Global positioning system

Editor's Notes

• #10: http://www.gps.gov/systems/gps/space/ The GPS space segment consists of a constellation of satellites transmitting radio signals to users. GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km. Each satellite circles the Earth twice a day http://en.wikipedia.org/wiki/Global_Positioning_System#Space_segment As of December 2012,[65] there are 32 satellites in the GPS constellation.
• #11: http://www.gps.gov/systems/gps/control/
• #12: http://en.wikipedia.org/wiki/Global_Positioning_System#User_segment
• #15: How a Receiver Determines Its Position Traveling at the speed of light each satellite PRN signal takes a brief, but measurable amount of time to reach a GPS receiver. The difference between when the signal is sent and the time it is received, multiplied by the speed of light, enables a GPS receiver to accurately calculate the distance between it and each satellite, provided that several factors are met. Those factors are: Good satellite signal lock by the GPS receiver (already covered) A minimum of four satellite signals (discussed next) Good satellite geometry (discussed later) When a GPS receiver is turned on it immediately begins searching the sky for satellite signals. If the receiver already has a curent almanac (such as one acquired on a previous outing), it speeds up the process of locating the first satellite signal. Eventually it locates and acquires its first signal. Reading this signal the receiver collects the Navigation Message. If the receiver does not have a current almanac, or was moved more than 300 miles while turned off, it must collect a new almanac, which will take about 12-13 minutes after the first satellite signal is acquired. Why the need for a new almanac if the receiver is moved more than 300 miles while turned off? Beyond 300 miles from its last used location the receiver is presumed to be using different GPS satellites, and therefore should download a new almanac to reflect the new PRN codes. If the receiver is turned on and collecting satellite signals while moving over 300 miles, its almanac is automatically updated. In the above graphic, the GPS receiver calculates a rough location somewhere on this three dimensional sphere, which is actually thousands of miles in diameter. All the receiver can really do at this point is collect system data and search for more satellites.
• #16: How a Receiver Determines Its Position (cont.) In a perfect world, where both satellite and receiver clocks were perfectly synchronized with each other, an accurate position could be determined from just two satellites. However, most receivers are incapable of calculating an accurate position using just two satellites. The dot in the example represents the approximate location of where the receiver thinks it is based on the information provided by two satellites. At least now the receiver knows that it is somewhere at the intersection of those two satellite signals. But that’s the only improvement in its position calculations. The satellite signal spheres should intersect at precisely the receiver’s location, but don’t because the clock in the GPS receiver isn’t yet synchronized with GPS Time. So the receiver estimates a “pseudo-range” to each satellite.
• #17: How a Receiver Determines Its Position (cont.) Three satellites can provide only a two-dimensional (2D) position. Without manually entering the receiver’s exact elevation (most GPS receivers don’t allow elevation to be entered manually), the rendered 2D position may be off by several kilometers on the ground. If the exact elevation of the GPS receiver is known, entering that elevation into a receiver with this capability replaces the need for a fourth satellite signal to allow a receiver to triangulate a precise position. The receiver essentially uses elevation in lieu of a fourth satellite, and makes the appropriate adjustments to trilaterate a reasonably good 3D position. But without manual elevation correction most GPS receivers must rely on a fourth satellite to provide the final clock correction information necessary to calculate a 3D position. Until a fourth satellite signal is acquired the receiver will not be able to determine x and y horizontal, and z vertical positioning (a true 3D position). This is because the fourth satellite signal is used by the receiver not to provide more position data, but, rather, the final time correction factor in its ranging calculations. As a rule, 2D positions should always be avoided whenever possible. Use 2D positioning only when a 3D position is not possible, but be aware of the horizontal error inherent in any 2D position. The inability of a GPS receiver to triangulate a 3D position may be due to a variety of factors, including user error, poor satellite geometry, and harsh landscape conditions (tall buildings, canyons, and dense tree cover among others). As will be shown later in the course, all GPS receivers provide some means for informing the user which mode they are operating in. It’s up to the user to be aware of the errors associated with 2D positioning.
