Re: Peruvian 757 crash -- possible caus

Date:         23 Nov 96 03:36:22 
From: (McLELLAN Alexander, DED/1)
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In article <airliners.1996.2359@ohare.Chicago.COM>, says...

>I own a gps that costs 300 bucks, and can display location, speed, and
>altitude with reasonalbe accuracy.  Is every airliner equiped with a gps
>now, and if so why not?

There are problems with GPS - even for the equipment that is fitted to
transport aircraft.

There are 'holes' in GPS coverage that move around the world (including the
US), so you can't guarantee accuracy - although you can predict where the
holes are.

Another problem is that the NAVSTAR (and the GLONASS) failure rate is too
big, and you don't know when you have big errors and when you don't.  These
are  military systems, and the integrity and continuity requirements that
are applied  to civil aeronautical systems were not taken into account
during development. Augmentation systems such as WAAS, LAAS and EGNOS will
help solve this problem.

The position data for the country over which you are flying has to be
available in
co-ordinates referenced to a 'shape' of the Earth called WGS84 - otherwise
GPS says you are in one place but looking out of the window tells you
something else!

States have a legal duty to supply aero-navigational data within their own
airspace. With ground-based systems, a State can avoid liability problems by
ensuring the safety of the system that they provide. With GPS, only Russia
and the US can control the safety (or otherwise!) of the system - everyone
else has to rely on them to do the decent thing!

This is by no means an exhaustive list why GPS isn't present in all
aircraft, but it gives an idea of some of the main problems.

You can probably find out more from the NAS Architecture Home Page:


Alex. McLellan

(not speaking for Eurocontrol)

All these problems are soluble, and work is going on to solve them.  But
until they are solved, GPS won't be certified for 'sole means' navigation
for all phases of flight.