Satgen 199  Using Doppler Pt 1      by GM4IHJ                17th Jan 93
The BID of this msg SGEN199    PLEASE DO NOT CHANGE IT IF U RETRANSMIT
Please Please no more duplicates such as OZ5... WHO ADDED  4k ADDRESSES TO
TEXT OF SGEN 196 ,ALTERED THE BID AND SENT HIS DUPLICATE AROUND THE WORLD
AGAIN 3 WEEKS AFTER THE ORIGINAL. Hamlet is clearly not the only, mentally
disadvantaged Dane.

Most Amateur satellite users avoid all mention of Doppler shift,
presumably for the twin reasons that it is mathematically complex, and,
not of much consequence.This is a mistake. Several modern sat tracking
programs calculate doppler shift and, properly applied this knowledge is a
great help to operators. Starting with HF sats - Consider RS10 in a low
elevation pass of GM4IHJ. The signal shift can be up to 680 Hz or so at
29.357 MHz . Very awkward if you momentarily lose valuable DX and have to
tune around to find it or wait for it to come out of a fade. With a
readout of doppler you know exactly where to tune your receiver at any
time.
Even worse than simple RS10 above the horizon DX, is RS12 below the
horizon DX. How do you know the doppler shift from a satellite half a
world away ? RS12 coming up towards UK from Capetown goes through an
awkward doppler high ( when it is pointing straight at GM4IHJ ) when it is
still south of the equator on some orbits.
So good computer software will save you a lot of wasted DX. Most computers
calculate doppler from rate of change of slant range between the sat and
your station. So even if you just have the micro print out you have solved
one big problem. There is however as you will discover the problem of what
frequency you tell your computer it should measure at closest point of sat
approach to your station. At this CPA point relative velocity of sat to
your station is nil and hence doppler shift is nil and you hear the actual
transmitted frequency unstretched or, uncompressed by doppler, though
purists will tell you you should make allowance for earth rotation because
even at cpa the earth is turning your station towards or away from the sat
- a point we will leave for now as it is not that important.
What you need to know is the sat frequency you will hear at CPA - PLEASE
NOTE IT IS NOT NECESSARILY THE ONE GIVEN IN THE SAT DATA FILES. Many sats
have their Tx frequencies checked before they go into space , and when
they get into space, temperature and other changes can alter the frequency
. So to be correct you need to accurately measure the frequency yourself
when the sat is at cpa. The best way to do this is to measure satellite
doppler rate over a complete pass near your station. You find out the
moment when peak doppler rate of change occurs and you note the frequency
zero beat you got at the centre time of that doppler rate measurement.
Remember you measure frequency by finding the signal zero beat , but you
track the satellite using a tuning point 600 to 800 Hz below the zero beat
for comfortable audio tracking. In future satgens I will detail the
procedure for doing these measurements , and, I will discuss how doppler
affects users of, satellites of various types , at VHF, UHF and Microwave,
and, discuss some gentle doppler theory. 73 de GM4IHJ @ GB7SAN