From: Bob Noel
Subject: Re: GPS and IFR Approaches
Organization: The MITRE Corporation
Date: Thu, 31 Dec 1992 13:33:12 GMT

In article <barnett.725771194@convex.convex.com> barnett@convex.com (Paul
Barnett) writes:
>In <1992Dec31.011252.1593@oakhill.sps.mot.com> mpaton@oakhill.sps.mot.com
(Michael J Paton) writes:
>
>>In article <1992Dec30.124200.4248@linus.mitre.org> Bob Noel writes:
>>>
>>>it turns out that the really important DOP is the VDOP (the _Vertical_
>>>DOP).  A VDOP supporting vertical accuracy almost as good as ILS Cat I
>>>is available only 99%. (that number is for a full constellation and DGPS)
>>>
>>Could you clarify this a bit please. I believe that I understand that 
>>you are saying that the best GPS VDOP results in vertical accuracy
>>almost as good as the Cat 1 glideslope. Is it not however true that
>>if one assumes that the glideslope were perfectly accurate, and one 
>>could track it perfectly, then the main problem in avoiding the ground
>>is the use of a Decision Height determined in the Cat 1 ILS by a 
>>barometric altimeter which may be off by + or - 75 feet. 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?
>
>I also have an additional question, which may or may not relate to the
>answer to this one: As I understand the implementation of the
>localizer/glideslope in ILS, the sensitivity increases as you approach
>the decision height.  In other words, a deviation of a given distance
>from the "centerline" of the desired course and descent path will
>result in a larger deflection of the indicators as the aircraft draws
>closer to the runway (and the ILS antennas).  As I said before, I know
>nothing about MLS, so I'm not sure the same phenomena occurs.
>
>If this is indeed true, then I presume the comparison of ILS Cat I to
>whatever is at the minimum decision height of 200 feet.  How much do
>the "numbers" change if one backs up/off to 300, or even 400 feet?
>
>


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.

4.  Current measures of approach system accuracies are measured in angles,
except
at the MAP (or DH) or Threshold.  Since GPS provides (x,y,z) and not angles,
then
it (GPS) is more accurate as one moves away from the threshold (DH or MAP, 
whatever is the reference point).  However, as one moves away from the
threshold,
there is more margin for error (in the current design of approaches) so the
accuracy advantage of GPS is less significant because it isn't needed for
obstacle avoidance.



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