Satgen 169   Satellite Navigation     by GM4IHJ               21st June 92

There are no sign posts in the middle of the Atlantic, or in Space. The
best terrestrial Navigation using radio or the stars, can sometimes get
you a position within plus or minus a mile of somewhere, but not much
better. Then came Polaris submarines . To fire a missile at point X you
need accurate measurement of your own position. So in the 1960's Johns
Hopkins University came up with the Transit system using the doppler shift
of signals on 150 and 400 MHz from low earth orbit satellites.
Transit suffered from propagation effects . So the military went looking
for a replacement. This duly appeared in the 1980's and is known as
Navstar Global Positioning system GPS. It is designed to use about 20 sats
in high 20000km orbits, and it produces repeatable positioning accuracy of
a very high order. The Russians use a similar GLOSNASS system. Being
military , these systems have only recently become available to the
general public.
There are lots of situations where GPS might be useful. From keeping track
of rental cars, lorries and buses, to monitoring the rate at which the
great geological rift in East Africa, is pulling the continent apart. In
August 92 a Uosat clone satellite system build under license by Marconi
Matra starts trials with the launch of S-80-T which goes up on Ariane
together with TOPEX, POSEIDON and KITSAT ( an Amateur Radio store and
forward packet Uosat clone which will assist Uosat 5 with its 9600 bps
International packet linking).
Eventually sats of the S-80-T kind will carry GPS to say where they are
and they will keep check of lorries fitted with GPS and inform their
owners where they are. The GPS units in the vehicles are the size of a
small packet TNC. So there is no reason why we should not put something
similar up in our Amateur radio sats, so that they can tell us exactly
where they are. GPS can of course be fitted in your car. Back seat drivers
will love GPS. Imagine the scene . "Turn here dear ". " You are sending me
down a one way street "." Yes Officer it is my wife's fault . Please give
her the ticket. She is navigating "
Meanwhile S-80-T is reported to be carrying equipment to measure noise in
the band 148 to 149.9 MHz . So we may find this system becomes a close
neighbour of 2m amateur radio. A rather poor choice I suggest , in light
of possible QRMing of 2m Amsats. IARU to monitor please.
QRM is already a problem with GLOSNASS the Russian version of GPS . It
uses frequencies around 1250 and 1603.5 MHz and modulates them with a
several megahertz wide Random noise modulated spread spectrum signal.
Unfortunately the Russian system seems to be wider than necessary. There
are several reports that Radio Astronomers want it switched off because it
spreads across the naturally generated radio astronomy signals transmitted
by distant quasars, galaxies and gas clouds. There is no QRM from the
American GPS on 1227.6 and 1575.42 MHz. The US birds use a different
method of identifying their satellites so that they do not have a wide
frequency spread.  
GPS sats are poor test targets because their wide signals sound just like
noise. By contrast, the still operational Transit birds are excellent
targets for radio education experiments as the RSGB Space Radio Handbook
Exp 8.01.04, 8.01.05, 8.01.07, 8.01.08 report.73 de John GM4IHJ @ GB7SAN