Localization of mobile devices

1. Localization of mobile devices Xian Zhong March 10, 2003

2. Overview • Introduction • Location Sensor Technologies • Selected Systems • GPS • ORL Ultrasonic Location System - The Cricket Location-Support System CS691

3. Introduction • Background - wide use of sensor networks - Each sensor is self-sufficient to sense its environment, perform simple computation and communicate with its peers and observers -Some sensors are unaware of their position and required to be localized • Context-aware Applications CS691

4. Context Awareness • What is context? - Who - What - When - Where - How • Context-aware applications need to know the location of users and equipment, and the capabilities of the equipment and networking infrastructure CS691

5. What is Location? • Absolute position on geoids • e.g. GPS • Location relative to fixed beacons • e.g. LORAN • Location relative to a starting point • e.g. inertial platforms • Most applications: • location relative to other people or objects, whether moving or stationary, or the location within a building or an area CS691

6. Location Sensor Technologies • Electromagnetic Trackers: • High accuracy and resolution, expensive • Optical Trackers: • Robust, high accuracy and resolution, expensive and mechanical complex • Radio Position Systems (Such as GPS): • Successful in the wide area, but ineffective in buildings, only offer modest location accuracy • Video Image (Such as the MIT Smart Rooms project): • Location information can be derived from analysis of video images, cheap hardware but large computer processing CS691

7. GPS • History When: 1973 start, 1978-1994 test Who & Why: • U.S. Department of Defense wanted the military to have a super precise form of worldwide positioning • Missiles can hit enemy missile silos… but you need to know where you are launching from • US subs needed to know quickly where they were • After $12B, the result was the GPS system! CS691

8. GPS • Approach • “man-made stars" as reference points to calculate positions accurate to a matter of meters • with advanced forms of GPS you can make measurements to better than a centimeter • it's like giving every square meter on the planet a unique address! CS691

9. GPS System Architecture CS691

10. GPS System Architecture • Constellation of 24 NAVSTAR satellites made by Rockwell • Altitude: 10,900 nautical miles • Weight: 1900 lbs (in orbit) • Size:17 ft with solar panels extended • Orbital Period: 12 hours • Orbital Plane: 55 degrees to equitorial plane • Planned Lifespan: 7.5 years CS691

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12. GPS System Architecture • Ground Stations, aka “Control Segment” • The USAF monitor the GPS satellites, checking both their operational health and their exact position in space • the master ground station transmits corrections for the satellite's ephemeris constants and clock offsets back to the satellites themselves • the satellites can then incorporate these updates in the signals they send to GPS receivers. • Five monitor stations • Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs. CS691

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14. GPS Signals in Detail • Carriers • Pseudo-random Codes • two types of pseudo-random code • the C/A (Coarse Acquisition) code • it modulates the L1 carrier • each satellite has a unique pseudo-random code • the C/A code is the basis for civilian GPS use CS691

15. GPS Signals in Detail (contd.) • the P (Precise) code • It repeats on a seven day cycle and modulates both the L1 and L2 carriers at a 10MHz rate • this code is intended for military users and can be encrypted and called "Y" • Navigation message • a low frequency signal added to the L1 codes that gives information about the satellite's orbits, their clock corrections and other system status CS691

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17. How GPS Works • The basis of GPS is “trilateration" from satellites. (popularly but wrongly called “triangulation”) • To “trilaterate," a GPS receiver measures distance using the travel time of radio signals. • To measure travel time, GPS needs very accurate timing which it achieves with some tricks. • Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. • Finally you must correct for any delays the signal experiences as it travels through the atmosphere. CS691

18. Earth-Centered Earth-Fixed X, Y, Z Coordinates CS691

19. Geodetic Coordinates (Latitude, Longitude, Height) CS691

20. Trilateration • GPS receiver measures distances from satellites • Distance from satellite #1 = 11000 miles • we must be on the surface of a sphere of radius 11000 miles, centered at satellite #1 • Distance from satellite #2 = 12000 miles • we are also on the surface of a sphere of radius 12000 miles, centered at satellite #2 • i.e. on the circle where the two spheres intersect CS691

21. Trilateration (contd.) • Distance from satellite #3 = 13000 miles • we are also on the surface of a sphere of radius 13000 miles, centered at satellite #3 • i.e. on the two points where this sphere and the circle intersect • could use a fourth measurement, but usually one of the point is ridiculous (far from earth, or moving with high velocity) and can be rejected • but fourth measurement useful for another reason! CS691

22. Measuring Distances from Satellites • By timing how long it takes for a signal sent from the satellite to arrive at the receiver • we already know the speed of light • Timing problem is tricky • the times are going to be awfully short • need some really precise clocks • on satellite side, atomic clocks provide almost perfectly stable and accurate timing • what about on the receiver side? • atomic clocks too expensive! • Assuming precise clocks, how do we measure travel times? CS691

23. Measuring Travel Times from Satellites • Each satellite transmits a unique pseudo-random code, a copy of which is created in real time in the user-set receiver by the internal electronics • The receiver then gradually time-shifts its internal code until it corresponds to the received code--an event called lock-on. • Once locked on to a satellite, the receiver can determine the exact timing of the received signal in reference to its own internal clock CS691

24. Measuring Travel Times from Satellites (contd.) • If that clock were perfectly synchronized with the satellite's atomic clocks, the distance to each satellite could be determined by subtracting a known transmission time from the calculated receive time • in real GPS receivers, the internal clock is not quite accurate enough • an inaccuracy of a mere microsecond corresponds to a 300-meter error • The clock bias error can be determined by locking on to four satellites, and solving for X, Y, and Z coordinates, and the clock bias error CS691

