Satgen 508  Sat Fingerprints Pt3  by GM4IHJ  (BID SGEN508) 1998-12-19

As mentioned in Satgen 507, the signal fingerprint of a satellite
displayed using AF9Y software , can reveal characteristics of the
doppler shift which assist in identifying its orbit type. In the
normal mode of the display , 3 minutes of time appear vertically on
the left of the screen, 3 minutes of signal amplitude appear on the
right of the screen, and a band of audio 200 to 2700 Hz wide appears
horizontally. However by using the INTEGRATION control the display can
be set up to record the 12 minutes of a typical low earth orbit
satellite pass or an even longer period of tracking of a high altitude
satellite. 

Only satellites downlinking at 29 MHz or lower in frequency will stay
inside the audio band . But it is possible with a little practise to
track satellites of up to 1700 MHz downlink , using regular step
tuning of your receiver. You do not have to evaluate the record as it
appears in real time. You can concentrate on tuning to get a complete
record . Then print out and study the record at your leisure.

DOPPLER. Used in this way doppler rate = the highly diagnostic change
of frequency with time, can easily be read off the print out. Allowing
comparison with computed doppler rates for different parts of a wide
range of possible orbits. So that the type of orbit, its period ,
satellite height and other parameters can be estimated with reasonable
accuracy and then fully confirmed over several orbits.

SIGNAL AMPLITUDE. At the side of the AF9Y display is a section
recording changes in signal amplitude. With a print out of the
display, it is possible to examine the variations in signal level and
obtain information about the satellite. 
a.  Regular periodic changes in amplitude.  Can indicate satellite
spin characteristics. Eg the regular set of 3 consecutive fades , each
slightly different in shape , present on Oscar 10 beacon records
revealed that a set of 3 occured every single revolution of the
satellite ( rather than 3 revolutions). Thereby providing an excellent
record over many months as the satellite spin rate steadily decreased,
until the satellite had lost so much spin that it was no longer
stable. At which point the fade pattern became very irregular.
Reasons for amplitude fading are numerous. They can be due as in the
case of Oscar 10 , to the fact that the two metre antenna on the
satellite did not have a perfectly circular pattern - a not surprising
outcome of the satellites triagonal 3 pointed shaped and disposition
of the antenna elements. Equally possible, some satellites display
very revealing spin modulation patterns on the satellite downlink
amplitude because the satellite antenna field is obstructed by other
antennas, which at some parts of the spin cycle, shield one another.
b. Much sharper changes can occur when a satellites spin temporarily
causes a part of the satellites body to interupt the signal path,
c. While irregular fading in downlink amplitude can indicate that the
satellite is tumbling and moving unpredictably.
d. Last but not least changes in the satellites environment - eclipse
or reduced solar flux, rise or fall in temperature , can also produce
signal amplitude changes , slowly in some cases but quickly in others.