PH>WD> Over the oceans there is no tracking. Aircraft virtually fly in the
PH>WD> blind.
 
PH> But there's GPS isn't there?
 
Yes. It tells you where 'you' are, but it doesn't tell you where other planes are.
 
GPS in aviation basically helps the crew and the plane cross uncharted
territory without any known NAV-markers, but since there still is no
automatic loop-back system for the GPS-signal, a plane cannot be tracked
yet once it's outside radar-boundaries.
 
An example.
 
When a plane flies from London, England to New York then at some point it
will receive its oceanic clearance from Shanwick EGGX, these are the
powerfull transmitters from Shannon, Ireland operated from a control center
in Prestwick, Scotland.
 
Shanwick and Gander CZQX have contact twice a day and based on current
wind-systems and weather-patterns will determine 7 oceanic westbound tracks
called A->G (eastbound tracks are S->X).
 
Each track also has layers (5 I think). These tracks are separated by about
50 nautical miles and I think there's a vertical separation of 500ft.
 
Once planes get their oceanic clearance they depart on the assigned track
and fly it with GPS. The accuracy is astonishing and after 4-5 hours over
water they have a max deviation of a few meters. Prior to GPS they did it
with inertial navigation and they could swerve about 2 miles off track but
with the 50 nautical miles separation they were safe until picked-up by
radar again at the end of their over-water run.
 
Once beyond the curvature of Earth, there is "NO" radar tracking
anymore and they're on their own. Every 5 degrees each plane needs to
report passing that longitudinal waypoint and give an estimated time of
when it will pass the next 2 waypoints. That's the only thing that Shanwick
and Gander have to keep planes separated over the Atlantic.
 
ATC is blind and needs to rely on airplane reports and planes themselves
are also blind with 2 remarks:
 
1) They have GPS and they know where they are
2) They also have TCAS (Traffic Collision Avoidance System) which
automatically communicates with the TCAS of other planes in the vicinity
and knows where they are and what they're doing. In case of a potential
collision it will order one plane down and another plane up.
 
Added issues for Shanwick and Gander is that all planes do not fly at the
same speed. A 747's cruising speed is higher than a 767/757/777 which then
again is faster than the 737s being used in transatlantic service. They
need to take care for example they do not put a 747 behind a 737 because
that will result in a "fender-bender".
 
There is also a very northerly track system for flights originating in
Europe and going non-stop to the North American westcoast. There are
southerly tracks over the Azores and then there are the tracks for South
America.
 
A similar track-system also exists for example over the Pacific and on
USHawaii flights.
 
This all means that once a plane is behind the curvature of the Earth and
radar cannot see it anymore (it needs a direct line of view) ATC has no
clue what happens to it nor where it is. We all rely on the progress
reports by the pilots, but if there's a rogue pilot or something really
unexpected/unknown occurs, then it is looking for a needle in a haystack.
 
Take care,
 
 \%/{at}rd

--- D'Bridge 3.99