From: Bob Noel
Subject: Re: GPS and IFR Approache
Organization: The MITRE Corporation
Date: Wed, 30 Dec 1992 13:13:02 GMT

In article <1992Dec30.045040.5727@craycos.com> ewv@craycos.com (Eric Varsanyi)
writes:
>>GPS is line-of-sight.  To provide coverage down to the DH it (the
>>DGPS ground station) will be sited in manner similiar to MLS.  Thus,
>>you can't have wide-area DGPS.
>
>Wide-area DGPS is in the research state by the Coast Guard. Regular DGPS
>which provides 100-200 NM coverage per station is implemented along the
>eastern seaboard and in one area of the great lakes (I don't remember where,
>the paper is at home). The accuracy they REQUIRE for harbor approach and
>traffic control is 8-12 meters absolute. With a GDOP > 3.2 (which is what
>you get 24 hours a day, worldwide starting in mid 93) they exceed this
>requirement. With a good GDOP (<3.2) they are MEASURING accuracies of
>about 5 meters close to the stations (20NM). They also get corrected
>velocity accuracy of .01 Knots. With DGPS you know in less that 1 sec 
>that a satellite signal is unhealthy. Since the current DGPS signal is
>in the low frequency AM area (2??-4?? Khz) you don't really need line
>of sight. There is no dependence on the DGPS signal as anything other
>than a data link (100 bps), you can do all sorts of evil things to it
>(including non-detministic delays) and still use it.
>
>I have an idea, instead of beating on DGPS without actual facts, call
>the coast guard GPSIC (GPS information center, 703-866-3806) and order
>the papers they've written on DGPS, read them, then comment on the
>system. They even have a bulletin board system you can download the
>papers from. I'm witholding technical comment on MLS since I don't know a
>lot about it (other than the obvious scaling and functionality issues).
>


>>2. There's good evidence that DGPS will *not* have the availability
>>of current nav systems for precision approaches wrt sufficiently 
>>low VDOP.
>
>Please quote a reference. With a full constellation the VDOP should be
>sufficient to give 5 M accuracy. If you can get me within 10 meters of the
>end of the runway I'll have a pretty damn good chance of seeing it.
>

actually, comment (2) above was in response to an _unreferenced_ claim
that DGPS would have availability superior to today's nav systems.  

in any case, do the math.  5M _veritical_ accuracy is inferior to MLS Cat I.
10 M veritical accuracy stinks in the world of precision approaches.  

>>3. availability <> integrity
>
>Integrity is carefully monitored in the DGPS scenario, read up on it, greater
>minds than me have actually thought about this stuff.
>

this DGPS scenario you refer to, is this the Coast Guard's DGPS? If so, then
I question it's applicability to aviation.  The integrity required for
avaition is vastly different then for the (relatively) slow moving surface
ships.
Aviation systems must detect problems within a couple of seconds.  The 
"ground" system for precision approaches cannot contribute more than
one bad landing in 10,000,000.  Is the integrity requirement(s)
for surface ships even close to this?  Also, surface ships don't have
to worry about the veritical axis (at least as long as they are sea-worthy).


>>by precision approach systems.  My general impression is that GPS
>>proponents are  puzzled as to why the FAA isn't embracing GPS 
>>(after all, it's a very accurate system).
>
>I think its less being puzzled and more being disgusted with the expected
>10-20 year red tape cycle. With GPS/DGPS this does seem to be moving along
>amazingly fast so there is hope. 
>
>I still think there is incredible power (financial) in being able to
>use equipment for navigation that will also be useable by ground
>vehicles. There is relativly little chance that MLS will ever be
>useable by ground based traffic. As an analog, which parts are cheaper
>on your typical Cessna, the Delco alternator or the McAuley prop?
>

geez, of course MLS won't be used for ground equipment.  We don't use B52's
for ground transportation either, but does that make B-52's a waste?

btw, which things are cheaper, TSO'd equipment or the equivalent automotive
equipment?


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