From: dick.zeitlin%acc1bbs@ssr.com (Dick Zeitlin) 
Newsgroups: rec.aviation.ifr
Subject: Re: GPS and IFR Approache
Date: 30 Dec 92 21:46:00 GMT
Distribution: world
Organization: Advanced Computer Concepts BBS, New Rochelle, NY 914-654-1981
Reply-To: dick.zeitlin%acc1bbs@ssr.com (Dick Zeitlin) 

O> .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.

You misunderstand the principle of DGPS.  The correction signal can be
sent over any frequency.  The Coast Guard is using DGPS
corrections transmitted over existing NDB beacons and usable at
the limits of the NDB reception range.  Line of sight is not a factor.
The FAA has successfully tested wide-area DGPS at up to 250 miles for
approach use. (I don't know what frequency was used, but you can be sure
it wasn't subject to line-of-sight considerations.)

O> .2. There's good evidence that DGPS will *not* have the availability
 > .of current nav systems for precision approaches wrt sufficiently
 > .low VDOP.

There's equally good evidence that it will.  There are several
techniques that are being discussed for dealing with potential VDOP
concerns.  The availability of sufficient SVs for adequate RAIM in some
mountain environments is an issue, but a pseudolite at or near the
airport seems to be a more-than adequate solution.  A pseudolite is
cheaper to install and maintain than an ILS/DME.

O> .4. what I meant to get across was:  I lot of what I read about GPS (of
 > .any flavor) gushes about the accuracy (usually under ideal conditions)
 > .and completely ignores the other requirements that must be met
 > .by precision approach systems.

Talk to anyone that's got a GPS unit in their aircraft now.  I know of
several people that have pulled their LORANs in favor of GPS.  That's
under real-world, current conditions, without the full constellation of
SVs up.  GPS at its worst is better than LORAN at its best.

O> .                               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).

But the FAA _IS_ embracing GPS.  And at a pace far faster than they have
with any other advance.  (Even TCAS!  <~> )  The first GPS TSO is
released, and there _will_ be many overlay non-precision approaches
certified in less than a year.  Compare this to LORAN, where there are a
handful of non-precision approaches, but all NOTAMed OTS.  (And I
suspect they always will be...)

There's no question that the GPS environment has the potential to
completely replace the current VOR/DME environment as the primary
enroute nav. source.  There's also no question that it can be used
for CAT-I approaches.  There are no apparent issues preventing its use
for CAT-III approaches.  (This includes both integrity and availability
issues...)  At least one manufacturer has said outright that their
current unit could easily handle the database for the precision
approaches without major change.

Expect to see LOTS of movement from the FAA towards GPS in the near
future.  It's happening...

        D/
---
 . QMPro 1.0 41-3345 . Bring back A-N airways!!    Dick.Zeitlin%acc1bbs@ssr.com
                                                                                                         
Newsgroups: rec.aviation.ifr