25. Extra Satellite Measurement to Eliminate Clock Errors • Three perfect measurements can locate a point in 3D • Four imperfect measurements can do the same thing • Pseudo-ranges: measurements that has not been corrected for error • If there is error in receiver clock, the fourth measurement will not intersect with the first three • Receiver looks for a single correction factor that will result in all the four imperfect measurements to intersect at a single point • With the correction factor determined, the receiver can then apply the correction to all measurements from then on. • and from then on its clock is synced to universal time. • this correction process would have to be repeated constantly to make sure the receiver's clocks stay synced • Any decent GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously CS691

26. Where are the Satellites? • For the trilateration to work we not only need to know distance, we also need to know exactly where the satellites are • Each GPS satellite has a very precise orbit, 11000 miles up in space, according to the GPS master Plan • GPS Master Plan • spacing of the satellites are arranged so that a minimum of five satellites are in view from every point on the globe CS691

27. Where are the Satellites (contd.)? • GPS satellite orbits are constantly monitored by the DoD • check for "ephemeris errors" caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites • satellite’s exact position is relayed back to it, and is then included in the timing signal broadcast by it • On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment CS691

28. GPS Technology Status • Standard Positioning Service (SPS): C/A code with SA • Horizontal accuracy of ± 100 m (95%) [30m without SA] • Vertical accuracy of ± 156 m (95%) • UTC time transfer accuracy ± 340 ns (95 %) • Precise Positioning Service (PPS) : P code • Horizontal accuracy of ± 22 m (95%) • Vertical accuracy of ± 27.7 m (95%) • UTC time transfer accuracy ± 200 ns (95 %) CS691

29. GPS Technology Status (contd.) • Differential GPS • Horizontal accuracy of ± 2 m • Vertical accuracy of ± 3 m • Requires a differential base station within 100 km • Real Time Kinematic GPS • Horizontal accuracy of ± 2 cm • Vertical accuracy of ± 3 cm • Requires a differential base station within 10-20 km CS691

30. GPS Technology Status (contd.) • The size and price of GPS receivers is shrinking • World’s smallest commercial GPS receiver (www.u-blox.ch) • Differential GPS receivers are inexpensive ($100-250) • Differential GPS available in all coastal areas • Real Time Kinematic GPS receivers are expensive • GPS needs line-of-sight to satellites • does not work indoors, in urban canyons, forests etc. CS691

31. So, we need indoors location system CS691

32. ORL Ultrasonic Location System • Measurements are made of time-of-flight of sound pulses from an ultrasonic transmitter to receivers placed at known positions around it. • Transmitter-receiver distances can be calculated from the pulse transit times. • A small wireless transmitter is attached to every object that is to be located CS691

33. Distance Calculation • For each receiver, the interval; Tp between the start of the sampling window and the peak signal time represents the sum of several individual periods CS691

34. Position Calculation (4 spheres) CS691

35. Position Calculation (Cont’d) CS691

36. Position Calculation (Cont’d) CS691

37. Position Calculation (Cont’d) • In the ORL system all the receivers lie in the plane of the ceiling, and the transmitters must be below the ceiling. This allows calculation of transmitter positions using only three distances rather than the four required in the general case. • Occasionally, however, the direct path may be blocked, and the first received signal peak will be due to a reflected pulse. In this case, the measured transmitter-receiver distance will be greater than true distance. • The difference between two transmitter-receiver distances cannot be greater than the distance between the receivers. CS691

38. Applications • The teleporting system: Redirect an X-window system environment to different computer displays. We can use location data to present a user’s familiar desktop on a screen that face them whenever they enter a room. • Nearest printer service: offered to users of portable computers. Tags placed on the computer and printers report their positions, and the computer is automatically configured to use the nearest available printer as it is moved around a building. CS691

39. The Cricket Location-Support System (to be contd.) CS691

40. Bibliography • "A New Location Technique for the Active Office", Andy Ward, Alan Jones, Andy Hopper, IEEE Personal Communications, Vol. 4, No.5, October 1997, pp. 42-47. • “Special Issue on Global Positioning System”,Proceedings of the IEEE, Vol.87, NO.1, January 1999 • “The Cricket Location-support System”,Nissanka B. Priyantha, Anit Chakraborty, and HariBalakrishnan, MIT Laboratory for Computer Science, Cambridge, MA 02139 • “The Global Positioning System”, I.A.Getting, IEEE Spectrum, Vol.30, December 1993 • “Adaptive Beacon Placement”, N.Bulusu, H.John, E.Deborah CS691

41. Bibliography (contd.) • “The Active Badge Location System”, Want, R., Hopper, A.,Falcao, V., And Gibbons, J.,ACM Transactions on Information Systems 10, 1 (January 1992), 91-102. • “The Cricket Compass for Context-Aware Mobile Applications”,Nissanka B. Priyantha, Allen k.L. Miu, Hari Balakrishnan, and Seth Teller, MIT Laboratory for Computer Science • PowerPoint--“Location Sensing for Context-Aware Applications”,Mani Srivastava,UCLA –EE Department, mbs@ee.ucla.edu • PowerPoint– “Localization”, Huei-Jiun JU(Laura) & Yichen Liu, UCLA-EE Department CS691