From: mikec@tabasco.lcs.mit.edu (Mike Ciholas)
Subject: Re: DGPS performance (was GPS -- A Potentially Dangerous Technology)
Organization: MIT Laboratory for Computer Science
Date: Wed, 15 Apr 1992 19:27:00 GMT

In article <65423@apple.Apple.COM> stadler@Apple.COM (Andy Stadler) writes:
>In article <1992Apr13.155425.5586@bronze.ucs.indiana.edu>
> greg@bronze.ucs.indiana.edu (Gregory R. TRAVIS) writes:
>> [deleted list of good GPS uses]
>
>Another consideration, though, is expense.  MLS, like ILS, is a one-per-
>runway kind of installation.  Like VOR, however, D-GPS allows a single ground
>station to provide multiple non-precision approaches to multiple runways AND
>multiple airports.  

You really don't need a DGPS station for non-precision approaches.
The accuracies (50-70 meters horizontal, 100 meters vertical) are
sufficient, especially if you use good old air pressure for altitude.

>Through carefully chosen siting, a single D-GPS station, on a hill in sight
>of multiple airports, could provide approaches to all of them.  You can even
>"tune" the minimums by allowing lower approaches at the airports closest to
>the D-GPS, gradually reducing the quality as distance increases between the
>station and the runway.
>--Andy    stadler@apple.com

Three comments:

1. DGPS corrections apply per *satelite* and have meaning to *all* GPS
receivers that can hear that satelite (about 40% of the earth's
surface at any one time).  How much meaning that correction factor has
depends on several factors:

	A. Ionospheric activity
	B. Method of implementing SA (slective availability)

The ionosphere introduces a variable delay in the GPS signals that
varies about 10-15 meters.  Some of the ionosphere variation can be
predicted by time of day, latitude, and time of year.  This removes
about half of the variation, leaving about 5-7 meters left.  The
remaining factors are removed perfectly for GPS receivers located
close to the DGPS station since they are receiving signals that have
traveled through the same ionosphere.  At further distances, the
ionosphere starts to change.  In general, the ionosphere has
variations over several 100km or so and varies little within 50km.
Thus, a DGPS correction factor would have meaning over a fairly wide
area.

The method of implementing SA (the purposeful degrading of accuracy)
can also be a factor in the range of DGPS value.  If SA is implemented
by skewing the clock in the satelite (that is, the satelite lies about
when it transmitted its signal) then the DGPS correction value will
properly correct this time value and the factors will apply everywhere
(since the time distortion applies everywhere).  If, on the other
hand, the satelite skews its position data (the satelite lies about
where it is) then the DGPS factors will gradually have less value the
further from the DGPS station you get.  The reason is that as you get
further from the DGPS station, your realtive geometry to the satelite
changes, and thus the position corrections "look different" than the
DGPS station would see.

I believe the DoD uses the time skew method now, but will or has plans
to switch to a time and position skew combination.

2. The other comment is that, while we would certainly like to have
one station for a wide area, guaranteeing radio visibilty to all
approaches within an area is hard.  If we have line of sight RF
properties, then the transmitting antenna for the DGPS station must be
mounted on a high tower.  Television towers are obvious places.  You
could also design approaches that have MDA at the point where loss of
contact with the DGPS station is imminent.  Since DGPS really only
helps a great deal when you want MDA below 500 feet, this might be a
pretty small region where MDA is based upon DGPS reception.

3. On the expense issue, I can't positively say since I haven't built
a DGPS station!  But, the calculation of DGPS correction factors is
quite trivial once you have calculated your range to the satelite (its
pretty much just the difference) and since every GPS receiver
calculates the satelite ranges, then it does not take extraodinary
cost to do this.  In addition, you need only a simple low power
omnidirectional antenna to broadcast this data.  We need about
10kilobits per second of correction factors, so data rate isn't a real
bear.  A true FAA approved system would need a second monitoring
station that would receive the DGPS correction and listen to the GPS
satelites.  If the corrected position differs from the calculated one,
then it shuts down the DGPS emitter and the airport loses that
approach.  I'd make a comfortable living selling DGPS stations (with
monitor and emergency backup even) for less than $25K.  Tell me how
much MLS costs.

DGPS will easily allow ILS Cat I approaches.  Take a Magnavox GPS
engine which has 1.5 meter range measurement noise (and costs about
$700 for the board).  A full constellation of satelites would have a
geometry factor (called PDOP, a measurement of how badly the satelites
are arranged) of about 4 (meany 4 times worse, see note), that would
give less than 10 meters inaccuracy.  Certainly this is good enough
for horizontal positioning (it is about 20% of runway width) and
altitude to within 10 meters is pretty good at 200 feet MDA, but you
could always use the air pressure if so desired.

Note about PDOP:  Dr. Tom Herring has a simulation of PDOP he showed
me for when the constellation is complete.  It showed PDOP values from
1.01 (1 is prefect) to about 1.5.  I think 4 is quite an over estimate.

Mike Ciholas
mikec@lcs.mit.edu