Реферат: Gps Essay Research Paper The new Avionics

Gps Essay, Research Paper

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.

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