Satgen359   Going Digital Pt7    by GM4IHJ               10 Feb 96

Once you have your digital pacsat station working, there are still one or
two unexpected obstacles to be overcome, which result from the complexity
of the satellites themselves and the environment in which they operate.

Digisats are full of computer electronics. They generally orbit at quite
low altitude around 900 kms where they are below the worst effects of the
Van Allen radiation belts, but their orbits still take them through the 
South Atlantic Anomaly several times each day. Fortunately Amsat designers 
have allowed for this. The satellite computer memories are refreshed at 
frequent intervals . None the less software glitches do occur when 
radiation hits a memory chip and alters one or more bits of memory. As a 
result you are lucky if a Pacsat gives more than 60 to 90 days continuous 
service before failure. At which point it will be out of action for a few 
days until Ground Station controllers sort out the problem.

Fortunately there are now enough satellites in orbit, to ensure that one
or two are usable at any one time. Most users naturally concentrate on the
more reliable satellites, so do not be surprised if you find quite
different rates of traffic flow to one satellite or another.

Years ago it was claimed that satellites " Would provide the first
worlwide radio amateur service unaffected by the vagaries of ionospheric
propagation ". Unfortunately this claim has proved false, and users of
digisats will soon discover that they too are not immune from problems as
their signals transit both up and down through the ionosphere.

Most satellite operators have experience of Faraday Rotation of the plane
of polarisation of their signal as it passes through the ionosphere. In
general Faraday affects the lower frequency uplink on 2m more than it
affects the 70cm downlink. So switchable uplink antenna polarisation can
be useful.

Far less predictable and much more annoying is Sporadic E SpE. The
ionisation of patches of the ionosphere at around 90 kms altitude.
Unfortunately a patch just 30 kms across at 90 kms height, can cast a
shadow 300 kms long across the digisat orbit at 900 kms altitude. So do
not be misled by terrestrial operators who talk about "small SpE clouds" .

Even small clouds can obscure uplink access for minutes at a time .
Sporadic E is prevalent in the Northern hemisphere from May to September
and again from December to February. Indeed as this is being written on
5th Feb 96, SpE is clearly evident south of Scotland with 48.491 Mhz TV
from Spain loud and clear here at IHJ, whilst access to parts of the
orbits of passing digisats is very poor indeed.