Location Sensing for Context-Aware Applications

1. Location Sensing for Context-Aware Applications EE206A (Spring 2001): Lecture #10 Mani SrivastavaUCLA - EE Departmentmbs@ee.ucla.edu

2. Required Reading for this Lecture • Ward, A.; Jones, A.; Hopper, A. A new location technique for the active office. IEEE Personal Communications, vol.4, (no.5), IEEE, Oct. 1997. p.42-7.

3. Location in Mobile Computing • Goal of mobile computing: User’s applications should be available wherever that user goes, in a suitably adapted form • User interfaces of the application follow the user • These applications are called Follow-me applications • Special case of Follow-me applications: Context-aware Applications • Ability to adapt behavior to a changing environment • e.g. Adapt to available proximate peripherals • e.g. Adapt to location of the user

4. Context Awareness • What is context? • Who • What • When • Where • How • Continuous vs. Registered context information • Continuous: always available and appropriate to the situation • Registered: physical mapped to virtual • Context-aware applications need to know the location of users and equipment, and the capabilities of the equipment and networking infrastructure

5. Location-Aware Services • Multicasting selectively only to specific geographical regions • defined by latitude and longitude • e.g. sending an emergency message to everyone who is currently in a specific area, such as a building or train station • Providing a given service only to clients who are within a certain geographic range from the server • server may be mobile itself • say within 2 miles • Advertising a given service in a range restricted way • say, within 2 miles • Teleporting • Ability by a user to access his desktop environment from any networked machine

6. Location-Aware Services (contd.) • Providing contiguous information services for mobile users when information depends on the user's location • location dependent bookmarks • provide user with any important information which happens to be local (within a certain range) possibly including other mobile servers. • Emergency 911 from cellular phones • FCC’s E-911 mandate requires 125 m RMS accuracy 67% of the cases • Other location services in cellular systems • location sensitive billing • fraud detection • resource management • Fleet management and intelligent transportation services [Stilp96]

7. Other Apps of Location Sensing • Monitoring large numbers of sensors dispersed over an area for nuclear, biological, or chemical threats • Synthesis of large aperture antennas for tight beam communication, using scattered transceivers that know their precise relative location and synchronization • Keeping track of mines, armaments, equipment, vehicles, etc. • Keeping track of personal items, such as one’s children, pets, car, purse, luggage, etc. • Inventory control in stores, warehouses, shipyards, railroad yards, etc. • Safety - finding fire fighters in a burning building, police officers in distress, or injured skiers on a ski slope. • Sports - arbitrating rules in a game, playback of motions for coaching, or viewing the re-creation of an event. • Home automation - keyless locks and rooms that adjust the light, temperature, and music sound level. • Motion pictures - automatically adjusting camera focus and motion-tracking for matching digital effects

8. What is Location? • Absolute position on geoid • 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 • Range and resolution of the position location needs to be proportionate to the scale of the objects being located

9. Self-positioning vs. Remote-Positioning • Self-positioning • Mobile node formulates its own position • e.g. by sensing signals received at the mobile from the transmitters in the infrastructure • Remote-positioning • Position of mobile node calculated at a remote location • e.g. by using signals received from the mobile by sensors in the infrastructure • Indirect positioning • Using a data link it is possible to send position measurements from a self-positioning receiver to a remote site, or vice versa • A self-positioning system that sends data to a remote location is called indirect remote-positioning • A remote-positioning system transmitting an object’s position to the object is called indirect self-positioning

10. Techniques for Location Sensing • Measure proximity to “landmarks” • e.g. near a basestation in a room • example systems: • Olivetti’s Active Badge for indoor localization • infrared basestations in every room • localizes to a room as room walls act as barriers • Most commercial RF ID Tag systems • strategically located tag readers • improved localization if near more than one landmark • Estrin’s system for outdoor sensor networks • grid of outdoor beaconing nodes with know position • position = centroid of nodes that can be heard • # of periodic beacon packets received in a time interval exceeds a theshold • a problem: not really location sensing • it really is proximity sensing • accuracy of location is a function of the density of landmarks • Location accuracy = O(distance between landmarks)

