Satgen338  Satellite Software Pt5   by GM4IHJ                16 Sept 95

COMPUTER MAPS continued.  The previous satgen 337 discussed the distortion
of Azimuth and distance present on a Mercator map display. An alternative
to the mercator sometimes seen in amateur satellite software, is the great
circle map, where your station or some other spot such as the satellite ,
is at the centre of the map , with true azimuth and distance shown to the
rest of the world or part thereof, radially out from the centre.

Unfortunately this great circle treatment grossly distorts the geography
on the map , with relatively small island groups like New Zealand
stretching a third of the way around the circumference of a GC map centre
on England. None the less it is useful where in the case of an English
user it provides true azimuth and distance and avoids the worst of the
possible distortion, because the antipodeal point on the opposite side of
the earth from England  ( and that of stations in USA) is far from any
coast line, out in the sea.

For stations in New Zealand , Spain, Chile and the Argentine, there is
land at their antipodeal point so a full great circle map showing the
whole earth is something of a mess around the circumference. In which case
the simple solution is to reduce the maximum distance shown by the great
circle map to say 18000 kms ( Eg AO13 maximum communications link range )
, thereby avoiding the total geographic distortion present at longer range
,whilst still providing useful coverage for almost all amateur satellites.

When it is necessary to both track the satellite and monitor say, the
solar terminator line , as in the special case of RS12 mode K sub horizon
propagation. The great circle map centred on your station becomes
essential , because contrary to many diagrams in radio publications the
solar terminator line cannot be a great circle ( the Sun wtuld need to be
an infinite number of miles away for this to be the case ). Plotted on a 
great circle map the curve of the solar terminator at long range becomes
obvious, Indeed if GC maps are plotted at say ten minute intervals it is
easy to see why time windows for sub horizon contacts UK to say Wellington
NZ, or, New York to Perth in Western Australia are open for very short
periods often of 20 minutes or less in a particular day. Equally obvious
if this map formula is used , is  the interesting fact that , the Dunedin
to UK path is open at a time a few minutes different from Wellington to UK
path. Though Dunedin is less than 650 kms from Wellington

Old time satellite user will all remember another version of the great
circle map which showed a hemisphere of the earth centred on either the
north or south pole. This kind of display was useful in pre computer days
because it permited the use of the mechanical track plotter which could be
set to show the satellite track, from an Equator crossing at a particular
time ut and longitude west. There is no obvious reason to use polar
centred maps (which do not of course show straight line paths satellite to
ground station), because a 486/pentium computer can plot a gc map based on
the station or the satellite with true linear radio paths, in a few
seconds, thereby allowing the map to be updated and repositioned every 1
or 2 minutes. So polar centred gc maps are probably a thing of the past.