The new Avionics Modernization Program (AMP) systems installed in the F-111E and EF-111A have
raised their share of questions, so I have decided to continue my series of "Everything You Always Wanted
To Know" handouts to you pilots, navigators, and maintenance technicians on how the cotton-picken' thing
works. This informational pamphlet is an overview of the GPS system as a whole, NOT the system-
specific hardware that you find in your respective aircraft. I'll cover the basic theory of operation here, and
if there proves to be sufficient interest in platform-specific installation, that will be covered in a later

GPS works by triangulation, the process of finding where you are by the angle to fixed known points. In
the old method of DME position determination, you would tune one DME channel and draw a circle on
your chart around the DME transmitter, the radius of which was your DME reading in nautical miles. Then
you'd tune in a second DME station and repeat the process. On your chart at this point there would be two
circles whose lines intersected at two points. Even a vague guess of your whereabouts would be enough to
discard the bogus point, and you'd be left with a pretty good idea of your position. Better yet, take a cut
from a third DME transmitter and draw a third circle on your chart. Now you'd have three intersecting
circles and your position would be inside the little triangle formed by the intersection of the three circles.
Got the picture? This is basically how GPS triangulates, except that instead of circles, we're dealing with
intersecting spheres.

Think of GPS satellites as floating DME stations. They move along in orbit and that complicates things but
forget about that for the moment. How can we measure distance?

The satellites in the GPS are some 10,900 miles up, but they're not geostationary (they'd have to be much
higher and thus would require more power to reach earthbound GPS receivers) and they travel along at a
ground speed of about five miles a second. Like DME, GPS measures the time that it takes the signal to
reach the receiver. However, unlike DME, the receiver doesn't have the benefit of a returning pulse from an
interrogation to act as a baseline. It relies purely on one-way timing. You can see right away how it begins
to get complicated. The speed of microwave communication is roughly the speed of light, and from 10,900
miles up, any pulse from the GPS takes about 1/17 (0.059) of a second to reach us. The math is simple
enough. All we need to know is exactly when the signal left the satellite. And I do mean exactly. An error
of a mere .001 of a second would trash the fix by a factor of 180 miles or so. Obviously, very accurate
clocks are required.

Each satellite carries four atomic clocks internally, each of which uses the oscillation of cesium and
rubidium atoms to keep extremely accurate time, accurate to within one second over more than 30,000
years. (For you graduates of the USAF Academy, that's one part in 1013, or one part in
10,000,000,000,000). All satellites in the system are synchronized at exactly the same time and they are
kept within 176 nanoseconds of the Universal Time Code (UTC), plus accumulated jump seconds to
account for things like solar time. Navigation messages from the satellites announce the difference
between GPS time and UTC, providing self-recalibration of the clocks.

Okay, we have accurate clocks in the satellites. Now all we need are accurate clocks in our GPS receivers,
synch 'em up and we're in business. Of course, if your el cheapo K-Mart GPS receiver had a cesium clock,
it'd cost about $200,000 and be about the size of a desktop computer. The way around that was to develop
internal receiver clocks that are consistently accurate over relatively short periods of time, as long as they're
reset often enough to keep them synched. Here's how the receiver clocks are reset: Remember how we
explained that DME business, with three intersecting circles? Well, GPS does the same thing, only it uses
three intersecting spheres to determine position. Let's for a moment assume that the