The new Avionics Modernization Program (AMP) systems installed in the F-111E and EF-111A have raised their share of questions, so I have decided to continue my series of “Everything You Always Wanted To Know” handouts to you pilots, navigators, and maintenance technicians on how the cotton-picken’ thing works. This informational pamphlet is an overview of the GPS system as a whole, NOT the system- specific hardware that you find in your respective aircraft. I’ll cover the basic theory of operation here, and if there proves to be sufficient interest in platform-specific installation, that will be covered in a later supplement. THE BASICS GPS works by triangulation, the process of finding where you are by the angle to fixed known points. In the old method of DME position determination, you would tune one DME channel and draw a circle on your chart around the DME transmitter, the radius of which was your DME reading in nautical miles. Then you’d tune in a second DME station and repeat the process. On your chart at this point there would be two circles whose lines intersected at two points. Even a vague guess of your whereabouts would be enough to discard the bogus point, and you’d be left with a pretty good idea of your position.
Better yet, take a cut from a third DME transmitter and draw a third circle on your chart. Now you’d have three intersecting circles and your position would be inside the little triangle formed by the intersection of the three circles. Got the picture? This is basically how GPS triangulates, except that instead of circles, we’re dealing with intersecting spheres. TIMING IS EVERYTHING Think of GPS satellites as floating DME stations. They move along in orbit and that complicates things but forget about that for the moment. How can we measure distance? The satellites in the GPS are some 10,900 miles up, but they’re not geostationary (they’d have to be much higher and thus would require more power to reach earthbound GPS receivers) and they travel along at a ground speed of about five miles a second. Like DME, GPS measures the time that it takes the signal to reach the receiver. However, unlike DME, the receiver doesn’t have the benefit of a returning pulse from an interrogation to act as a baseline. It relies purely on one-way timing. You can see right away how it begins to get complicated. The speed of microwave communication is roughly the speed of light, and from 10,900 miles up, any pulse from the GPS takes about 1/17 (0.059) of a second to reach us. The math is simple enough. All we need to know is exactly when the signal left the satellite. And I do mean exactly. An error of a mere .001 of a second would trash the fix by a factor of 180 miles or so. 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).
The Term Paper on The GPS Technology
Throughout time people have developed a variety of ways to figure out their position on earth and to navigate from one place to another. Early mariners relied on angular measurements to celestial bodies like sun and stars to calculate their location. The 1920s witnessed the introduction of more advanced technique-radio navigation-based at first on radios that allowed navigators to locate the ...
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.
The Essay on Heald College Dress Code
Heald College in Fresno, California has rules that change and get stricter every quarter. Dress code is one of those rules that are constantly changing at the Fresno’s campuses. Students are required to dress professional Monday through Thursday, and Friday through Saturday is allowed to wear a Heald t-shirt, jeans, and sneakers. Before the rules were changed, students were allowed to wear a Heald ...
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 signal left the satellite. Just having a clock set to satellite time isn’t enough. The receiver 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).
The Term Paper on Global Positioning System (GPS)
... Earth. Each satellite continually transmits a microwave radio signal composed of two carriers, two codes, and a navigation message. When a GPS receiver is switched ... tracking the GPS satellites in order to determine and predict satellite locations, system integrity, behavior of the satellite atomic clocks, atmospheric data, the satellite almanac, and ...
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 large area.
The Essay on gps navigation
GPS: The Future of Navigation and Technology As we enter the 21st century, we are constantly being bombarded with new technologies. From the wireless community to operations that once took weeks to recover and now only take a day or so, our world will never be the same. Another technology that is revolutionizing the world we live in is the Global Positioning System or GPS. The first GPS satellite ...
It’s so low-powered that it’s completely hidden in the RF background hash of cosmic rays, car ignitions, neon lighting, computer drive fuzz and so forth. That’s where random code comes in. The receiver starts generating its own code and listening for matches in the background noise. Once it has enough matches to recognize the satellite’s transmission, it drags the signal out of the background muck and locks onto the signal using Automatic Gain Control (AGC) circuits. When three satellites are locked up, navigation can begin. This is why a receiver can get by with a very small, relatively non-directional antenna. The new handheld GPS units have antennas that are only a couple of inches square or perhaps the size of a cigar. Not coincidentally, pseudo-random code and low power makes the GPS system very hard to jam. For military purposes, this is obviously very desirable. A BIG SYSTEM That’s the theory, and it works. In fact, it works very well indeed. But it takes a whole lot of effort and money to keep it working. The GPS system consists of three major parts: the user segment (that’s us), the ground or control segment (the DOD geeks who run the thing) and the space segment.
The space segment will eventually be composed of 21 satellites, with three in-orbit spares. Right now, as this is being written in August of 1992, there are 19 satellites in orbit, 18 of which are usable. Three more are due for launch by the end of the year. The satellites are now being launched by Delta II expendable rockets. At one time, the Shuttle was supposed to launch GPS satellites but that plan went down 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).
The Essay on Auto Data System Cost Inventory
XYZ QUICK LUBE Co. The purpose of this exercise is to design a single user system, to solve the needs of a startup business. The business I chose is a service oriented, automotive oil change company. This system will need to satisfy the business needs of a small company, including database queries, inventory management, customer tracking, form generating, accounting, and many other functions. This ...
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.
The Essay on Gps 3
... any satellite. Ephemeris data is the precise satellite positioning information that is used by the GPS receiver to compute its position. Each satellite transmits its own ephemeris data. It ... 'amplify' the GPS signal. As well as the GPS signal containing a pseudo random code, every satellite also transmits almanac and ephemeris data. The almanac data is general ...
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
