Satgen 560  Doppler=Vital Evidence  by GM4IHJ (BID SGEN560) 1999-12-18

For 20 years or so Radio Amateurs have been aware that DX is not
always achieved on the straight and narrow great circle path between
us and them.The classic case of this was exploited by VE1YX as far
back as the 10m/6m crossband qsos ,two solar cycle peaks back in 1979.
When contacts into Europe were regularly obtained provided both
stations aimed their antennas at the Caribbean. If either or both
stations went back to pointing along the great circle path they got
nothing.

So there appeared to be a back scatter anomaly somewhere above
latitudes 20N to 45N over the Atlantic. Indeed subsequent results
suggest it extends all around the world. But has anyone seen it or
plotted its extent ?

Answer yes, In the 1980s , the Russians had a space tug called Cosmos
1686 which operated a beacon on 19MHz, which provided excellent
doppler shift data for locating the anomaly. Whereby as the satellite
went south towards the anomaly with its direct signal coming off the
back of the satellite producing doppler low at northern receivers,.
The anomaly reflected a signal from the front of the satellite doppler
high, back to the north. RSGB Space Radio Handbook p164 refers.

Now in 1999 you can do it with pictures. Track RS13 beacon 29.458 MHz
as it goes south in DAYLIGHT, towards the anomaly, and an audio
spectrum analyser FFTDSP display will show beatiful feather like
curves of descending signal doppler as the satellite approaches and
overflies the anomaly. Noting that the signal start doppler high
sometimes 200 to 400 Hz above the direct signal coming off the back of
the satellite. Then as the satellite nears the anomally the signal is
coming off the satellite to the anomaly at an increasing angle on the
satellites bow ie a decreasing doppler getting closer to the frequency
of the direct signal off the back of the satellite.

Alternately, try this when RS13 is coming north in your daylight, as
it will be for the next few weeks,and you get the picture in
reverse.You must do this in daylight. It is clear that the anomaly is
a daylight only phenomenon at least for IHJ . Equally clear the
anomally is not always singular. Some records at IHJ suggest up to
three seperate and distinct doppler feathers, pointing to, up to 3
anomalies between 18N and 45N at times. So do not be surprised if you
see more than one doppler feather on your display during the same
satellite pass.

At IHJ the anomalies have been traced stretching from 309W, westward
through the Greenwich meridian to 56W. But DX results from American
and Japanese stations suggest that the anomaly/anomalies are worldwide
in extent when the Sun is reasonably active as at the present time.

Whether similar anomalies exist in souther latitudes is yet to be
revealed.