• #18: How a Receiver Determines Its Position (cont.) Unfortunately, accessing only two or three satellite signals, the clock in the GPS receiver cannot yet be synchronized precisely with GPS Time. The pseudo-range spheres (the diagram here shows only two satellites for simplification), as interpreted by the GPS receiver, will either be just a little too large (if the receiver’s clock is running faster than GPS Time) or too small (if the receiver’s clock is slower than GPS Time). The spheres will not intersect with each other. In this example, the “do not” could be the false pseudo-range position if the GPS receiver’s clock is running faster than GPS Time, or the dot is the position if the receiver’s clock is slower than GPS Time. For the purpose of this example, we’ll pretend that the receiver’s clock is running a little fast, so the dot is the true location.
• #19: How a receiver determines its position (cont.) For a GPS receiver to achieve three-dimensional (3D) positioning it needs to acquire four or more satellite signals. A 3D position is comprised of X and Y (horizontal), Z (vertical) positions, and precise time (not varying more than a few hundred nanoseconds). The receiver’s processor uses the fourth satellite pseudo-range as a timing cross check to estimate the discrepancy in its own ranging measurements and calculate the amount of time offset needed to bring its own clock in line with GPS Time (recall the radio station and record player simultaneously playing the same song). Since any offset from GPS Time will affect all its measurements, the receiver uses a few simple algebraic calculations to come up with a single correction factor that it can add or subtract from all its timing measurements that will cause all the satellite spheres to intersect at a single point (x, y, and z). That time correction synchronizes the receiver's clock with GPS Time. Now the receiver essentially has atomic clock accuracy with the time correction factor needed to achieve precise 3D positioning. The pseudo-ranges calculated by the GPS receiver will correspond to the four pseudo-range spheres surrounding the satellites, causing the four spheres to intersect at precisely the receiver’s location (the dot in the diagram).
• #21: Sources of Signal Interference (cont.) Selective Availability (see previous slide). Control Segment blunders due to computer glitches or human error can cause position errors from several meters to hundreds of kilometers. Checks and balances by the Air Force Space Command virtually eliminates any blunders in the Control and Space segments of the Global Positioning System. User mistakes account for most GPS errors on the ground. Incorrect datum and typographic errors when inputting coordinates into a GPS receiver can result in errors up to many kilometers. Unknowingly relying on a 2D position instead of a 3D position can also result in substantial errors on the ground. A GPS receiver has no way to identify and correct user mistakes. Even the human body can cause signal interference. Holding a GPS receiver close to the body can block some satellite signals and hinder accurate positioning. If a GPS receiver must be hand held without benefit of an external antenna, facing to the south can help to alleviate signal blockage caused by the body because the majority of GPS satellites are oriented more in the earth's southern hemisphere. Errors in GPS are cumulative, and are compounded by position dilution of precision (PDOP) (covered later). It is the user’s responsibility to insure the accuracy of the data being collected with the GPS.
• #22: [R]: Features that makes the GPS so popular are: One, and the most important. It is free. It can precisely predict the location. GPS never fails, we can rely on it. It can work in all weather. No matter we are in the middle of sea or in the desert, it’ll work anytime, anywhere. It can handle unlimited number of users at a time, without any problem.
• #23: Attack – jmming sig. or altering iran capture US drone
• #24: [D]: Now, where GPS can be used. It’s applications. It can be used in numerous places, like Agriculture Timing Government uses it Surveying and Mapping It has number of application in space Road, Highways for navigation Can be helpful in Disaster Control Used in railways to prevent clash between trains For recreational activities like, hiking, trekking In marine industry, And lastly, in aviation industry.
• #25: [R]: And. That’s all we have for you as of now. [D]: Thank you, have a good day ahead.