From: rh@craycos.com (Robert Herndon)
Newsgroups: rec.aviation.ifr
Subject: Re: GPS and IFR Approaches [long]
Date: 25 Dec 92 01:36:39 GMT
Organization: Cray Computer Corporation

Get out the axes, Ma!  I got the sharpener going good!

Bob Noel & Eric Varsanyi (I hope I've got the attributions right) argue:

BN:  Will the availability of the vertical accuracy of GPS (I believe the
BN:  term is VDOP) be sufficient for Cat II or III requirements?  btw, an 
BN:  availability of "most of the time" isn't good enough.

EWV:  Well, I would argue that 'most' of the time for GPS is much better than
EWV:  'most' of the time for any ground based system like ILS or MLS.  

BN:  from the numbers I've seen, I can't agree.

then:

EWV:  When
EWV:  the 24 Sats are up (mid 93) there are 4 small locations (about 1 degree
EWV:  square it appears) on earth that will have a regular outage (defined as
EWV:  less that 4 sats in view). The outage will last less than 30 seconds at
EWV:  each location. 2 of the locations are in the Antarctic, over water (or
EWV:  ice I suppose) and the other 2 are in a corresponding location in the
EWV:  Northern hemisphere (there is one East of Alaska, way up north and one
EWV:  in Greenland).

BN:  4 birds in view is *not*, repeat *not*, a sufficient condition for
BN:  a GPS geometry with a low enough VDOP to support Cat I (or better)
BN:  approaches.  The number of satellites is not really the issue, the
BN:  important thing is the precision offered by the satellite geometry
BN:  which, of course, is strongly influenced by the number birds in view).
BN:  You can have situations where 5 birds in view is inferior to other
BN:  times with only 4 birds in view.


EWV:  I think all the other factors you could list that would affect
EWV:  availability (weather/icing, jamming, electrical failure, equipment
EWV:  breakage) would equally affect GPS and MLS, perhaps it would even
EWV:  be worse for MLS since they are on the ground and vulnerable to bad
EWV:  drivers.

BN:  For GPS to provide precision approaches, it appears that there must be
BN:  a ground station.  Thus, both systems will be vulnerable to unintentional
BN:  threats.

and again:

BN:  If, repeat IF, GPS can provide the integrity required for Cat I,II or III
BN:  approaches (and vertical accuracy), it won't be much of a contest.  

EWV:  Yes, I can easily see how you can compare an NDB or VOR approach with a 
EWV:  Cat III ILS approach. Not.

BN:  I'm not comparing NDB or VORs with GPS and MLS.  In this thread I've been
BN:  consistently dealing with precision approaches.

BN:  and as usual, nobody seems to worry about the integrity of GPS or DGPS.

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Pardon my ignorance, but I really do not understand Mr. Noel's (rabid?)
defense of MLS.  As someone has pointed out, for the large majority of
aircraft out there, the choice will NOT be between GPS and MLS, it will
be between GPS and GPS + MLS.  MLS is *only* a landing system (it is
apparently incapable of assisting en-route navigation), and is likely
to be present only at large airports.  In spite of the fact that MLS
requires much less landscaping, antenna geometry, etc., I do not see
many (any?) airports switching from ILS to MLS.

Even United Air Lines, apparently one of the biggest MLS backers, has
informed the FAA that it doesn't want MLS, and will not install receivers
in its aircraft.  If the U.S. is required to support MLS for international
operations, then most likely the U.S. will do the minimum required by
treaty and let it go at that.

Yes, maybe MLS *is* better than ILS, VOR, and NDB approaches.  Nonetheless,
it is expensive, and the gains are quite marginal compared to an ILS.
Even if it can give ILS accuracy for approaches to airports now only
capable of NDB and VOR approaches (i.e., NOT international airports)
I see no reason for these approaches to be installed.  Yes, maybe MLS
is better, and if all VOR, NDB, and ILS approaches could be replaced
with MLS systems, it'd be great.  But there are an awful lot of ADFs,
VOR receivers, and glideslopes out there that *have to be kept anyway*
(well, maybe not the glideslopes) for enroute navigation.

As R. P. Feynman noted, innovation is never easy.  To displace existing
technologies, something has to be either an awful lot *cheaper* than
what it replaces, or an awful lot *better*, and usually both.  MLS fails
this test because:  (1) it's expensive; and (2) it can't replace VORs
or NDBs.

GPS, on the other hand, may pass on both accounts.  It is now about
as cheap as LORAN, and is getting cheaper rapidly.  It is fully capable
of supporting both enroute navigation and landing approaches.  Thus,
even though it may not replace ADFs and VORs, it does at least provide
backup for them and could conceivably replace them.

I grant that non-differential GPS may lack the accuracy that the FAA
insists upon for precision approaches.  I grant that other countries
may not certificate GPS since it's administered by the U.S. department
of defense rather than the FAA.

For the former, differential GPS may meet the FAAs accuracy requirements
for precision approaches.  EVEN IF IT DOES NOT, it appears to be
substantially better than NDB and VOR approaches, and should in some
fashion be integrated into the U.S. approach structure.  That it can be
used to provide "ILS approaches into grass strips" with an accuracy
comparable to a VOR approach, WITHOUT ANY LOCAL GROUND EQUIPMENT AT ALL,
is undeniably a very useful and powerful capability.  Failure to
acknowledge and exploit this capability, which will undoubtedly soon
be present in a large fraction of the world's aircraft, would be
foolish.

On the issue of reliability, there are some valid concerns about GPS.
What is its availability?  What are its failure modes?

Since GPS is a global system, it has the potential for simultaneous
global outages, which for the most part is not true of the current
system of VORs, NDBs, and ILSes.  It may also have the potential for
developing significant errors if (somehow) the GPS constellation
started transmitting erroneous information.

The potential for outages implies that our system of NDBs, VORs, etc.,
cannot go away overnight.  Should the GPS constellation fail for some
reason, aircraft will need redundant measures to complete their
operations.  This possibility should not prove traumatic for GPS;
certainly the ILSes, VORs, and NDBs I've navigated with have had
their share of outages.  The nature of failures with GPS, however,
is different, and some adaptations will have to be made.

Regarding accuracy, the potential for satellite positioning errors MAY
limit GPS approaches to DGPS approaches, where a ground station can monitor
the GPS satellite signals for integrity, and provide a valid/invalid
signal of some sort.  Most likely, until GPS's accuracy characteristics
are established, we'll see higher than necessary MDAs and decision
heights that may be relaxed as time passes and the characteristics
become known.

Certainly, GPS's accuracy can be easily measured by ground stations.
Again, since it is a global system, it should be easy to monitor on
a regional basis.  I am sure that the FAA will wish to do so (it would
seem to be a simple part of a DGPS transmitter), and possibly issue
flight notices and/or NOTAMs and/or adjust descent minima should
out-of-tolerance inaccuracies be measured.

Regarding GPS's international use, I hope that the U.S. Government can
come to some arrangement suitable to foreign governments.  That lies
in the realm of politics, but there are many reasons to hope and believe
that arrangements can be made.  I, at least, am optimistic.

If the Bob Noel now wishes to return to arguing the merits of MLS, he's
welcome to do so.  But I believe that MLS will not stand on its own
merits, and certainly think it will not for GA.  The gains are just
too small and the costs too great.

-r

----
Robert Herndon		    --	not speaking officially for Cray Computer.
rh@craycos.com			   --|--		(US) 719/540-4240
AOPA, EAA, & SSA member		-----o-----		C-172A N7511T
PP-ASEL,I 			    " "			Glider student