Satgen518  Great Circles by GM4IJ    (BID SGEN518)     1999-02-27

A question often asked by Scottish school children is , " Why, every
afternoon, being 500 miles north of London do we see so many jet
trails from airliners, flying north over Scotland. Surely these planes
cannot all be going to Greenland or Iceland " ? 

Tell them that the planes are going to Vancouver, Los Angeles or Japan
,and they point out that the (Mercator) map at school ,shows America
is west not north and, Japan is even further away to the south east.

This misconception is not confined to Scottish children . A lot of
radio amateurs on both sides of the Atlantic, are unaware of it. But
take a globe of the world and a tape measure, and show that the
shortest route from London to Los Angeles goes up over Scotland,
Iceland, Greenland, Baffin Island, and Hudsons Bay, and some get the
point that living on a sphere produces some unexpected answers.. Great
circles are the shortest and least expensive in fuel, routes for
airliners, and, they are the only way to go, for radio signals.

Fortunately for most satellite users, their tracking software reports
great circle azimuths, without them being aware of it. The only
sources of confusion which can occur , are, on the computer which
displays a distorted Mercator projection of satellites position, or ,
the natural geography or buildings around their station, which the
mercator map induced misconceptions suggest will block their signals
to far away places. At IHJ there is a large ( fortunately) extinct
volcanic peak directly to the east. Several visitors over the years
have commented that signals to satellites offering connections to
Japan and the Far East must be poor. When in fact, Japan is to the
north east, clear of the obstruction , but Australian qsos are rare
because an easterly obstruction blocks the great circle route to
satellites best placed to reach Australia.

In a quite different sense , the great circle map is a vital component
in planning at IHJ , when as many satellite paths do from this
northern location , the signals transit across the auroral zone. on
their way to and from many excellent DX locations. With 17 radio
auroras so far this year at IHJ, the problems this presents are all
too frequent. So for a station in this situation the computer
generated, personal, own station centred, great circle map, is a must.
Not only does it help assess propagation problems , but , it also
helps with warning of auroral storms long before they come above the
Scottish horizon. In that, satellites far away towards Siberia and
even Alaska, are often heard on northern bearings with clear evidence
of auroral distortion of the signal. So it is then no surprise when 4
or 5 hours later, the auroral activity becomes evident above the
Scottish horizon. Unfortunately these events do not always help
Scottish stations . As for example when a recent aurora gave excellent
Auroral Es propagation from stations in southern England up to
northern Scandinavia. But gave no help to GM stations, because any
auroral Es forward propagation from them finished up putting their
signals down much further north, on natures biggest parking lot, the
empty polar pack ice.