GPS in remote sensing,P K MANI

Editor's Notes

• #3: developed by the US Department of Defense as a worldwide navigation resource for military and civilian use.
• #7: The MCS, at Falcon AFB, is responsible for the overall command and control, which includes maintaining the exact orbits of each satellite and determining any timing errors that may be present in the highly accurate atomic clocks. The errors are corrected by Falcon with updated orbits and clock corrections relayed once a day to each satellite via four ground antennas. In the future, more monitor stations will be added to further refine the system. Military customers get better than required accuracy as a result. It is important to note that even though the Air Force operates the constellation of satellites on a day-to-day basis, the overall GPS program is jointly managed. The unit that develops the hardware (both military receivers and the actual satellites) is the GPS Joint Program Office (JPO) at Los Angeles AFB. There are, among others, Army, Navy, DoT, and even DMA Deputy Program Managers.
• #19: For simplicity sake, we calculate our position by understanding where a satellites is in orbit and then further computing how far we are from those satellites. If we know we are a certain distance from satellite A (11,000 miles). This narrows down where we are in the whole universe. This one range tells us we are located somewhere on the sphere of the range of this one satellite. If at the same time we also know that we are 12,000 miles away from a second satellite. This narrows where we are in the universe even more. The only place we can be 11,000 miles from A and 12,000 miles from B is on a circle where those two spheres intersect. Then if we measure to a third satellite we can pinpoint our location. If we know that we are 13,000 miles from satellite C then there are only two points in the universe that this can be true. One point is where the sphere enters into the circle formed by satellites A and B and the second is where the sphere of satellite C exits the circle formed by satellites A and B. By ranging from three satellites we can narrow our location down to one of two points. We decide between these two points typically by one of two methods. In some cases the software built into the GPS receivers have various techniques to determine which point is correct and which point is ridiculous. Technically, trigonometry says we really need four satellites to range to unambiguously locate ourselves. In other words the fourth satellite can be used to determine which of the two points derived from three satellites is the correct choice. The fourth satellite has other functions that relate to time that will be discussed later.
• #22: Since GPS is based on knowing your distance to satellites in orbit, a method must be devised to figure out how far we are from the satellites. The GPS system works by timing how long it takes a radio signal to reach us from a satellite and then calculating the distance from that time. We calculate this by measuring distance or the velocity times travel time. Radio waves travel at the speed of light: 186,000 miles per second. If we can then compute exactly when the signal started at the satellite and ended at the receiver then we have our required distance. We then multiply the time by the velocity to derive the distance.
• #24: Okay, we have accurate clocks in the satellites. Now all we need are accurate clocks in the receivers, sync 'em up and we're in business. Of course, if your discount GPS receiver had to have 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. 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; it's geometrically impossible for four mutually intersecting spheres to merge at the same point unless the clock is spot on. The receiver assumes, then, that because the fourth line doesn't jive with the others, the receiver's internal clock must be out of sync. The receiver then runs a simple little 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 that's going on when your receiver has just been turned on and you're waiting for it to initialize.
• #27: This is a diagram that shows how radio signals work in general. Each SV generates a plain sine wave called the Carrier 1575.42Mhz and 1227.6Mhz. This carrier is Modulated (altered) by adding three signals to it. (The C/A code, P-Code and the Navigation message). In this case, it is that particular SV's Pseudo-Random Noise (PRN) Code this is essentially the P-Code and the C/A code for each SV. The PRN code looks like a square wave because it's just 1's and 0's; digital information that is Phase Modulated (PM). Then what the SV actually broadcasts is that modulated signal. Your receiver picks this up, subtracts the carrier (it can generate a copy of the carrier on its own), and is left with a copy of the original signal. All radios works this way. The significance for GPS is that the signal that is produced in the receiver is delayed by a small amount of time. For GPS, the information in the signal is not the only important part - as we've seen, it's the time delay that we really care about. In fact, GPS receivers already have to have a copy of the signal (most of it anyway) just to pick it up in the first place. Remember, all the SVs are talking on the same frequency, so we can't pick them out of the chatter without knowing what to expect ahead of time. 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 SVs weigh just less than 2,000 pounds. That means that power requirements are limited and the radiated signal power is also quite low, 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. Big problem. For comparison, a typical communication satellite has much more power and it radiates a very directional signal that you need a satellite dish to receive. For obvious reasons, ships, planes, cars and other moving vehicles, can't have dishes. Who wants a plane that looks like a West Virginia sharecropper's double-wide trailer? Besides, they'd blow off in the slipstream. Rather than directing a high power signal, then, a GPS satellite spreads a very low power signal over a large area. It's so low-powered that it's completely hidden in the 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 SV's transmission, it drags the signal out of background muck and "locks on." When three SVs are locked up, navigation can begin. This is why a receiver can get by with a very small, relatively nondirectional antenna. Handheld GPS units have antennas that are only a couple of inches square or perhaps about the size of a cigar. One other thing: using pseudo-random code and low, low power makes it very hard to jam a GPS signal. For military purposes, this is obviously very desirable.
• #30: i.e., a highly accurate measuring device will provide measurements very close to the standard, true or known values.Example: in target shooting a high score indicates the nearness to the bull's eye and is a measure of the shooter's accuracy. Refer to pictures below: It is an indicator of the scatter in the data. The lesser the scatter, higher the precision. The pictures given below clearly describe Accuracy and Precision. Precision is not the same as accuracy. Accuracy is a measure of departure from the true value of a quantity. Precision, on the other hand, is a measure of the "repeatability" of the data. The difference between accuracy and precision is known as "bias" or "systematic error". Taking large amounts of data will improve the precision of a sample mean, but will not remove systematic error. Accuracy is a very desirable measure. However, it is generally quite difficult to obtain. It requires strict control over sources of systematic error
• #42: Although at certain times of the day there may be up to12 satellites visible simultaneously, there are nevertheless occasional periods of degraded satellite coverage (though naturally their frequency and duration will increase if satellites fail). "Degraded satellite coverage" is generally defined in terms of the magnitude of the Dilution of Precision (DOP) factor, a measure of the quality of satellite geometry. The highest the DOP value, the poorer the satellite geometry. There are a variety of DOP indicators, such as GDOP (Geometric DOP), PDOP (Position DOP), HDOP (Horizontal DOP), VDOP (Vertical DOP), etc. PDOP, or Precision Dilution of precision, probably the most commonly used, which is the dilution of precision in three dimensions. Some- times called the Spherical DOP.
• #44: To detect systematic error, one needs reference coordinates; preferably, as error free as possible. A typical source of such control is a monumented point surveyed by a geodetic agency or a state highway department. With "traditional" DGPS, a base station receiver is set up on a location where the coordinates are known. The difference between the known location and the calculated location based on satellite signals is used to determine individual corrections Provided that the exact location of a reference receiver (on a point that has been very accurately surveyed), and the location of satellites in space are known, it is possible to compute a distance between the receiver and each satellite. The reference receiver then divides that distance by the speed of light and gets a time for how long the signals should have taken to reach it. This theoretical time is compared with the time the signals actually took to reach it, and the difference is the error in the satellite’s signal.
• #46: system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.