Реферат: Gps Essay Research Paper The new Avionics

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