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Navigation & Positioning Systems

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  1. Navigation & Positioning Systems Lecturer: Michael O'Grady Course: MSc Ubiquitous & Multimedia Systems Unit: Context Sensitive Service Delivery Lecture: GPS

  2. Objectives • Introduce some principles of electronic navigation • Describe state-of-the-art in satellite navigation • Introduce techniques based on cellular telephony • Outlined current developments in Satellite Based Augmentation Systems

  3. Why the sudden interest? • Location-aware services • also called proximity services • Examples include: • personal locator services • asset tracking • Emergency services provision • E-911 • E112 Exercise: Find some research reports and see what they say regarding market projections and types of services required

  4. Classification • Terrestrial Radio • Satellite • Cellular Network • Hybrid

  5. Terrestrial Radio - Techniques • Radio Direction Finding • directional antenna • Tune into radio station with known coordinates • Determine bearing • Repeat with a second station • Construct two lines using appropriate bearing • Intersection indicates position • Hyperbolic Technique • Tune into two stations at known positions • Calculate time difference between signals • Repeat with a second station • Construct hyperbola • Intersection indicates position

  6. Terrestrial Radio - Examples • DECCA • 1937 in USA • Problems • short range signal • large number of transmitters • expensive to implement and maintain • LORAN-C • Successor to DECCA • 1950 in USA • Deployed worldwide • Civilian use since early 1990 • Now operated by North West Europe LORAN-C System (NELS)

  7. Satellite - History • 1950s • Russian space program • Doppler Shift Measurements of the Doppler Shift in the signals broadcast by a satellite following a known and well defined orbit could be used to estimate position • Example: Transit • Navy Navigational Satellite System • 5 satellites following low altitude (1100km) polar obits • Decommissioned in 1996 ( due to GPS!)

  8. Satellite - GPS • Global Positioning System (GPS) • Navstar GPS • US Department of Defense (DoD) • Military ownership and control • History • Initial tests in 1973 using ground transmitters • First satellites launched in 1978 • 10 satellites launched by 1989 • 24 satellites in operation • 1995 - Full operational capacity • Services • Navigation (accuracy variable but 20m is realistic) • Time

  9. GPS - Architecture • GPS comprises of three segments • Space Segment • Control Segment • User Segment

  10. GPS - Space Segment (I) • GPS Satellites • Space Vehicles (SVs) • 24 SVs in constellation • 3 spares • Orbit every 12 hours • Altitude of 20,200 km • Space Vehicle Characteristics • 2 solar panels • I small rocket • 4 types (Block I, II etc) • 4 atomic clocks (2 caesium & 2 rubidium)

  11. GPS - Space Segment (II) • Constellation structure • 6 equally spaced (60 degrees) orbital planes • 4 SVs per plane • Each plane inclined at 55 degrees with respect to the equatorial plane • Objective • Ensure that between 5 and 8 SVs are visible from any point on earth at any given time

  12. GPS - Space Segment (III)

  13. GPS - Control Segment (I) • Network of 5 monitoring stations • Function • Track SVs • relay satellite time and ranging information to master control station in Colorado • At the master control station • orbit and timing parameters calculated for each SV • information then uploaded to the appropriate SVs Note: All SVs tracked 92% of time due to the geographic spread of the monitoring stations

  14. GPS - Control Segment (I)

  15. GPS - User Segment • Two groups of users • Military • standard military applications • Civilian • Commercial • Surveying • Vehicle monitoring • Precision agriculture • Recreational • Geocaching

  16. GPS - Theory (I) • Trilateration a basic geometric principle that allows the position of an object be determined if its distance from three objects is known In brief…. if distance to one object is known I am on a sphere with that object is centre. if distance to a second object is known I am on a circle that denotes the intersection of the two spheres. if distance to a third object is known I am on either of two points where the three spheres intersect.

  17. GPS - Theory (II) But clock on receiver NOT synchronised with those on the satellites! • So fourth satellite required to eliminate clock bias • 4 equation- 4 unknowns • 3D fix (longitude, latitude, altitude), time bias eliminated • Extra satellites can be used to improve calculation • World geodetic System 1984 (WGS84) • If altitude assumed fixed - three satellites adequate for a reading (2D fix) Note: distance to satellite referred to technically as a pseudorange

  18. GPS - Signals • Each satellite broadcasts at 2 frequencies • L1 • 1575.42 MHz • Civilian users • L2 • 1227.6 MHz • Military users • Encrypted

  19. GPS - Sources of Error • clock and ephemeris errors • ionospheric & tropospheric delays • multipath & shading • satellite geometry or Geometric Dilution of Precision (GDOP) • receiver noise & delay Note: Selective availability has been removed since May 2000 Note: One nanosecond of inaccuracy results in 30 centimeters error approximately

