Satgen337  Satellite Software Pt4   by GM4IHJ                 9 Sept 95

COMPUTER MAPS . The Earth is a partially flattened sphere. Its surface
extends in 3 dimensions. If we try to peel off its surface and present it
as a flat 2 dimensional map we get distortion of distance and direction.
This situation must be bourne in mind by anyone using computer maps.
Software adverts which show, satellite radio paths as linear features,
satellite footprint outlines as perfect ellipses or circles, or ,
beautiful symetrical sine waves defining , satellite ground tracks or
solar illuminated zones, should be treated with the greatest suspicion.

Consider first the MERCATOR or ORTHOGRAPHIC projection map. This shows the
outlines of continents on a rectangular map using parallel, linearly
spaced , latitude lines, and longitude lines. On this map the software can
faithfully produce the sub satellite point and your station location. BUT,
any suggestion that a straight line joining these two points represents
the path a radio signal would follow from sat to station, is quite wrong.

Taking the example of a satellite overhead New York . Seen fom Scotland a
Mercator map suggests an azimuth heading of roughly 250 degrees. But take
a piece of string and put it around an earth globe from GM to New York and
the great circle azimuth the signal takes is clearly nearer azimuth 290.
So if you believe a mercator map you will miss by more than your antenna
beam width. Fortunately most software prints the figure for true great
circle azimuth to the satellite somewhere on the edge of the map.

Equally prone to error oo some mercator maps are the outlines of the
satellite footprint ( that area of the world the sat can see and talk to
). Several software ads seem to suggests that footprints are always
perfect circles or ellipses, but on a mercator projection this cannot be
the case because distance east west is grossly exaggerated the further you
get from the equator . So unless the sat is over the equator the footprint
will not be a balanced figure north /south , and it will appear wider away
from the equator finishing up looking more heart shaped, like the shield
carried by knights of old, rather than an ellipse. To see the reality of
this situation compare the size of Greenland on an earth globe, with its
exaggerated width on a mercator projection.

All these things conspire to make a Mercator map very misleading . Indeed
matters can get even worse if a mercator map is used to display the path
to a satellite concurrent with the position of the Solar Day/night
terminator line. Too often in some amateur radio aids these are presented
as straight line path over a sine wave terminator line, which is very
misleading and useless for assessing ionospheric propagation to the
satellite, from your station, and from any desirable DX location.

Should we therefore abandon Mercator maps ? Unfortunately there are no
preferred alternatives. Which ever way we try to turn 3 dimensions into 2
we get distortion. BUT other types of maps have different kinds of
distortion. So if we provide, as the better amateur software does, a
choice of say 3 map types and we remember the different potential for
distortion in each kind of map, we can perhaps avoid mistakes. Next sgens
will discuss other map types. 73 de GM4IHJ @ GB7SAN ,or, gm4ihj@delphi.com