Реферат: Gps Essay Research Paper The new Avionics

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