From: mikec@ginger.lcs.mit.edu (Mike Ciholas)
Newsgroups: rec.aviation.ifr
Subject: Re: GPS vs DGPS info needed
Date: 12 Jan 93 04:41:53 GMT
Distribution: usa
Organization: MIT Laboratory for Computer Science

In article <1993Jan12.014925.16271@rwwa.COM> witr@rwwa.com writes:
>In article <1993Jan11.153539.7780@mintaka.lcs.mit.edu>,
>	mikec@ginger.lcs.mit.edu (Mike Ciholas) writes:
>| While DGPS does nullify the SA errors, its primary value is to improve
>| the basic accuracy of the positioning information.  Even if SA where
>| turned off permanently, straight GPS would not be good enough for
>| precision approaches, whereas GPS with SA turned on might (most
>| likely) be adequate for non-precision approaches, and is certainly
>| accurate enough for enroute navigation.
>
>OK.  Let's separate things out so I can make sense of this (all questions
>obviously apply to IFR use):
>
>1) For enroute use, is GPS with SA accurate enough?

Using a barometric altimeter, yes.

>2) What standards apply to the above?

It seems pretty obvious to me that VOR and NDB airways don't begin to
approach the accuracy of GPS with SA (+/- 100 meters).  If loran is
good enough for enroute use, then GPS with SA should be as well.

>3) For approach use, is GPS with SA accurate enough for *any* approach?

It is my opinion that GPS with SA turned on is accurate enough for any
non-precision approach.  Another way to say that, GPS with SA performs
as well as any present non-precision system with the possible
exception of the localizer.

>4) What about GPS without SA?

The accuracy gets better, but it ends up in the gray area where it
isn't good enough for precision approaches, and is not needed for
non-precision approaches.  It might be good enough to do the localizer
now.

>5) What standards apply to 3 and 4.

There are significant standards for instrument approaches, most
outlined in the TERPS manual (how to build an instrument approach,
basically).  The manual outlines the required obstacle clearances and
equipment performance.

>My back of the envelope calculations seem to indicate to me that GPS with SA
>is more accurate than VOR navigation enroute, except when quite close to the
>transmitter (when you consider the allowable error in a VOR check).  Thus
>I don't understand your implication that DGPS would be required for enroute
>GPS use...

Ooops, I didn't mean to imply that at all so I am sorry if I misled
you.  I agree 100% that enroute is a no brainer for GPS.

>Likewise, for approach use my kneejerk reaction is that a GPS (even with SA)
>approach is likely to be more accurately flown than, say, a ndb approach,
>given typical pilot performance.  By this I mean that the resulting approaches
>would be, on the average, closer to the ``nominal'' approach.

We can see evidence of this now when you track a non-precision
approach using loran or GPS.

>In the precision approach case, is your claim that DGPS is not accurate
>enough in the ``localization'' aspect or the glidepath aspect, or
>both?

Neither.  I believe DGPS of the proper type can do at least as good as
Cat I ILS.  My belief includes the glideslope path.

>Again, given the allowable errors in the onboard instrumentation, it seems
>that DGPS would certainly be accurate enough in the localization plane, but
>I am not sure about the glidepath aspect...

We will see.  I expect we will have several tests of such systems,
some with DH determined by DGPS, some with barometric altimeters.  I'd
like to see both used.  I've come to appreciate that the barometric
altimeter is a quite remarkable and reliable instrument indeed.

A great deal of the arguments surrounding DGPS applied to precision
approaches have to do with fault detection, not accuracy.  This issue
will have to be settled as well.

FAA has published a TSO for GPS receivers.  I believe this means you
can install a GPS receiver which is TSO'd and get IFR enroute approval
if you test it in some way (Lorans can be done this way as well).

Mike Ciholas
mikec@lcs.mit.edu
Newsgroups: rec.aviation.ifr