Satgen 206   Doppler Part 4   by GM4IHJ                       6th March 93
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Satgens 199, 201, and 203 explored the reasons for doppler shift of a
radio signal from a low earth orbit satellite coming towards or going
away, from your station. For a 29 MHz satellite the shift is about 620 Hz
high on direct approach from the horizon for an overhead pass, and 620 Hz
low when the satellite has passed overhead and has nearly reached your far
horizon.
If we receive a satellite using 145 Mhs = 5 times higher frequency. We get
5 times more doppler shift. 3.1 kHz high to 3.1 kHz low , a total shift of
6.2 kHz on an overhead pass . Most of the change in frequency is
concentrated in the middle "overhead " section of the satellite pass . So
you can face a situation were you need to tune at around 2 kHz/min in the
centre of the pass. Just the worst place it could happen because you are
also trying to track the satellite with your antenna at that point when
the antenna tracking rates can be quite fierce.
It is therefore no surprise that very few people even try tracking Dove or
Pacsat ( Ao17 or Ao16) microwave downlinks on 2401 MHz where the shift can
be 2401/29 x 2 x 620 Hz = 130 kHz. With a doppler rate of change of about
30 kHz per minute in the middle of the pass.
Please remember however that high doppler rates only apply to low altitude
near earth orbiters. A TV geosat is moving at 3 kms/sec and using a
frequency of 10 Ghz ( 4 times higher than Dove ) but with respect to your
station the geosat is stationary . It goes round its circle the same
number of degrees /min as does your earth station . So you stay lined up
with no relative velocity and no doppler. Though you do need auto
frequency control because 10Ghz systems can drift a lot in frequency.
Oscars 10 and 13 which go out from the earth at their apogee high point,
to as far as the geostationary orbit , have almost no doppler shift over
their several hours around apogee, as they imitate a geosat . But after a
few hours they descend from apogee and dive around the earth at low
altitude Indeed Oscar 13 comes lower than most low earth orbit satellites
at its present perigee height of around 600 kms. So its doppler shift
around perigee can be quite fierce. Users of Oscar 10 and 13 must remain
aware of this sawtooth shaped doppler curve . Climbing up to apogee and
then slowly dropping back to earth these satellites display very little
doppler . Their curve just gradually rising in frequency very slowly. But
as they dive past perigee near the earth , the shift which has taken 10
hours to build up, can dissappear in 30 minutes of high speed perigee
dash.
Anyone wishing to experiment with elliptical orbiter doppler curves can
perhaps try Fuji Oscar 20. This satellite has a slightly elliptical orbit
and though you can never hear more than a brief portion of it at any pass
, it does vary enough in doppler shift to make an interesting practice
target before you go on to try the much more difficult Phase 3 Oscar 10
and 13.
In the next essay on doppler I want to tacle the much more difficult
problem of two way doppler . How your uplink ,to the satellite closing
your station, is shifted up at the sat Rx antenna , transponded in the
satellite electronics then fired back at you getting another dose of
increasing doppler before it gets into your station receiver. 73 de GM4IHJ
@ GB7SAN