Localization

1. Localization Xian Zhong March 12, 2003

2. Overview • Introduction • Location Sensor Technologies • Selected Systems • GPS (Global Positioning System) • 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 -Determining physical location of a sensor node is a critical service in these wireless sensor networks • 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 geoid • Location relative to fixed beacons • Location relative to a starting point • 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 Somenewtechnologiesaredeveloping 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 CS691

11. CS691

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

13. CS691

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

16. CS691

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. Geodetic Coordinates • The Cartesian coordinates, though convenient for calculations, are not practical for representations on maps • Maps historically have used Geodetic coordinates (latitude, longitude, and height above a reference surface) • Positions obtained from GPS can be converted into a local datum with an appropriate transformation CS691

21. 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

22. 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 • the fourth measurement useful for another reason! CS691

23. 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 -300,000kilometers/second • 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

24. 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

25. 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

26. Extra Satellite Measurement to Eliminate Clock Errors • Three perfect measurements can locate a point in 3D • Four imperfect measurements can do the same thing • 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

27. 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

28. 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

29. Differential GPS • Error in a measurement can be estimated if the receiver location is known • These error estimates computed at a reference receiver, if made available to other GPS users in the area, would allow them to mitigate errors in their measurements. • To be usable for navigation, such “differential corrections” have to be transmitted in real time over a radio link---DGPS CS691

30. Differential GPS (contd.) • DPGS can provide meter-level position estimates depending upon the closeness of the user to a reference station and the latency of the corrections transmitted over the radio link • Such performance can meet the requirements of much of land transportation and maritime traffic; DGPS services, both commercial and federally provided, are now widely available CS691

31. 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%) CS691

32. GPS Technology Status (contd.) • Differential GPS • Horizontal accuracy of ± 2 m • Vertical accuracy of ± 3 m • Requires a differential base station within 100 km CS691

33. 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 • GPS needs line-of-sight to satellites • does not work indoors, in urban canyons, forests etc. CS691

34. we need indoor location system CS691

35. 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. CS691

36. ORL Ultrasonic Location System Structure • A small wireless transmitter is attached to every object that is to be located • Consist of a microprocessor, a 418MHz radio transceiver, a Xilinx FPGA and a hemispherical array of five ultrasonic transducers • Each prototype mobile device has a unique 16-bit address, is powered by two lithium cells, and measures 100mm*60mm*20mm CS691

37. ORL Ultrasonic Location System Structure (contd.) • A matrix of receiver elements is mounted on the ceiling of the room to be instrumented • Each receiver has an ultrasonic detector, whose output is being digitized at 20KHz by an ADC which is controlled by a Xilinx FPGA, which can monitor the digitized signal levels. • Receiver also are individually addressable and are connected in a daisy-chain to a controlling PC CS691

38. ORL Ultrasonic Location System Structure (contd.) • A controller connected to the PC transmit a radio message consisting of a preamble and 16-bit address in every 200ms • The PC dictates which address is sent in each message • The transceiver pick up the radio signals and decode it by the on-board FPGA • The single addressed device broadcast an ultrasonic pulse • The controlling PC sends a reset signal to receivers at the same time as each radio message is broadcast CS691

39. ORL Ultrasonic Location System – Structure (contd.) • The FPGAs on each receiver then monitor the digitized signals from the ultrasonic detector for 20ms, calculating the moment at which the received signals peak for the first time • The short width of the ultrasonic pulse ensures that receivers detect a sharp signal peak • The controlling PC then polls the receivers on the network, retrieving from them the time interval between the reset signal and detection of the first signal peak (if any signal was detected) CS691

40. 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

41. Position calculation CS691

42. Position Calculation (Cont’d) CS691

43. Position Calculation (Cont’d) CS691

44. 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

45. 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

46. The Cricket Location-Support System • Cricket Indoor Location System • Support for mobile, indoor applications • Location-aware scenarios • Active maps • Resource discovery and interaction • Way-finding and navigation • Stream redirection CS691

47. Design Goals for Cricket • Operates well indoors • Different regions distinguishable • Preserves user privacy (listeners are passive) • Decentralized administration: owner of space installs and configures beacons as needed • Operates with low energy, • Easy to deploy and administer • Low cost, both H/W and installation • Granularity: This will really depend on beacon placement CS691

48. Where am I?(Active Map) CS691

49. What’s near me? Find me a …(Resource Discovery) Location by “intent” “Print map on a color printer.” System response Locates nearby free color printer Sends data there Tells you where it is CS691

50. What’s over there?(Interaction) Viewfinder Point-and-use interface CS691