Re: Can't use GPS on Alaska Airlines

From:         frensley@utdallas.edu (William Frensley)
Organization: The University of Texas at Dallas
Date:         18 Sep 96 13:50:35 
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In article <airliners.1996.1901@ohare.Chicago.COM>, boyd@france3.fr (Boyd Roberts) writes:
|> I flew with American Airlines from Paris to New York and back in the last
|> two weeks and was surprised to see that they post the flight waypoints
|> (or some such aproximation) on maps stuck on galley external walls,
|> tagged with the flight time.  When I saw this I programmed them into my
|> GPS and created a route.  Reading the doc I saw that they allowed some
|> electronic devices to be used but prohibited _VHF_ radios and thought
|> to myself 'hah, GPS is _UHF_'.
|>

But, as has been pointed out, the frequencies of the local oscillators,
the various intermediate frequencies, and image frequencies may very well
fall within the aircraft bands.

|> On the sector to New York I had no problem with the cabin crew and suprisingly
|> no problem with my Magellan 2000, which locked up after a couple of minutes.
|> I was suprised to see that most of the waypoints were at 'well known' points;
|> 10 degree lon and whole or half degree lat.  I was further suprised that
|> the GPS indicated only small n (5-10) kilometer differences when nearing
|> waypoints.  However, when I was tracking 4 satelites the altitude was out by
|> 10's of thousands of feet.

That is because in high-altitude jet flight, "altitude" really refers to
air pressure (due to the use of barometric altimeters).  When one is near
the ground, taking off or landing, absolute altitude is of course a big
issue, and you assure that your altimeter gives you the right answer by
obtaining an "altimeter setting" from the local air traffic control facility.
This is just the current atmospheric pressure reduced to sea level.  At
higer altitudes, there is little concern for absolute altitude, but a
great deal of concern for relative altitudes between different aircraft.
Thus all aircraft are required to reset thier altimeters to the standard
setting (29.92 inches Hg) when passing from the low-altitude airspace
system to the high-altitude system (at 18,000 ft in the US).  Altitudes
above this are referred to as "Flight Levels", because they don't really
represent true altitudes, but rather constant air-pressure surfaces.
When the captain announces that "We will be cruising at an altitude of
33,000 feet," he really means that he has been assigned to Flight Level
330.  This can depart from the true altitude by 1000's of feet, but
10s of thousands seems a bit much.

--
Bill Frensley
Electrical Engineering
University of Texas at Dallas
P.O. Box 830688, MS. EC-33
Richardson, Texas 75083-0688