From: barnett@convex.com (Paul Barnett)
Subject: Re: GPS and IFR Approaches
Date: Tue, 29 Dec 1992 18:44:13 GMT
Organization: Engineering, CONVEX Computer Corp., Richardson, Tx., USA
X-Disclaimer: This message was written by a user at CONVEX Computer
              Corp. The opinions expressed are those of the user and
              not necessarily those of CONVEX.

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.

In particular, the VOR/DME 26 approach into Anoka County, MN, takes
one right past the highest towers in the area: 1500 feet AGL.  I used
to look at them as I drove to the airport... if I could see the top of
them, the ceiling was high enough for flight lessons (at least in the
pattern).  The approach begins around 2600 feet MSL, just about the
tops of the towers.  If I remember correctly, RADAR REQUIRED.  That
means Mpls approach will let you know if you wander too far off the
beaten path.

Looking around me in the vicinity of a DH, I usually see lots of
towers.  200 feet is pretty high near an airport, and clears most
nearby obstructions.  My main concern would be the shadowing or
distortion of the line-of-sight signal at inopportune times by tall
buildings that sprout up after the initial installation.

>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).

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).

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?
--
Paul Barnett
MPP OS Development     (214)-497-4846
Convex Computer Corp.  Richardson, TX