From: mikec@tabasco.lcs.mit.edu (Mike Ciholas)
Subject: Re: GPS -- A Potentially Dangerous Technology
Organization: MIT Laboratory for Computer Science
Date: Mon, 13 Apr 1992 21:02:03 GMT

In article <1992Apr13.155425.5586@bronze.ucs.indiana.edu>
	greg@bronze.ucs.indiana.edu (Gregory R. TRAVIS) writes:
>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
[deleted]
>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.

Okay, assume VORs are about 0.1 degree accurate, then 10nm would be
about 30 meters.  This is slightly better than uncorrected and
distorted GPS can do (about 50-70 meters).  If you decide that VORs
can be relied upon for 1 degree accuracy (multipath can cause worse
errors), then your region of VOR superiority is within 1.2nm.

>       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.

If you have a DME that reads out in 0.1nm increments, then your
digital "noise" (+/- 1 digit) is more than GPS accuracy.  DME accuracy
does not depend very much on the distance to the station as it is a
time of flight measurement (yes, the atmosphere causes distortion, but
pretty small, for GPS signals coming from space, the extra delay due
to the atmosphere corresponds to less than 3 meters).

>[differential GPS used for LOC/DME]
>
>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.

Altitude and time are also GPS outputs.  I also believe DGPS can do
all the instrument approaches as well as the native systems do.

>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

GPS does provide altitude.

> - a PRECISION curved approach is possible
>with MLS.  As far as I know, no other navaid has this capability.

Differential GPS accuracies under a meter are not only achievable, but
aren't that hard.  The geologists here at MIT routinely do DGPS
measurements to the sub centimeter level in a few minutes.  They can
get meter accuracy instantaneously.  Over long periods of time (like a
week), they can measure the plate movements of the Earth.

I would suspect that any navigational system that can position you in
three space to within a meter is sufficient to be called PRECISION.
I'd guess that it would be quite a bit better than ILS.

The curve approach issue is simple, if your GPS unit can generate
curved trajectories, then you can do curved approaches.  So a little
math will generate any arbitrary approach you want.  You can customize
for your airplane as well (MLS may be able to do this, I dunno) where
you can choose the glide slope (as long as it is above a minimum
value, of course :-).

>Glossary:
>ICAO		- Internation Civial Aviation Organization
>GPS		- Global Positioning System
DGPS		- differential GPS
>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.

How do you get a subscription?

>Gregory Reed Travis					D P S I

Mike Ciholas
mikec@lcs.mit.edu


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