Satgen259  Satellite Spotter Part 7  by GM4IHJ                 12th Mar 94
BID of this msg is SGEN259  Please use this BID if you retransmit this msg

Callsigns, voices and signal patterns, can be a useful guide to satellite
identification. Some Geosats transmit a wide variety of signals, which
immediately reveals that you have a comsat with telephony, voice and
digital data capability acting as either a fixed trans ocean relay or, a
repeater for mobile stations on the sea, on the land , and in the air.
Given the rapid increase in scale of this traffic. You can hear a Swedish
truck driver, reporting back to his home base, giving his GPS location and
the state of his deliveries and cargo Eg the temperature of refrigeration
etc. Or, you can hear an oil rig superintendent reporting coded drilling
results to head office, and , now becoming more frequent, passengers
aboard trans oceanic airliners phoning home.

Non commercial satellites are much more difficult to classify. Most voice
traffic is digitised and then further encoded. But the end result is
generally recognisable as a digital signal which is unreadable.
Recognition of this kind of traffic on several adjacent channels emenating
from the same source , invariably spells out - Military or Scientific
satellite. In complete contrast to this , all amateur radio satellites
tell you exactly who they are at regular intervals using standard
callsigns.

What is clear as you listen to the multiplicity of satellite downlinks ,
is that the original idea "That International Radio Traffic is Never
Encrypted", has disappeared for ever. Nobody ? actually deliberately ?
encrypts traffic so that you cannot even identify the source. But they do
achieve the same effect by using highly effective modulation codes. Eg GPS
sats are sorted one from another by looking for the unique modulation code
which each one uses. So you cannot read them unless you have access to the
modulation codes. Indeed even if you know the codes some GPS signals are
deliberately blurred, so that other countries military or civilians
cannot use the system at maximum accuracy.

It will be obvious from the above that , you have to classify satellite
types by knowing what their signals sound like , but never understanding
what they are saying, at least in some cases. This situation is not going
to improve. Given the adverse publicity earned by peeping toms who have
published details of other peoples private phone calls. Indeed moves to
code and thereby enhance the privacy of satellite communication channels
are thoroughly understandable. 

So the satellite spotter needs to learn, what " unreadable " signals
represent, what types of satellites. Bird watchers do this all the time .
Recognising their avian visitors by their singing even though we will
never be likely to know what and why they are singing. Satellite listeners
will just have to get used to doing the same type of classification by
sound or signal spectral characteristics. This latter process can involve
a great deal of equipment much of it far outside the scope of a humble
student of satellites, but an ordinary oscilloscope or simple spectrum 
analyser can be a great classification aid.
Note.. Not all national authorities permit detailed satellite monitoring.
Please do make sure you are legal before you start. 73  GM4IHJ @ GB7SAN