Re: Martin Marietta's APALS

Organization: Cambridge University Engineering Department, UK
Date:         30 Oct 94 21:29:41 
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	According to 5-11 October 1994 issue of Flight International,
Martin Marietta is attempting to market a new approach and landing aid
designed to chnage the approaches are made. The system is called APALS
(Autonomous Precision Approach and Landing System). The way it works
is it enhances the aircraft's existing weather radar to produce ground
images of sufficient resolution to allow matching with a terrain
database. The correlation of actual radar image and the database scene
produces an error which is used to update the aircraft's Inertial
Navigation System (INS). The new position is used to predict what the
next radar image will be, and so on. APALS does not require ground
aids such as ILS or airport lighting but does require an airborne
global-positioning-system (GPS) and the creation of a terrain database
for all approaches to be flown. Full benefits of the system can be
realised if used with a head-up-display (HUD) which recreates an image
of the approaching runway including touchdown markers.

	I would like to point out the advantages and disadvantages of
this system. Firstly the fact that it does not require any ground aids
means that you can land your aircraft on any airport in case of
emergency provided the system database has the image data. This system
is ideal for airports like Katmandu in Nepal where there are limited
ground aids and poor visibility. This system could be another solution
to the replacement of ILS in Europe where the VHF band will reach
saturation by 1997.

	However the idea if being totally independent of ground aids
can be dangerous in the case of system malfunction. Due to its
reliance on GPS the accuracy cannot be guranteed unless high acurracy
high cost GPS recievers are used. The database should cover all of the
main airports throughout the world if it is to be used internationally
and the terrain data should be constantly updated since features such
as roads, building edges are used for correlation. All of this means
that the operating cost of APALS could be very high and higher than
operating the existing ground based approach and landing systems.

Any comments are welcome.

It should be noted that this article is intended to point out the
advantages and disadvantages of using APALS and not to criticise
Martin Marietta's products.