Satgen257  Satellite Spotter Part 5  by GM4IHJ                  26 Feb 94
BID of this msg is SGEN257  Please use this BID if you retransmit this Msg

A further good clue to satellite classification and identification,
concerns fading of the strength of the signal- sometimes with a regular
pattern. The most obvious of these fading patterns is due to Faraday
Effect whereby the natural rotation of the signals polarisation as it
comes through the ionosphere produces repetitive fades on a fixed
polarisation receiving antenna. RS10 and 12 at 29MHz fade 20 or 30 times
in a pass of say 17 minutes . Fades occur frequently when the signal is
near your horizon, less frequently when RS is near overhead.

At 145.82 MHz Dove and RM1 !45.98 MHz have 5 times higher frequency than
RS and so suffer 5 times less frequent fading due to Faraday. But, the
fades are much longer and deeper than the faster RS fades.

At 435 Mhz Pacsats and Fuji have only one or at most two fades per pass
but the fade is usually a long one  and can happen any time in the pass.

For sats at 1269 MHz and above you will not notice Faraday Effect fading,
but all these sat signals RS 29 Mhz upwards often have a flickering or
scintillating signal - a radio version of twinkle twinkle little star. 
This scintillation is least for satellites over middle latitudes, and
worst for sats over the Arctic, Antarctic and the Equator.

Aside from the above natural sources of signal fading, the satellite
itself is often spinning , continuously changing the aspect of its
antennas with respect to your station. Oscar 13 on 145.81 down shows very
clear spin modulated fading, as do Dove and Lusat. In fact if you compare
say telemetry of the satellites solar panel current , with the spin
modulated fading you will note that there is a correlation, albeit
multiplied when the satellite antenna lobes are distinctly " three fold"
as in the case of Oscar 13.

Geosats suffer signal scintillation but their antennas are "de spun" to
counteract satellite spin, thereby eliminating spin modulation of their
signals. The frequencies they use are however generally too high for
Faraday fading, except in the case of the lowest geosat frequencies such
as those around 260 MHz used for military communications , there, all spin
and faraday fading is eliminated by using dedicated circular polarisation
on the satellites downlink antennas.

Extra copies of this satgen series are available from GM4IHJ @
GB7SAN.#78.GBR.EU , if you miss any parts.
I regret my printer scrambled the request from " Chris in New Jersey" for
earlier parts of this series. Please repeat your message Chris and I will
supply.
73 de John GM4IHJ @ GB7SAN