11. Techniques for Location Sensing (contd.) • Dead reckoning: position relative to an initialization point • work as supplement to a primary location sensing techniques • resynchronize when the primary location sensing technique works, and takes over if the primary fails • e.g. supplement GPS during signal outages • Use wheel and steering information in vehicles • Integrating accelerometers mounted on gyroscopically stabilized platforms • Point Research’s Pointman Dead Reckoning Module • inertial measurement unit for personnel on foot • Latitude and longitude relative to the start point • magnetic compass + MEMS-based electronic pedometer + barometric altimeter + DSP • position error of 2-5% of total distance traveled since last resynchronization • no drift with time • U. S. Patent No. 5,583,776. • www.pointresearch.com

12. Pointman Dead Reckoning Module Size: 1.9" x 2.9" x 0.6“ Weight: 1.5 oz. Power: 0.5 Watts @ 3.3 V (250 mW in new low-power DRM)

13. Trackman Personnel Locator • Combines a DRM with a GPS and a radio transmitter to provide continuous location tracking • Kalman filter is used to combine the dead reckoning data with GPS data when it is available • Specifications: • Size: 3.2" x 7.5" x 2.3" • Weight: 12 oz. • Range: 0.25 miles

14. Techniques for Location Sensing (contd.) • Measure direction of landmarks • Simple geometric relationships can be used to determine the location by finding the intersections of the lines-of-position • e.g. Radiolocation based on angle of arrival (AoA) measurements of beacon nodes (e.g. basestations) • can be done using directive antennas or antenna arrays • need at least two measurements BS 2 BS 1 MS 3 BS

15. Techniques for Location Sensing (contd.) • Measure distance to landmarks, or Ranging • e.g. Radiolocation using signal-strength or time-of-flight • also done with optical and acoustic signals • Distance via received signal strength • use a mathematical model that describes the path loss attenuation with distance • each measurement gives a circle on which the MS must lie • use pre-measured signal strength contours around fixed basestation (beacon) nodes • can combat shadowing • location obtained by overlaying contours for each BS • Distance via Time-of-arrival (ToA) • distance measured by the propagation time • distance = time * c • each measurement gives a circle on which the MS must lie • active vs. passive • active: receiver sends a signal that is bounced back so that the receiver know the round-trip time • passive: receiver and transmitter are separate • time of signal transmission needs to be known • N+1 BSs give N+1 distance measurements to locate in N dimensions

16. Radiolocation via ToA and RSSI x2 d2 BS BS x1 MS d1 d3 BS x3

17. Techniques for Location Sensing (contd.) • Measure difference in distances to two landmarks • Time-difference-of-arrival (TDoA) • Time of signal transmission need not be known • Each TDoA measurement defines line-of-position as a hyperbola • hyperbola is a curve of constant difference in distance from two fixed points (foci) • Location of MS is at the intersection of the hyperbolas • N+1 BSs give N TDoA measurements to locate in N dimensions

18. Radiolocation via TDoA

19. Algorithms for Location • Depends on whether ToA (RSSI is similar) or TDoA is used • Straightforward approach: geometric interpretation • Intersection of circles for ToA • Intersection of hyperbolas for TDoA • But what if the circles or hyperbolas do not intersect at a point due to measurement errors?

20. Sources of Errors • Multipath • Introduces error in RSSI, AoA, ToA, TDoA • RSSI • Multipath fading and shadowing causes up to 30-40 dB variation over distances in the order of half a wavelength • Shadowing may be combated by using pre-measured signal strength contours that are centered at BSs (assumes constant physical topography) • AoA • Scattering near and around the MS & BS will affect the measured AoA • Problem even when there is a LoS component • In macrocells, basestations are elevated so that signals arrive in a relatively narrow AoA spread • In microcells, signals arrive with a large AoA spread, and therefore AoA may be impractical • ToA and TDoA • Conventional delay estimators based on correlation are influenced by the presence of multipath fading which results in a shift in the peak of the correlation

21. Sources of Errors • Non line-of-sight (NLoS) • Signal takes a longer path or arrives at a different angle • Can be disaster for AoA if received AoA much different from true AoA • For time-based, the measured distance may be considerably greater than true distances • in GSM system, ranging error due to NLoS propagation is 400-700 m • Multiple-access interference • Most problem in CDMA where high power users may mask the low power users due to near-far effect • Power-control is used in CDMA • But, MS is not power controlled to other BSs • So signal from MS may not be detectable at enough BSs to form a location estimate • A possibility is to temporarily power up MS to maximum, thus mitigating the near-far effect

22. Location Algorithms in Presence of Errors • Geometrical algorithms fail • resort to estimation • 2D scenario • MS is located at • BSs are located at • vector of noisy measurements, , from a set ofBSs can be modeled by where is an measurement noise vector, generally assumed to have zero mean and ancovariance matrix • The system measurement model depends on the location method used

