Satgen 623  Doppler Revisited  by GM4IHJ  (BID SGEN623)  2001-03-03

As Amateur Radio is forced out of VHF and UHF frequencies by the
steady encroachment of new " must have " commercial gadgets, which by
definition " must also have ", amateur radios long cherished
frequencies. There is an inescapable tendency to move what is left of
amateur radio still higher and higher in frequency. Which for
satellite users means dealing with several new problems, not the
least of which is previously unheard of doppler shifts of frequency.
Study of a set of rough examples of the doppler shift and rates of
change experienced when using low earth orbit LEO sats, produces the
following approximate values of total frequency shift to be expected
on a near overheadf LEO satellite pass, together with the very
important maximum rate of change of frequency, encountered as the
satellite makes its closest approach to your station.

Wavelength Frequency  Approx total Shift   Approx Leo maxrate
                      -ve near Leo pass     CPA -ve change
 10m         29Mhz         900 Hz               17Hz/sec
  2m        144Mhz        3.9 Khz               83Hz/sec
 70cm       450Mhz       11 Khz                250Hz/sec
 23cm      1260Mhz       31 Khz                700Hz/sec 
 12cm      2400Mhz       60 Khz               1360Hz/sec
  5cm      5850Mhz      140 Khz               3140Hz/sec
  3cm        10Ghz      250 Khz               5020Hz/sec
 12mm        24Ghz      650 Khz              12000Hz/sec

Note.. These results do not apply to Phase 3 elliptical orbiting high
altitude apogee, satellites. They experience much smaller doppler
shifts and rates, particularly during their lengthy time near apogee.
In addition please note that doppler shift from elliptical orbiters
is often positive . Only going negative doppler shift when passing
through perigee. Unlike LEOsats where doppler shift is always
negative when the satellite is above your horizon.

Even at frequencies as low as 450 Mhz, doppler rate of change during
an overhead pass defeats many SSB voice operators. Although SSB
doppler can be followed on passes near your station horizon when the
doppler shift is much reduced and equally important that is where the
DX stations will be encountered. At other modulation modes eg CW
doppler at 450 Mhz can be followed more easily . While at 450Mhz PSK
automatic frequency control AFC can be used to provide steady
tracking of the signal. The use of subtractive mixing at the
satellites transponder ( 2m up doppler subtracts from 70cm down
doppler), does reduce the problems somewhat. But at higher
frequencies some form of AFC is desirable provided the signal type
provides a carrier for the AFC to track. Knowing which way the
doppler is going to go and how much it will change, does help.
Several versions of satellite tracking software calculate the shift
for you and allow you to manually or in some cases automatically
correct for it. Not all software versions are accurate enough to deal
with frequencies from 1260 Mhz upwards.You can get a guide to the
software accuracy by noting whether its predictions are irregular
minute by minute, or ( on good software) are smooth. Better still you
can check it on a near overhead pass.