Satgen 593  Doppler 4 (HF SATS)   by GM4IHJ  (BID SGEN593) 2000-08-05

>From the point of view of a station located near a polar auroral
zone, the ionosphere over the pole is an ever present source of HF
propagation anomalies which can be investigated by monitoring
satellite signal returns from both above and below the station
horizon. These anomalies come in several kinds of distinct doppler
patterns , and they occur quite frequently .

Doppler Feathers -  Feathery wide band traces on an FFT display
accompanied by rough broad band T1 - T3 signals , very similar to
those seen over lower latitudes. However unlike them, these are
present on an irregular basis on satellite transits near the poles,
what ever part of the solar cycle we are in.

A typical example occured on 27 Jan 00 ,when RS13 passed west of
Scotland remaining above the horizon until 1137z, producing via a
direct line of sight path a signal with the expected slowly
decreasing signal gradualy dopplering lower in frequency. But at
1128z RS13 overflew the edge of and entered into the polar auroral
oval producing a second signal at a higher frequency ( ie with near
zero doppler shift). Then as the satellite gradually got further
inside the auroral edge its signal coming via this second path at
first rapidly then more slowly dopplered much lower in frequency .
Producing a near horizontal feather trace on the FFT display which
then gradually reduced in doppler rate as the signal coming from the
satellite to the anomaly came off at a decreasing angle to the
satellites stern. Such that the feather gradually turned vertically
down the FFT trace , finally indicating a doppler shift very similar
to that coming from the original direct signal. At which point the
direct signal disappeared as the satellite went below the station
horizon, while the signal via the auroral anomaly continued for
several minutes.

Roof Tile pattern -  A quite different mode of signal propagation
coming from RS13 when it is above the polar regions , but sub horizon
from Scotland, follows the pattern of an event which occured on 11
Nov 99 from 1220 to 1228z. At this time the satellite tracked from
76N 217W ( near New Siberia Isles), passing well north of Alaska, to
pass the pole 82N 144W and then descend over Arctic Canada 73N 93W (
Somerset Island). 

Through out this sub horizon pass, the FFT display showed a curious
but familiar signal pattern . Whereby in one minute, frequency
shifted 350 Hz lower, then immediately jumped back 220 Hz higher,
before starting another downward slide in frequency taking 2 minutes
to drop 430Hz, before once again jumping back about 200Hz higher and
starting another slide downwards. 

At this time, this author cannot pretend to know why this happens .
This pattern variously described as like a set of overlaping tiles on
a sloping roof or thick slate slabs on an old cottage. Was copied
more than 20 times in just 3 months November 99 to January 00, and 
has been seen several times since.