Реферат: Gps Essay Research Paper The new Avionics

and so on streaming down from space en route from one global place to another. The GPS data

stream, on the other hand, is just the opposite; very little information spread out over a wide,

non- directional signal. If satellite signals were soup, a communications satellite would be a rich,

thick minestrone, while the GPS would be chicken broth, and a pretty thin one at that. The GPS

data stream trickles down from each satellite in 1500-bit frames, each co! mposed of five

subframes 300 bits long. Subframes 4 and 5 are subcommutated 25 times each, which is a fancy

way of saying that to get a complete data message requires that 25 full frames be sent. A full

1500-bit frame takes 30 seconds to send. Do the math here and you’ll realize that the GPS data

rate is slower than slow – it’s 50 (yes, fifty) baud. If your computer downloaded this article at 50

baud, it would take about six hours. You could read the damn thing c-h-a-r-a-c-t-e-r by

c-h-a-r-a-c-t-e-r. The data subframes contain various housekeeping information. Subframes 1,

2, and 3 contain time and date information, user range accuracy, satellite health status messages,

clock correction, ephemeris data and some other odds and ends. Subframes 4 and 5 contain the

almanac, which, as we noted, is the location in space of all of the satellites in the GPS

constellation. It’s a fair amount of data and that’s why it’s subcommutated. If it weren’t and the

almanac were transmitted conti! nuously until complete, a GPS receiver would take about 12

minutes to initialize, every time you turned it on. Oh… and no navigating while you’re waiting.

WHAT’S IT DOING? So you hop into your newly-revamped USAF jet, turn on the GPS and…

it just appears to sit there. You read the manuals and learn that it needs a current almanac if one

wasn’t downloaded within the past nine months or if the receiver was moved more than a 1000

miles without having been initialized. What’s it doing? Well, for one thing, it’s looking for a satellite

so it can grab an almanac, which it must have in order to find the three or four satellites it needs to

fix a position. If the receiver is “dumb” and has no almanac at all or an outdated almanac, it’ll take

12.5 minutes to download. Why? Well, remember, the almanac is in subframes 4 and 5, each of

which takes 6 seconds to send. Because there are five subframes, though, the almanac is coming

through only 2/5ths (40%) of the time. It takes 25 full data frames to get a full almanac. Each full

frame takes 30 seconds, so 25 frames takes 12.5 minutes, which is why your manual gives 12.5

minutes as the download time. In cas! e you’re wondering, here’s what an almanac (or at least a

portion of one) looks like: Epoch: 48871.0000 MJD (almanac reference time 9-6-1992 0h