GLOBAL POSITIONING SYSTEMS. Knowing where and when

Transcription

1 GLOBAL POSITIONING SYSTEMS Knowing where and when

2 Overview Continuous position fixes Worldwide coverage Latitude/Longitude/Height Centimeter accuracy Accurate time

3 Feasibility studies begun in 1960 s. Pentagon appropriates funding in First satellite launched in System declared fully operational in April, The History of GPS (or, Global Navigation Satellite Systems GNSS)

4 Location - determining a basic position Navigation - getting from one location to another Tracking - monitoring the movement of people and things Mapping creating and updating maps/gis Timing - bringing precise timing to anywhere in the world Typical GPS Applications

5 Global Positioning Systems The default Global Positioning System is the US NavStar system, developed by the US Department of Defence. The first satellites were sent into orbit in 1978, although GPS as we currently recognize it wasn t available until after The Russian Federal Space Agency developed their own system, GLONASS, around the same time as the US system. (here) China has its own system BeiDou in 2000, and currently is establishing a NavStar-like system (by 2020). (here) Europe is also developing its own system Galileo started in 2011, with plans to complete the system by (here) India (IRNSS) and Japan (QZSS) also have regional GPS in place.

6 GPS Segments (or components) Space Segment: satellite constellation (24+ active satellites in space). Control Segment: ground stations located on earth (originally 5, now up to 16). User Segment: GPS receiver units that receive satellite signals and determine receiver location from them.

7 Space segment User segment Control segment

8 Space segment is the satellite constellation. 31 satellites with a minimum of 24 operating 98% of the time (22 additional satellites have died over the years) (currently, 27 are active in the constellation in order to provide more accurate fixings) 6 orbital planes (inclination ~55 ) Circular orbits ~20,000 km above the Earth's surface 11 hours 58 minute orbital period Visible for approximately 5 hours above the horizon Space Segment

9 Falcon AFB Colorado Springs, CO Master Control Monitor Station Hawaii Monitor Station Ascension Island Monitor Station Diego Garcia Monitor Station Kwajalein Monitor Station Control Segment: Ground Monitoring Stations

10 Orbits of GPS satellites need to be continually updated because they do not stay circular without adjustments. Adjustments are required because: Other objects exert gravitational force on each satellite (e.g., sun, moon). The effect of gravity is non-uniform (geoide). Radiation pressure (due to solar radiation). Atmospheric drag. Other effects. GPS Satellite Orbits

11 User segment: the receivers that have been designed to decode signals transmitted from satellites for purposes of determining position, velocity or time. Receiver must perform the following tasks: select one or more satellites in view recover navigational data acquire GPS signals measure and track signal Third Component: User Segment

12 Military Search and rescue Disaster relief Surveying, geophysics (continental drift) Marine, aeronautical and terrestrial navigation Remote controlled vehicle and robot guidance Satellite positioning and tracking Shipping Geographic Information Systems (GIS) Recreation (e.g., geocaching) User Segment

13 Satellites transmit Ephemeris and Almanac data obtained from the monitoring stations to the GPS receivers. Ephemeris data contains important information about the status of the satellite (healthy or unhealthy), the current date and time. This part of the signal is essential for determining a position. Almanac data tells GPS receiver where each GPS satellite should be at any time throughout day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system. Important Concepts

14 The fundamental principle behind the GPSystem is that time can be used to determine position. Therefore, the most important element of the GPSystem is determining the Time Of Arrival (TOA) of the signal in the GPS unit. Determining position

15 GPS use the concept of time of arrival (TOA) of signals to determine user position. This involves measuring the time it takes for a signal transmitted by an emitter (satellite) at a known location to reach a user receiver. Time interval (signal propagation time) is multiplied by the speed of the signal (speed of light) to obtain satellite-to-receiver distance. TOA Concept Pseudo Random Number Code [PRNC}

16 Signal leaves satellite at time T Signal is picked up by the receiver at time T + 3 T T + 3 Distance between satellite and receiver = 3 times the speed of light

17 In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. Accurate time measurements are required. If it is off by a thousandth of a second, at the speed of light, that translates into almost 200 miles of error. Synchronizing Clocks

18 To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in each receiver. But atomic clocks cost somewhere between $50,000 and $100,000, which makes them too expensive for everyday consumer use. The scientists behind the GPSystem developed a clever solution to this problem. Every satellite contains an expensive atomic clock, but each receiver uses an ordinary quartz clock, which is constantly reset. Synchronizing Clocks

