Реферат: Gps Essay Research Paper The new Avionics

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 the year. The satellites are now being launched by Delta II expendable rockets. At one

time, the Shuttle was supposed to launch GPS satellites but that plan went down with the

Challenger. There have been no successful launches of GPS satellites by the Shuttle and probably

never will be. You’ll hear the term “Block I” and “Block II” used to describe satellites. Block Is

were the initial R and D birds and a few (four or five) are still ope! rating. They’re smaller than the

Block II production satellites and don’t have the same amount of military spook stuff aboard.

They’re also not selective-availability equipped. The full constellation of GPS satellites is due to

be in orbit by mid-1993, if all goes according to plan. (No guarantee there). Until then, the

system is deemed by the Defense Department to be under construction. THE BOYS IN BLUE

The U.S. Air Force’s 2nd Satellite Operations Squadron at Falcon AFB in Colorado maintains

the GPS system. These guys are the ground segment. They have monitoring stations at several

points on the globe, from which they keep track of satellite health, maintenance and so forth.

Make no mistake about it, GPS is a high-maintenance system. The satellites require regular

tweaking including data uploads, orbital positioning adjustments and clock maintenance. If the

ground segment stopped doing this constant maintenance, it’s said that the system would