Satgen 590  Doppler 2 (HF Sats)  by GM4IHJ  (BID SGEN590) 2000-07-15

Typical variations in the signal received from an HF satellite as its
velocity with respect to your station resulted in doppler shift of
frequency as the satellite passed above your station horizon, were
described in Satgen 589. When however the satellite is sub horizon
from you , between you and the equator. You may still sometimes
receive its signal, via ionospheric propagation. But, the pattern of
doppler frequency changes you will hear, or see if you have FFT
spectrum analysis , will be very different from those heard when the
satellite was above your horizon.

The principal ionospheric features responsible for these mid latitude
sub horizon events, are bands of ionospheric density anomalies
stretching east west across and below the path the satellite is
taking as it comes towards you from the equator, or as it goes away
from you towards the equator.

Considering first the effect this has on the signal from a satellite
coming from the equator towards you. Long before it reaches your
horizon it sees the anomaly ahead of it , and can use the anomaly to
forward scatter a signal to you. A signal which is doppler high
coming directly forwards from the satellite.  As the satellite nears
the anomaly however , the anomaly gradualy passes beneath it . So the
signal coming from the satellite to the anomaly begins to depart at
an increasing angle away from the satellites forward motion . So
velocity relative to the anomaly decreases and hence received doppler
signal decreases in frequency. Until as the satellite overflies the
anomaly the signal received has zero doppler shift. 

This passage up from the equator is therefore seen or heard as a
steady doppler high at the start which at first slowly then
increasingly quickly goes further and further doppler low until it is
lost at a frequency with zero doppler . Producing a trace on an FFT
display which looks like a feather curving lower in frequency. Then
as the satellite meets your horizon the signal comes back again at
the original doppler high. 

Going the other way , down to the equator the signal is lost doppler
very negative at the horizon. Only to reappear at zero doppler circa
400 Hz higher as the satellite overflies the anomaly and you begin to
get scatter back to you via the anomaly . Then immediately the signal
begins to get lower and lower in frequency as the signal to the
anomaly begins to come to you off the back of the retreating
satellite , finally reaching roughly the same frequency that you
heard as the satellite dropped below your horizon. So this time the
doppler feather trace on the FFT display starts high and then quickly
swings down in frequency to a straight trace until the satellite
losses sight of the anomaly. Please note that these ionospheric
anomalies appear to be seasonal. They are almost completely absent in
your local summer. In addition they appear to require a solar flux
level above roughly 130 units to sustain sub horizon propagation at
29 MHz. Although in the 1980s anomaly propagation from Cosmos 1686 at 
19 MHz was seen at lower flux levels.