From: Bob Noel
Subject: Re: GPS and IFR Approaches
Organization: The MITRE Corporation
Date: Wed, 30 Dec 1992 12:42:00 GMT

In article <barnett.725654653@convex.convex.com> barnett@convex.com (Paul
Barnett) writes:
>In <1992Dec29.174022.6514@linus.mitre.org> Bob Noel writes:
>
>>However, such a tower would be an obstruction that mustn't be
>>allowed to penetrate protected airspace.  A tower sufficiently
>>far away as to not be a navigational hazard would also be low
>>on the horizon.  In that case, you still need to worry about 
>>line-of-sight down to the DH (and below - for margin).  My gut
>>reaction is that such a tower would be too low on the horizon
>>for visibility for almost any runway.
>
>The FAA has specific criteria for the locality and height of
>obstructions in relation to airports and instrument approaches, and my
>observation is that they are more liberal than you might think.
>

not quite.  Having access to the criteria used by the FAA, I'm quite
aware of the siting criteria, especially in relation to ground based
equipment

stuff deleted

>>For non-precision approaches, the accuracy of GPS is sufficient
>>(DGPS would be overkill _for the accuracy_). 
>
>For non-precision approaches, I agree.  However, the prospect of
>precision approaches at any airport, without all the costs and hassles
>of an ILS/MLS, makes the issue worth further consideration.  I came
>into this discussion late, so forgive me if I am rehashing the same
>issues.
>
>The FAA has specific performance criteria for precision landing
>systems, which accounts for human factors, ground facilities, and
>(properly maintained) avionics.  All of that should translate into
>hard numbers that can be used as a starting benchmark that could be
>converted into a required GDOP (geometric dilution of precision).
>

it turns out that the really important DOP is the VDOP (the _Vertical_
DOP).  A VDOP supporting vertical accuracy almost as good as ILS Cat I
is available only 99%. (that number is for a full constellation and DGPS)

>Given the number and locations of the GPS satellites in view at any
>given moment, the GDOP can be easily computed, yielding the
>"potential" position error.  A DGPS station (or even a ground GPS
>transmitter, which could simply be treated as another "satellite in
>view") would simply further reduce the GDOP.
>
>Excuse me if I don't have the terms exactly correct, but my point is
>that a GPS receiver can display the equivalent of a warning flag if
>the GDOP exceeds a certain threshold.  What would that threshold be?
>Perhaps it could be set prior to an particular approach, preferably by
>calling the entire approach plan out of a database.
>
>GDOP's can also be forecasted, so it wouldn't be hard to NOTAM an
>approach out of service during periods of time that the GDOP is
>expected to be insufficient.  Another alternative: the decision
>heights for ILS's are routinely increased for a variety of reasons, so
>why not adjust the decision height of a particular precision approach
>via GPS, based on the worst case GDOP?  Maybe your navigation
>equipment could even tell you what a safe DH is at that moment? 
>(Yeah, I know, I can hear the FAA groaning already).
>

yep, a GPS receiver can detect some of the possible system failures
(induced by geometry or whatever).  


>I sense an insistence, either on the FAA's part, or our part (or both)
>to shape the proposed usage of the GPS constellation to conform to
>existing methodologies and procedures.  I wonder if we should give
>more consideration to how we can do things differently to take
>advantage of the unique capabilities of the system?

My insistence (for what it's worth, which aint much) is that we don't
go backwards in terms of safety.  I don't care how it's used, but GPS
must be as safe as MLS or I don't want any part of it.  This safety is,
in part, provided by system accuracy (which I still question) and 
the ability to monitor itself (and here I'm _not_ referring to the receivers).
All in all, there are still a number of things that must be
addressed before GPS can be shown to be better than MLS for
precision approaches.  


Bob Noel
my views are my own, not MITRE's

Newsgroups: rec.aviation.ifr