From: kerry@Ingres.COM (Kerry Kurasaki)
Subject: Re: GPS and IFR Approaches
Reply-To: kerry@Ingres.COM (Kerry Kurasaki)
Organization: Ask Computer Systems Inc., Ingres Division, Alameda CA 94501
Date: 5 Jan 93 00:46:19 GMT

In article <greg.726184317@saltydog> greg@saltydog.dpsi.com (Gregory R. Travis) writes:
>
>>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.
>
>Yes, but consider that there are often one to three "backups" to the altimeter
>determination of DH:
>
>1.  Using a LOC/DME, DH can be determined by DME distance to the localizer
>    antenna.
>2.  Using the middle marker as a reference, DH can be determined by activation
>    of the middle marker light*.
>3.  Using a radar altimeter, DH can often be determined by radar altitude
>    above terrain**.
>
>In a typical CAT-I ILS, I would tend to value the information from #1 above
>the most highly.
>
>*#2 suffers because: a) MM transmitters are often, but not always, co-located
>with GS DH intercept, b)  The size of the MM beam also makes EXACT DH
>determination difficult, AND c) MM in most countries (the USA included) are
>going away.
>
>**#3 is very good for approaches in which the terrain at DH is the same
>elevation as the runway threshold elevation.  Obviously an approach to a
>runway whose threshold began abruptly atop a 300' cliff would be a poor
>candidate for DH determination via radar altimeter.  But, if I knew I
>were making a tight approach into a airport on land as flat as a pooltable,
>I think I would rather use the RA to determine DH over all else.  Certainly
>if my RA said I was at DH and my GPS told me otherwise, I would go with the
>RA.
>

First, apologies for the massive inclusion of the previous posting,
but it seemed appropriate in this case.

Sorry Greg, but I have to disagree with your choice of words.  There is
no such thing as a "backup".  There is only one way to determine DA(H)
with a CAT I approach, and that's with the altimeter on the GS.

A better choice of words is "cross-check".  To the casual observer,
it seems like any of the three methods you describe would work, and
practically, they probably will, but in my training and understanding
of the FARs, only the altimeter is legal.

Now you may suspect that your altimeter is whacko due to the improper
altitude readout at the outer marker.  Some of your choices are to
use another approach, or the LOC (GS out) minima.

I just want it make absolutely clear that the examples you cited above
may NOT be substituted for the DH.  Someone might read your post wrong.

It is also my understanding, and someone can correct ME if I'm wrong,
that the 200' DH for CAT I approaches includes a possible 75' error
in the altimeter.  There is that much safety margin, provided the
approach is flown properly (on or above the GS, etc.).

Kerry
Newsgroups: rec.aviation.ifr