Date: 23 Nov 96 03:36:22 From: email@example.com (McLELLAN Alexander, DED/1) References: 1 Followups: 1
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In article <airliners.1996.2359@ohare.Chicago.COM>, firstname.lastname@example.org 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: http://asd.orlab.faa.gov Regards 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.