From: mikec@tabasco.lcs.mit.edu (Mike Ciholas)
Newsgroups: rec.aviation
Subject: Re: GPS -- A Potentially Dangerous Technology
Keywords: GPS, Navigation, Defense
Date: 8 Apr 92 15:34:02 GMT
Organization: MIT Laboratory for Computer Science

In article <167C14041.M21208@mwvm.mitre.org> M21208@mwvm.mitre.org writes:
>In article <101820007@hpcvra.cv.hp.com>
>randys@hpcvra.cv.hp.com (Randy Stockberger) writes:
>>If the good old USA is in imminent danger of attack the Feds can, and
>>probably will, turn off all aviation and marine navigation aids, foiling
>>the plans of our evil enemies.  This is written up in the AIM, I think.
>>Something to think about when you are solid IMC.
> 
>Nobody's too worried about what's going to IFR flight if there's a major
>attack on the U.S. - whether the next VOR is on the air will probably not
>be a prime concern.  The issue with GPS is that it is not primarily a civil
>system like VORs and NDBs are, and is not under the control of civil users.

The parallels between the present VORTAC system and GPS are striking.
First, nearly all VORs are VORTACs which includes three parts: a VOR
(radio compass designed primarily for civilian users, has magnetic
north), a TACAN (sp?) (radio compass for military uses, has true
north), and DME (distance measuring equipment, both users).  Almost
all VORs are VORTACs (there are some exceptions).  The DoD runs the
VORTACs, so claiming the system is primarily a civilian system isn't
quite true.  The DoD can and will shut off navaids when ever it feels
the need (including the plain VORs as well).

In GPS, there are several modes of operation.  Most are high precision
military modes, but one is dedicated to civilian navigation.  This is
the L1/CA mode which has SA (selective availability) which can degrade
the signal to 50-100 meter accuracy (otherwise it is about 10 meters).

The US published the coding for the L1/CA mode and has agreed to allow
the ICAO (international civil aviation organization, they govern
international flights) to use the GPS system for the next 10 years
without fees (originally, the scramble codes were going to be licensed
and fees collected for its use, maybe this is a 10 year "free trial
subscription" to hook everybody).

If your country doesn't have VORs, or LORAN, then you will certainly
look upon GPS as a major advance.  50% of all european airplanes carry
GPS.  Only 5% of US.

>DOD could decide to shut it off tomorrow, and there isn't a thing that the
>FAA, for example, could do about it.  Ditto for GLONASS.

Ditto for VORs and VORTACs.

>Until the
>availability and reliability of the satellite systems is permanently
>guaranteed, you can understand why the FAA might be a little  reluctant to
>make it the primary means of IFR navigation.

I agree that the system needs to be thoroughly tested.  But, there are
many checks and balances built into the GPS system that other systems
don't have.  For example, on final approach, typically you are
depending on one localizer and one glideslope transmitter.  You can't
really monitor the signals very well since you can't place a receiver
in the glide path!  With LORAN, you may have better redundacy in the
signals, but the variability of the transmission medium will never
allow LORAN to do precision approaches (how many of you have flown a
LORAN approach?)  In GPS, a satelite has to pass many sanity checks
before it can be used (both in a sense that the DoD sets it "usable"
by monitoring it for a while, and in the receiver in the airplane).  A
lot of this effect comes from the 4D nature of the system (lat, long,
alt, and time).  If those numbers don't make sense, then there is a
problem.  In otherwords, it is virtually impossible to generate a
condition that causes a systematic error that leads you to fly off
course a mile.  This would require extraordinary cooperation among the
satelites as they would have to warp their signal in just the right
way to fake out your receiver.  Any place else, the signals just
wouldn't make sense.

I am a little worried that the LORAN approaches will prove to be
pretty bad and that this will be used against GPS (a sort of guilty by
association).  NASA has done GPS instrument landings in a 737.  About
170 approaches were made, about 80 were made to touchdown, about 30
used the GPS to roll out on the runway centerline.  I believe this
work was to support a shuttle GPS landing system under consideration.

Mike Ciholas
mikec@lcs.mit.edu