19 In a nutshell, the receiver looks at incoming signals from three or more satellites and gauges its own inaccuracy (which = determining its exact position). The actual process of determining the position is called trilateration D?C B Synchronizing Clocks E

20 When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect. Synchronizing Clocks

21 - One time measurement narrows down our position to the surface of a sphere - Second time measurement narrows it down to intersection of two spheres - Third time measurement narrows it to just two points Trilateration

22 - Fourth time measurement will decide between the two points Fourth measurement will only go through one of the two points In certain situations, 3 measurements are enough (2 dimensional position: latitude and longitude; e.g., when on a boat). One point will be obviously incorrect: out in space or moving at high speed. A fourth measurement will provide a 3 dimensional position: latitude, longitude and elevation Trilateration

23 However, because the GPS unit s clock is off, even with 4 satellites you will not initially identify a single point. Showing only 2 spheres for clarity, but the principle is the same for 4. Determining the clock correction Clock error

24 The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites. Synchronizing Clocks

25 In order to properly synchronize clocks and figure out which PRNC signal it is listening to, the receiver has to know where the satellites actually are. This isn't particularly difficult because the satellites travel in very high and predictable orbits. Knowing Satellite Locations Pseudo Random Number Code [PRNC}

26 The GPS receiver stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly. However, the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals (ephemeris). Using Almanac Information

27 Ephemeris

28 Errors can be categorized as intentional and unintentional. Intentional errors: government can and does degrade accuracy of GPS measurements. This is done to prevent hostile forces from using GPS to full accuracy. Policy of inserting inaccuracies in GPS signals is called Selective Ability (SA). SA was single biggest source of inaccuracy in GPS. SA was deactivated in 2000 (but could be turned back on should the US military decide to do so, although the newer NavStar satellites have been designed without SA). Two Types of Errors

29 Sources of Unintentional Timing Errors

30 Ionosphere (a band of charged particles) Troposphere (our weather) Atmospheric interactions

31 Earth s Atmosphere Solid Structures Metal Multipath signals Electro-magnetic Fields

32 Source of Error Typical Error in Meters (per satellite) Satellite Clocks 1.5 Orbit Errors 2.5 Ionosphere 5.0 Troposphere 0.5 Receiver Noise 0.3 Multipath 0.6 SA 30 Errors are cumulative and increased by PDOP. Typical Errors

33 System and other flaws = < 9 meters User error = ± 1 km Receiver errors are cumulative

34 Positional Dilution of Precision (PDOP) Horizontal DOP Vertical DOP Relative positions of satellites can affect error and reduce accuracy standards N Y Data quality parameters

35 Good PDOP 4 Bad PDOP 8 PDOP

36 N W E Ideal Satellite Geometry S

37 Good Satellite Geometry

38 DOP Good Satellite Geometry

39 N W E Poor Satellite Geometry S

40 Poor Satellite Geometry

41 DOP Poor Satellite Geometry

42 A technique called differential correction can yield accuracies within 1-5 meters, or even better, with advanced equipment. Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point. Because physical location of base station is known, a correction factor can be computed by comparing known location with GPS location determined by using satellites. Differential GPS

43 Differential GPS

44 Source Uncorrected With Differential Ionosphere 0-30 meters Mostly Removed Troposphere 0-30 meters All Removed Signal Noise 0-10 meters All Removed Orbit Data 1-5 meters All Removed Clock Drift meters All Removed Multipath 0-1 meters Not Removed Receiver Noise ~1 meter Not Removed SA 0-70 meters All Removed Differential GPS Improvements

45 Geostationary WAAS satellites GPS Constellation WAAS Control Station (West Coast) Local Area Augmentation System (LAAS) Wide Area Augmentation System WAAS Control Station (East Coast)

46 With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.* +-15 meters meters Under ideal conditions a WAAS-equipped GPS receiver can achieve three meter accuracy 95% of the time.* * Precision depends on good satellite geometry, open sky view, and no user induced errors. How good is WAAS?

47 With GPS lat/lon, and its built-in clock, the receiver can give you several pieces of valuable information: o How far you've traveled (odometer) o How long you've been traveling o Your current speed (speedometer) o Your average speed o A "bread crumb" trail showing you exactly where you have traveled on the map o The estimated time of arrival at your destination if you maintain your current speed Using GPS Data

48 Summary GPS has fundamentally changed how location is determined Continuous position fixes Worldwide coverage Latitude/Longitude/Height Centimeter accuracy Accurate time anywhere

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