23. Location Algorithms in Presence of Errors (contd.) • System measurement model • ToA • TDoA • AoA • Note: • without loss of generality, TDoA are referenced to the first BS • if the time of transmission is needed to form the ToA estimate, it can be incorporated into as a parameter to be estimated along with and • the unknown parameter vector can then be modified to while the system measurement model becomes • The AoAs are defined by • Although not shown, , , and are nonlinear functions of

24. Location Algorithms in Presence of Errors (contd.) • A well known approach for estimating from a noisy set of measurements: method of least squares (LS) estimation • Weighted least squares (WLS) solution is formed as the vector that minimizes the cost function • LS methods can achieve the maximum likelihood (ML) estimate when the measurement noise vector is gaussian with and equal variances, i.e. • For unequal variances, WLS with gives the ML estimate • assume from now on…

25. Location Algorithms in Presence of Errors (contd.) • is a nonlinear function of the unknown parameter vector • The LS problem is a non linear one • One straightforward approach: iteratively search for the minimum of the function using a gradient descent method • an initial guess is made of the MS location, and successive estimates are updated according towhere the matrix is the step size,is the estimate at time , and denotes the gradient vector with respect to the vector

26. Location Algorithms in Presence of Errors (contd.) • In order to mold the problem into a linear LS problem, the nonlinear function can be linearized by using a Taylor series expansion about some reference point so that where is the Jacobian matrix of • Then the LS solution can be formed as • This approach can be performed iteratively, with each successive estimate being closer to the final estimate • a drawback: an initial guess of MS position must be made

27. Location Algorithms in Presence of Errors (contd.) • Problems with linearization: doesn’t work well if the linearized function does not represent the nonlinear function well • other approaches have been developed for TDoA that avoid linearization

28. Measures of Location Accuracy • MSE and Cramer-Rao Lower Bound • For location in M dimensions, the MSE (mean square error) is given by • Calculated MSE can be compared with the theoretical minimum MSW given by CRLB which sets a lower bound on the variance of any unbiased estimator • Circular Error Probability • Radius of the circle that has its center at the mean and contains half the realizations of a random vector • Measure of uncertainty in the location estimator relative to its mean • If the location estimator is unbiased, CEP is a measure of the the estimator uncertainty relative to the true MS position

29. Measures of Location Accuracy (contd.) • Geometric Dilution of Precision • GDOP provides a measure of the effect of the geometric configuration of the BSs on the location estimate • GDOP = ratio of the rms position error to thhe rms ranging error • where indicates the fundamental ranging error fo ToA and TDoA systems. • GDOP indicates the extent to which the fundamental ranging error is magnified by the geometric relation between the MS and the BSs • Furthermore:

30. Basic Multilateration (simplified) Residual of measured and estimated distance 1 Linearize using Taylor Expansion 2 a Linear form MMSE Solution 3 Repeat until δ becomes 0

31. Cooperative Networked Ranging for Ad Hoc Networks • Each node determines the range to every other node and then shares the information with members of the network • With 4 nodes knowing all the ranges between them, a rigid tetrahedral structure is determined (assuming no 3 nodes are collinear) • each node can then be in one of two 3-dimensional locations with respect to the other 3 nodes • having a 5th node resolves this ambiguity • Advantage: no fixed infrastructure • extensible, incremental, mobile, and survivable • Many open issues…

32. Some Other Technique/Systems for Location Sensing • Celestial • complicated • only works at night in good weather • limited precision • OMEGA • based on relatively few radio direction beacons • accuracy limited and subject to radio interference • LORAN • limited coverage (mostly coastal) • accuracy variable, affected by geographic situation • easy to jam or disturb • SatNav • based on low-frequency doppler measurements so it's sensitive to small movements at receiver • few satellites so updates are infrequent.

