Gps Essay

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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 supplement. THE BASICS 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.

TIMING IS EVERYTHING 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. DO YOU HAVE THE EXACT TIME? 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 receiver clock and satellite clock are

exactly in synch. The receiver times the signal, figures the distance from three satellites and where

the three spheres intersect...voila...that's our position. But, the receiver doesn't know for sure that

its clock is perfectly sy! nched up with the satellites. Remember, a lousy millionth of a second

translates to a thousand foot error. So, just to be sure, the receiver listens for a fourth satellite. If

the fourth line of position doesn't pass through the other three, the receiver knows something is

wrong, as it's geometrically impossible for four mutually intersecting spheres to merge at the same

point unless the clocking is perfect. The receiver assumes, then, that because the fourth line

doesn't jive with the others, its internal clock must be out of synch. The receiver then runs a

simple little software routine to adjust the clock until all four lines of position intersect the same

point. This is known as correcting clock bias and it's how the receiver resets its clock. That's one

of the things going on when your GPS receiver has just been turned on and you're waiting for it to

initialize. RUNNING HOT AND CODE So much for the clock synching. Pretty clever, eh? It

gets better. We said that in order to measure distance, the receiver has to know exactly when the

signal left the satellite. Just having a clock set to satellite time isn't enough. The receiver

determines range using something called pseudo-random code. Think of the code as looking like

the teeth on a carpenter's saw, with a few broken off at random points. Each satellite transmits its

own random code. The receiver has a code generator pre-programmed to generate the exact

same codes (in 32 variations). When the receiver detects a satellite, it matches up the code, much

like aligning the patterns of broken teeth on two saws. Since it knows that the signal carrying the

code left the satellite at a certain time, all the receiver does is generate its matching code at

exactly the same time, effectively zeroizing the time between actual satellite transmission and

receiver generation. Follow? It then measures how long it takes ! another burst of random code

from the satellite to arrive and converts this time lapse to a distance measurement. It does this for

four satellites and the rest is simple math. Earlier, I said four satellites are necessary, with the

fourth required to synch the clock and three others for lines of position. Actually, if the receiver

operator knows his altitude, he can plug that into the receiver and that serves as one line of

position. Then, only two other satellite ranges are required to determine position. The third

satellite is used to synch the clock. This is known as two-dimensional navigation. If a lock is only

available from one or two satellites, there is no GPS fix. From three satellites, a two-dimensional

fix is possible. With four satellites being received, three-dimensional fixes are calculated. I should

mention here that as far as the GPS system is concerned, the presence of the earth is incidental;

position is given in relation to the orbiting sphere of satelli! tes and then converted to latitude,

longitude, and altitude. HELLO, CAN YOU HEAR ME? There's another important reason for

random code; it relates to some basic GPS design limitations. In order to be affordable, GPS

satellites had to be relatively small and light. The Block II production satellites weigh just a little

less than a VW Beetle - about 2000 pounds. That means that power requirements are limited

and the radiated signal power is also quite low, something on the order of 40 watts. Think about

that. There's a 40-watt transmitter floating out there almost 11,000 miles away and it has to

blanket a very large portion of the earth's surface with a receivable signal. For comparison, a

typical communication satellite has much more power and it radiates such a directional signal that

a satellite dish is needed to receive and amplify the signal. For obvious reasons, ships, planes,

cars and other moving vehicles can't have satellite dishes sticking out all over the place. Rather

than directing a high power signal, then, a GPS satellite spreads a very low power! signal over a

large area. It's so low-powered that it's completely hidden in the RF background hash of cosmic

rays, car ignitions, neon lighting, computer drive fuzz and so forth. That's where random code

comes in. The receiver starts generating its own code and listening for matches in the background

noise. Once it has enough matches to recognize the satellite's transmission, it drags the signal out

of the background muck and locks onto the signal using Automatic Gain Control (AGC) circuits.

When three satellites are locked up, navigation can begin. This is why a receiver can get by with a

very small, relatively non-directional antenna. The new handheld GPS units have antennas that are

only a couple of inches square or perhaps the size of a cigar. Not coincidentally, pseudo-random

code and low power makes the GPS system very hard to jam. For military purposes, this is

obviously very desirable. A BIG SYSTEM That's the theory, and it works. In fact, it works very

well indeed. But it takes a whole lot of effort and money to keep it working. The GPS system

consists of three major parts: the user segment (that's us), the ground or control segment (the

DOD geeks who run the thing) and the space segment. The space segment will eventually be

composed of 21 satellites, with three in-orbit spares. Right now, as this is being written in August

of 1992, there are 19 satellites in orbit, 18 of which are usable. Three more are due for launch by

the end of...

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