From: greg@bronze.ucs.indiana.edu (Gregory R. TRAVIS)
Subject: Re: GPS -- A Potentially Dangerous Technology
Organization: Indiana University
Date: Mon, 13 Apr 92 15:54:25 GMT

In <1992Apr13.144135.17274@col.hp.com> davea@col.hp.com (David E Allen) writes:

>- To use GPS for precision approaches is another matter. GPS in itself is
>not accurate enough for that. This requires differential GPS (you know: with
>a ground based receiver that very accurately knows it's own position,
>compares that with the GPS-reported position, and makes the difference
>available as an error compensation signal to others). Now differential GPS
>is relatively new; experiments are going on and are very promising. But no
>standards have been agreed upon yet to my knowledge, so that's future for
>now.

From what I've read in the ICAO journal, even differential GPS is not
accurate enough for a PRECISION approach.

As I remember it:
	"Ordinary" GPS, under the best of circumstances, can be expected
	to provide an approach as accurate as a VOR approach where the
	VOR is ~10 miles from the airfield.  It is not as accurate as
	a VOR approach where the VOR is located ON the airfield and provides
	the MAP.  It is also less accurate than a VOR/DME approach as DME
	is considered more accurate within 10 NM of the DME station.

	With differential GPS a ground-based transciever constantly compares
	its own, firmly known, location to what the GPS system thinks and
	transmits the difference to nearby aircraft.  This system is
	expected to provide "localizer-like" azimuth guidance and allow
	approaches with equivalent precision to standard LOC/DME approaches.

	Even differential GPS is not expected to provide sufficient
	elevation information for even standard Cat I ILS minima.  The
	current thinking is that higher minimums GPS "precision"
	approaches are not warranted because a GPS "LOC/DME" type approach
	will provide the same, or lower,  minimums via letdown fixes.

The big wins from GPS are:
	1. Enroute navigation
	2. Higher precision approaches than that available from NDBs
	3. Arbitrary (curved) approaches with multiple stepdowns
	4. Approaches at virtually any airport.

Note that LORAN and MLS with also do #3 above (as will RNAV) with fancy
computers onboard.  LORAN won't do it with the precision of GPS and
MLS will be as precise, probably moreso, and provide constant
altitude information - a PRECISION curved approach is possible
with MLS.  As far as I know, no other navaid has this capability.
	
Glossary:

ICAO		- Internation Civial Aviation Organization
GPS		- Global Positioning System
VOR		- Radio beacon providing direct azimuth information
DME		- Radio beacon providing direct distance information
LOC		- Radio beacon providing direct and accurate azimuth info.
ILS		- Radio beacon providing altitude information as well as
		  highly accurate azimuth information.
MLS		- Like ILS, but based on microwave frequencies
PRECISION 	- An approach that provides constant altitude vs. distance
		  information to the pilot.
MAP		- Missed approach point.


greg, just parroting what he read in the ICAO journal.

-- 
Gregory Reed Travis					D P S I
	
Data Parallel Systems Incorporated	greg@cica.indiana.edu