From: mpaton@oakhill.sps.mot.com (Mike Paton)
Subject: Re: GPS and IFR Approaches
Organization: Motorola Inc., Austin Tx.
Date: Mon, 4 Jan 1993 20:12:07 GMT

In article <1992Dec31.133312.12126@linus.mitre.org> Bob Noel writes:
>
>I'll attempt to address both Paul's and Micheal's questions.....
>
>
>1.  Perhaps I should have stated:  A VDOP _at least_ good enough
>to support vertical accuracy almost as good as ILS Cat I is available
>only 99%.  It is correct to assume that there will be times when the VDOP is
>low enough to provide better than Cat I accuracy.  Better VDOPs have lower
>availabilities, which is (currently) bad in the world of precision approaches.
>
>2.  +/- 75 feet is a huge error for vertical accuracy.  And, the real system
>error of a baro altimeter is larger than that.  Thus, I wouldn't like to 
>trust my life to a baro altimeter when I'm that close to the ground.  
>(at least current aviation altimeters)
>
>>>If one assesses
>>>the accuracy of the ground installation combined with the airborne
>>>equipment, does the DGPS and full constellation not then come out ahead?
>
>not exactly.  At the DH, the vertical accuracy of DGPS and ILS 
>are just about the same (close enough not to make any real difference).  
>Actually, MLS is more accurate than ILS, but, again, it's not a 
>real big difference.
>
 
Hi Bob,
        I was going to follow up to this on the net, but for the second time
today, it gets an internal error when I try to post. Then I found that email to
you bounced as neither my mailer nor I could find your email address. So I
found another machine to post from. Hope it works. Perhaps you could attach
your email address to your postings.
 
I feel you've almost answered my question, but not quite. I'd like to consider
the total SYSTEM of vertical guidance on a precision approach. I'd also like
to cut out of the discussion the accuracy of the pilot's tracking ability,
perhaps by postulating a suitable coupled autopilot. All of the systems,
MLS, ILS and the DGPS we're speculating about can provide a straight line in
space, which for the purposes of argument we can call the glideslope. Assuming
this glideslope is tracked perfectly, then when do we abandon or "miss" the
approach when using ILS Category 1 standards. It seems to me that determination
of the DH is a critical part of the system. If the aircraft is on the
glideslope at all times, then the DH can be determined theoretically anyway
by distance in a horizontal or vertical direction. For the Cat 1 ILS it is
determined by a barometric altitude which is required to be tested every 2
years to a standard I don't have handy, and can be checked by the pilot on
the ground before flight as being within +- 75 ft. I have been ASSUMING all
along (and please correct me of course) that the DH on a DGPS approach
equivalent in category to the ILS Cat 1 would have the DH determined by the
DGPS itself and would not use the barometric altimeter.
 
My point is that the overall SYSTEM accuracy of the Cat 1 ILS is degraded by
the method used to determine DH whereas I assume that a DGPS approach would
not have any degrade as the DH would be determined with the same accuracy
as would the glideslope. I agree that +- 75 ft is a huge error, but it is the
current standard for Cat 1.
 
Michael
 
mpaton@tempest.sps.mot.com
 

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