Satgen 568  SatSignal Reception 4  by GM4IHJ  (BID SGEN568) 2000-02-12

As mentioned in Satgen567. Scintillation , irregular phase and
amplitude distortion, and, Auroral wide band doppler distortion, are
regular problems on satellites which orbit within 25 degrees of the
earths poles. It depends exactly where you are on the earth, just how
much your signals suffer. The considerable offset of the magnetic
poles of the earth with respect to the rotation poles, is responsible
for these differences , at one place as opposed to another.
Fortunately the offset makes its greatest impact over some very
lightly populated areas. With Canada and Alaska being worst affected.
But access to satellites over these areas, from Western Europe, can be
difficult at times. By contrast , the worst affected areas in the
Southern Hemisphere are the province of penguins rather than people.
So there is no great problem there.

Worse still, the scintillation problem does not end at the poles.
While the mid latitudes of the Northern and Southern hemispheres are
relatively free from scintillation , equatorial scintillation can be a
considerable nuisance on trans equatorial paths to and from
satellites. For as proved 20 years ago by satellite P76. Scintillation
can affect frequencies up to at least 2.5 GHz.

So what can be done to minimize the effects of scintillation and
aurora ? The best tactic appears to be - put your satellite up high,
so that reception is at high antenna elevation angles for as much of
the time as possible. Easy to say. But care is necessary. The Van
Allen radiation belts which inhibit orbits between 1200 and 20,000 kms
, cannot be used unless the satellites sensitive microchips are fitted
with expensive radiation shielding - which adds weight, or , are
radiation hardened , which is expensive.

Satellites like the Global Positioning system GPS, need to orbit at
high altitude to provide wide coverage , as well as improved
reception. As compared with their Transit system, low earth orbit 150
MHz, 400 MHz predecessors. Indeed Transit original began with just 150
MHz. But that experienced so many problems from both scintillation and
aurora . Particularly in the regions where the Polaris missile nuclear
submarines it was designed to assist, were wont to wander, that the
higher frequency became the frequency of choice . Albeit not the
perfect answer which did not appear until the GPS high flying birds
with their much improved methodology and performance , arrived on the
scene.

These points were not lost on the designers of Amsats Oscar 10 and 13.
Where the loiter at high altitude = high elevation for most folks ,
gave much better reception particularly at Mode B 2m, than would have
been possible with low altitude satellites. High apogee Molniya
elliptical orbits are favoured because they provide almost geo
stationary conditions for hours at a time. But the secondary benefit
of high altitude mostly clear of scintillation and aurora was not lost
on those of us who live at high latitude terrestrial sites.