Organisation: European Space Technology Centre (E.S.T.E.C)
Date: Thursday, 8 Oct 1992 11:21:44 CET
From: C R Francis <RFRANCIS@ESTEC.BITNET>
Newsgroups: rec.travel.air
Subject: Amsterdam (was Re: ILS? MLS?)
 <92275.235833RFRANCIS@ESTEC.BITNET> <1992Oct02.235449.23224@bnr.ca>
 <1992Oct3.121450.21282@cactus.org>

In article <1992Oct3.121450.21282@cactus.org>, rdd@cactus.org (Robert Dorsett)
says:
>
>In article <1992Oct02.235449.23224@bnr.ca> shams@bnr.ca (Shams) writes:
>>
>>What are the functions of ILS and MLS? What ILS stands for? How these two
>>systems compare to each other? It will be highly appreciated if you can
>>elaborate to shed a light on these.
>
>ILS = Instrument Landing System.
>MLS = Microwave Landing System.
>
>... followed by a lucid description of the systems and an outline of pros
>    and cons...
>              ...basically cost.
>
One of the chief virtues of MLS has always been the flexibility in approach.
It is perhaps invidious to mention it, but the recent Amsterdam crash
vividly illustrates this.  I should add that I feel this affair rather
deeply...I live less than 50km from Bijlmermeer, in about the same position on
the other side of Schipol.  Approaching planes are constantly flying low
over my house, but fortunately not last Sunday...

As in many places this (and some other) straight-in approach to Schipol is
over high density housing.  The ILS glidepath starts 9 miles out by the way.
The option of flying a curved approach can enable, in many cases, a lower-
risk route to be found.

Whether this would have helped over Amsterdam is arguable.  The pilot had
already rejected a proposal from Air Traffic Control (ATC) to use a
different runway, perhaps because he could not turn left.  He then continued
turning right, finally completing one and a half revolutions before
lining up for a staight approach to runway 06, from a distance of more
than 9 miles. Eyewitnesses (including a friend of mine) say that shortly
after this the aircraft came down very rapidly, not gliding at all.

Nevertheless the possibility is there.  If he had the option of a curved
approach he _may_ have needed fewer revolutions to line up, needing less
time, and _maybe_ could have made it.

>of the technology.  The nagging feeling that, perhaps, GPS/inertial
>approaches might provide a satisfactory solution, later on.  Not wanting to
>invest hundreds of millions in a technology that may not have ever really been
>viable.

On this question of GPS.  This is not used today even for trans-oceanic
navigation.  It should be used, and it is forseen that it will be.  However
normal GPS is not accurate enough for landing.  Errors of 10's of metres
are normal.  The so-called differential GPS is much more accurate (to
centimeters) but to get that performance you need to repeat observations
over several hours (or days) then enter an analysis which reuqires plenty
of manual intervention.  The process can take weeks.It's true there are some
research programs into using differential GPS for spacecraft docking, but
that sort of thing is many years off, and expensive.


-------------------------------------        Richard Francis
Internet: rfrancis@estec.estec.esa.nl        European Space Research and
Bitnet  : rfrancis@estec.bitnet                Technology Centre (ESTEC)
SPAN    : ESTERS::FRANCIS
smail   : C R Francis, OEE/OWP, ESTEC, Keplerlaan 1, 2200AG NOORDWIJK NL