow and inside the magnetic field - to full daylight outside the
magnetic field exposed to the full fury of the solar wind. Several early
geosats built up massive electric charges which caused local lightning on
the geosat, with disastrous results. Hence close monitoring of geosat
telemetry is vital if it is to be kept in good health - even to shutting
it down temporarily during big solar storms.
HOWEVER - don't just look for legitimate telemetry. You may detect other
signals , not just at band edges but also tucked into the edges of central
channels. These are pirates. It started when the Gorizont TVsat feeding
Cuba, stopped putting out signal when its feed from Mosco One ceased at
Russian midnight. Enterprising gents west and east of the sat found they
could up link signals into the vacant space and get hemisphere relay.
Today most geosats have security checks, but as far as I know , no one has
yet prevented the pirates from "say" uplinking a digi signal into Astra
type sats on the edge of an operational TV channel , and using the sat as
a relay to an office half way across Europe, at no cost except roughly 10K
sterling for the dishes ,receiver and single channel TWT uplink. 73 GM4IHJ