Реферат: Gps Essay Research Paper The new Avionics

whizzes along and completes one earth orbit every 12 hours, the Boys in Blue from Falcon talk to

it every few hours. Communications are uplinked in S-band at 2227.5 MHz and confirming

messages are downlinked on 1783.1 MHz. What do the ground guys tell the satellites? Well, we

mentioned basic maintenance items, including clock commands, power and! attitude messages,

new programming instructions. Occasionally, the satellite must undergo what’s called a

“momentum dump.” Each satellite has a series of gyroscopic wheels for stabilization. In space,

these wheels tend to accelerate and would do so indefinitely, eventually disintegrating. By

dumping the wheel energy periodically, this unpleasant scenario is avoided. ORBITAL

PERTURBATIONS Most of the uploading relates to routine navigation data, including almanac

and ephemeris information. Probably the most important is the ephemeris, which compensates for

the satellites normal orbital perturbations. As it circles the earth, each satellite is subject to several

major influences which cause its orbit to be less than perfectly circular. The major influence is the

earth’s equatorial bulge, but solar wind and other effects also take a toll. The GPS orbital

perturbations are defined by 16 constants and these are updated and uploaded at least once a

day (maybe more often) along with clock correction data. The satellite then rebroadcasts this and

your receiver decodes it as ephemeris data. The ephemeris tells the receiver exactly where the

satellite is so, when the receiver calculates distance, it’ll know exactly where the source of the

signal is; each satellite broadcasts its own ephemeris data. In addition, each satellite also

broadcasts what’s called an almanac. In! more general terms than does the ephemeris, the

almanac tells the receiver the location of all of the satellites in the GPS constellation. This lets the

receiver know when and where to look for satellites as it’s attempting to establish a fix. Your

receiver stores an almanac in its memory and that data is constantly updated when the receiver is

tracking satellites. If the receiver is turned off for several months, the almanac in memory will

usually remain usable enough for the receiver to find satellites and download a new almanac from

the next passing satellite. BITS, BITS, BITS Of course, all this data I’ve described here has to

find its way through 10,900 miles of space and atmospheric clutter and into your GPS receiver’s

computer memory. This is another one of the GPS’s elegant design features. Remember how we

explained that a communication satellite uses a relatively high powered, directional signal? Such a

signal allows for a rather dense data stream, which, when you think about it, is just what a