33. Short-range Radio Proximity Sensing Technologies • A variety of short-range radio-based technologies have been employed to track items indoors • identify objects with a sensor having a range of a few cms to about 3 m, depending on the technology. • Electronic article surveillance (EAS) systems • widely used in retail and library settings • simple tags respond to a matched electronic field by resonating when the resonance is detected • restricted range, lack id codes, and limited reliability • Radio frequency identification (RFID) • RFID tags are identified as they pass fixed sensors • detectable up to about 3 meters away • many applications • automated toll collection on highways • hands-free access control • replacement for bar codes in dirty or environmentally challenging environments

34. RFID Tags • Broadly categorized as active or passive • Passive tags require no battery, so that they tend to cost less but have shorter range • challenge of extracting operating power from the air • as they pass within range of an interrogator, their circuitry is charged either inductively (typically at 125 kHz) or electromagnetically (most commonly at 13.56 MHz) • once powered, passive RFID tags identify themselves to the interrogator using techniques such as frequency shifting, half-duplex operation, or delayed retransmission • range limited by the need for a nearby power source • few centimeters to 2-3 meters

35. RFID Tags (contd.) • Active tags require battery, but have longer read ranges and more features • more expensive • operate at higher frequencies, typically 900MHz or 2.4GHz ISM bands • many use modulated backscatter to communicate • the tags modulate their radar cross-section in a pattern to identify themselves to the interrogator • modulated backscatter tags have limited range, around 3 meters for the most part, • cannot be detected if blocked by a dense enough attenuator, such as a partition wall or a human body • Backscatter reflections from the tag overwhelmed by reflections from file cabinets, white boards, fluorescent lights, and other objects • RFID tags are fundamentally tied to a nearby power source

36. IRID Tags • Infrared counterpart of RFID tags • e.g. Olivetti/Xerox’s Active Badge • Tags periodically transmit their identification codes by emitting infrared light to readers installed throughout the facility • Problems: • tag prices are relatively high • installation is complicated by the large number of readers required to ensure a line of sight to every possible tag • reliability: IRID systems do not work at all under various common lighting conditions • a scarf or tie in the wrong position (or a party with balloons) can disable an IRID personnel tag

37. GPS • History • U.S. Department of Defense wanted the military to have a super precise form of worldwide positioning • Why? • Missiles can hit enemy missile silos… but you need to know where you are launching from • US missiles, unlike Soviet ones, were mostly sea-based • US subs needed to know quickly where they were • After $12B, the result was the GPS system! • 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!

38. GPS System • 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 • Current constellation: 24 Block II production satellites • Future satellites: 21 Block IIrs developed by Martin Marietta • Ground Stations, aka “Control Segment” • 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.

39. 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.

40. Earth-Centered Earth-Fixed X, Y, Z Coordinates

41. Geodetic Coordinates (Latitude, Longitude, Height)

42. 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 • 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!

43. 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 • if a satellite were right overhead the travel time would be something like 0.06 seconds • need some really precise clocks • if timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error • 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?

44. 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 • 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

45. 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

46. 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 • something that high is well clear of the atmosphere • it will orbit according to very simple mathematics • GPS Master Plan • the launch of the 24th block II satellite in March of 1994 completed the GPS constellation • four additional satellites are in reserve to be launched "on need." • spacings of the satellites are arranged so that a minimum of five satellites are in view from every point on the globe • 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

47. GPS Signals in Detail • Carriers • the GPS satellites transmit signals on two carrier frequencies • the L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing • The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random code • Pseudo-random Codes • two types of pseudo-random code • the C/A (Coarse Acquisition) code • it modulates the L1 carrier • it repeats every 1023 bits and modulates at a 1MHz rate • each satellite has a unique pseudo-random code • the C/A code is the basis for civilian GPS use • 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 • when it's encrypted it's called "Y" code • 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

48. GPS Signals

49. Encrypted GPS • Military maintains exclusive access to the more accurate "P-code" pseudo random code. • It has ten times the frequency of the civilian C/A code • potentially much more accurate • much harder to jam • When it's encrypted it's called "Y-code" and only military receivers with the encryption key can receive it • Because this code is modulated on two carriers, frequency diversity can be used to help eliminate errors caused by the atmosphere

50. Correcting Errors: Problems on the Way to the Earth • Speed of light is only constant in a vacuum • As the GPS signal passes through the charged particles of the ionosphere and then through the water vapor in the troposphere it gets slowed down a bit • this creates the same kind of error as bad clocks • Mathematical modeling can be used to predict what a typical delay might be on a typical day • it helps but, atmospheric conditions are rarely typical • Dual frequency measurements can be used to handle these atmospheric effects • low-frequency signals get "refracted" or slowed more than high-frequency signals • by comparing the delays of the two different carrier frequencies of the GPS signal, L1 and L2, we can deduce what the medium (i.e. atmosphere) is, and we can correct for it • requires a sophisticated receiver since only the military has access to the signals on the L2 carrier