.. 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 gracefully degrade to complete uselessness in about two weeks time.
So, as each satellite 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 multi-channel communications satellite needs.
There are lots of phone calls, fax bits, video pixels 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 UTC); ID# Type smaxis(km) eccentri inclina rt.ascen arg.peri mean-ano Hlth 02 GP 26560.0520 0.011080 54.9026 342.9035 194.5554 224.6108 0 03 GP 26560.2633 0.013058 64.3151 063.1001 142.6658 053.7576 0 11 GP 26560.3892 0.013453 63.8026 062.4385 231.0716 209.1055 0 12 GP 26560.3892 0.012450 62.7486 299.5745 340.7176 015.4047 0 13 GP 26559.9161 0.004059 63.5554 061.4368 214.5911 099.5112 0 14 GP 26559.7802 0.004146 55.0626 165.4253 067.8533 134.7840 0 15 GP 26559.8959 0.007275 55.1120 106.2742 109.0210 264.1008 0 Got that? Once the receiver’s got it, it can locate other satellites in the sky, download the ephemeris and other data and tell you where you are, within a few feet or so.
CLOSE ONLY COUNTS IN HORSESHOES So how accurate is the GPS? You hear all kinds of incredible claims about the GPS being accurate enough to locate a gnat’s ass while others say it’s only good for about 100 yards, give or take. Which is true? Well, it depends. GPS is generally said to be available in two forms, PPS and SPS. Depending on whose figures you want to believe, PPS or precision positioning service is accurate to about a six-foot CEP (circular error of probability), but this mode is proprietary to the military and is crypto-coded to keep it that way. SPS or standard positioning service is actually capable of the same accuracy, but the Pentagon can invoke something called selective availability (SA), which, in military jargon, can adjust SPS resolution to any degree necessary. SA currently degrades the SPS accuracy to about 100 yards CEP, and the GPS usually delivers on that promise. SA, by the way, is an intentional dithering of the clock accuracy and perhaps a contamination of the ephemeris dat! a. Since the armed forces paid much of the GPS research, development and launch costs, they insisted on having some sort of strictly military function for the system in order to get their money’s worth.
One last note about errors: I mentioned something called a single-frequency receiver. That’s a bit misleading because I didn’t explain that GPS satellites broadcast on two frequencies, called L1 and L2. L1 is at 1575 MHz, L2 is 1227 MHz. Military receivers generally receive both L1 and L2, then compare the results from each to greatly reduce the ionospheric errors affecting GPS signals passing through the atmosphere. Single frequency receivers used by civilian aircraft and maritime traffic use a fixed mathematical model to allow for ionospheric errors.
In the proverbial nutshell, that’s how the Global Positioning System works. It’s a great system now, and with projected improvements should be an integral part of aircraft navigation for the next fifty years or so. Everything You Always Wanted To Know back issues available: Number 1: Airborne Radar Principles Number 2: Interrogation Friend or Foe (IFF) Number 3: Radar Fairings and Radomes Number 4: Electronic Countermeasures Number 5: Tactical Air Navigation (TACAN) Number 6: Missile Guidance Techniques Phone the author on (44) 86923-2222 or on Autovon 263-4612. Comments welcome. Ich speche deutsch Hablo espaol Computers and Internet.