Satgen588   One man's Summer by GM4IHJ    (BID SGEN588)  2000-07-01

One man's summer is another man's winter, particularly if A lives in
Scotland and B lives in Australia. Nowhere has this been clearer
recently than in the observations of the RS13 satellites 29.485 MHz
signal when the satellite is far below the station horizon. Any sub
horizon reception needs ionospheric assistance . Leaving aside the
short range near field results from Sporadic E or Auroral
propagation. The only other propagation facility available at 29 MHz
is the F layer of the ionosphere. Which as summer approaches, gets
greater solar illumination and radiation , but perversely expands
upwards thereby cancelling any possibility of higher maximum usable
propagation frequency. So the rule should be " Less long range sub
horizon propagation to you, in your hemispheres summer.

A rule which at first sight might seem to be unbreakable. Were it not
for the fact that RS13 is putting a sub horizon signal into Scotland
in mid summer, on several orbits per day when it overflies or is near
the Antarctic Ice Cap.

Daily checking of these events , reveals that it is not "Antipodeal
propagation". The phenomenon whereby a station near your antipodes
has an infinite number of possible azumuths along which to propagate
a signal to you, and at least one of them should be usable. Two
months of plotting signal returns, reveals that signals from the
quarter of the southern hemisphere where the GM antipodes is located
are very rare indeed. Whereas propagation florishes from the region
30S to 83S latitude across the other 3 quarters of the southern
hemisphere 90W through 0W round to 180W. An area where northern
summer sun is replaced by southern winter darkness or a single
coherent strong winter time F layer. So after an easy pass down from
the satellite through the weak polar ionosphere the signal can
chordal hop up to a strong F layer as it continues further north.

By contrast sub horizon paths to GM from RS13 when it is on eastern
or western bearings, which were such a feature of last winters
results, have disappeared completely, as have the doppler feathers
produced by anomalies in the norther winter ionosphere between
latitudes 20N to 45N. Again this should be no surprise . The summer F
layer is not conducive to sub horizon DX and there is the additional
problem that east west contacts cross lots of time zones and
therefore encounter a great deal of ionospheric variation. A problem
which does not afflict sub horizon contacts from the Antarctic to
Scotland which travel up, rather than across the time zones, and
thereby experience little variation.

The above report has not considered, Field aligned Ionospheric
propagation. This can and does help trans equatorial propagation and
may assist south to north propagation from the Antarctic to Scotland.
But correspondents mentioning it recently appear to be unaware that
while FAIs are horizontal above the equator, they descend at the dip
angle as one nears the magnetic poles . So please do not report FAIs
to your south if you are in the northern hemisphere. Any FAI south of
a northern station is tilted to send your signal out into space , not
back to earth.