  20. GPS - DOP • Dilution of Precision (DOP) • Measurement of the geometry of the visible satellite constellation • satellites spread across sky gives good (small) DOP values • clustered satellites give poor DOP values ( • Geometric Dilution of Precision (GDOP) -position, time • Horizontal Dilution of Precision (HDOP)- Latitude, longitude • Vertical Dilution of Precision (VDOP) - altitude • Positional Dilution of Precision (PDOP) - position • Time Dilution of Precision (TDOP) - time • Examples: • 1 indicates perfection • if HDOP greater than 5, reading is rejected! • PDOP of 8 is acceptable provided HDOP less than 5

  21. Differential GPS (DGPS) 1. if the position of a GPS receiver is known, the error in the pseudoranges can be calculated 2. These “corrections” may be broadcast to nearby users thus improving their position readings accordingly DGPS demands • network of reference stations • Allocated FM frequency • specialist receiver • Readings of up to 5 meters may be obtained easily • sub-meter accuracy may also be obtained subject to cost and quality of the equipment

  22. DGPS

  23. Satellite - GLONASS • Soviet equivalent of GPS • Initial satellite launched in 1992 • Last satellite deployed in 1996 • Working constellation of 24 satellites • Architecturally similar to GPS • Control segment within borders of former Soviet Union

  24. Satellite - Galileo • Conceived in 1999 • Initial satellites scheduled for launch in 2004 • Initial services scheduled for 2006 • Full operational capacity by 2008 • Objectives • state-of-the-art positioning and timing services • guarantees regarding accuracy & availability • designed for civilian requirements

  25. Galileo - Architecture • 30 satellites in circular orbits • Orbit planes inclined at 55-60 degrees to equatorial plane • increase coverage in Northern Europe • Ground segment consists of 14 ground stations spread around the globe • Interoperable with GPS and GLONASS

  26. Cellular Techniques • Recall E-911 directive E-911 focused attention on how a cellular network’s features and topology might be used to ascertain a subscriber’s position

  27. Cell-ID • Also called Cell of Origin (COO) • Networks knows position of subscriber to cell level • Map geographic coordinates to Base Station • Advantages include: • speed & cost • Minimum modifications to network infrastructure • Disadvantages include • variable accuracy (as cell sizes vary) • Propagation effects may mean serving cell is not actually the nearest cell

  28. Timing Advance (TA) • GSM Timing Advance parameter • used for synchronising time slots • may be defined as the time delay (distance!) between handset and Base Station • 6 bits in length • precision of about 550 meters • Requires augmentation • other positioning mechanism • directional antenna

  29. Angle of Arrival (AOA) • Triangulation • Measure angles of signal at Base Station using sophisticated antenna • Repeat for second Base Station • Intersection of lines projecting outwards will indicate subscriber’s position • Disadvantages • Susceptible to interference and fading • Line-of-Sight (LOS) conditions essential • Will not work well in city • Advantage • may work very well in rural area

  30. Time of Arrival (TOA) • Similar to GPS • Measure distance between handset and Base Station • Repeat for two more Base Stations • if time is synchronized this is adequate • if not, a further reading is required • Advantages • easy to implement • Disadvantages • Accurate clocks required at each station • Susceptible to errors in urban environments • May not access enough base stations in rural areas

  31. Time Difference of Arrival (TDOA) • Uses Time Difference as distinct from absolute time • Hyperbolic curves must be constructed for at least two Base Stations • Intersection indicates position

  32. Enhanced Observed Time Difference (E-OTD) • Also called • Observed Time Difference (OTD) • Observed Time Difference of Arrival (O-TDOA) • Identical to TDOA except • Calculations are performed on the handset • Disadvantages • Significant modifications required on the handset

  33. Advanced Forward Link Trilateration (A-FLT) • In principle, identical to TDOA or E-OTD • Can be implemented on handset or network • Implemented only on CDMA networks • Recall: CDMA is time synchronized so time differences are easier to measure

  34. Location Fingerprinting • Combines signal features such that a unique signature (fingerprint) for a given location is created • Database is assembled by driving a vehicle through the area which transmits signals to the Base Station • Signature is generated by analyzing the incoming signal • By comparing the subscriber’s signal with the signatures in the database, a location estimate can be obtained

  35. Assisted GPS (A-GPS) • Utilizes GPS and topology of cellular network • GPS receivers are placed throughout the PLMN • These “advise” the handset on which satellites to observe • Information is then sent back to the server for position calculation • Advantages • quick & accurate • easy to incorporate DGPS • can track weak signals • Disadvantages • expensive

  36. Assisted GPS - Schematic

  37. Hybrid Positioning Systems • No technique capable of delivering accurate positions under all circumstances • But • Pragmatic combination may lead to better results sometimes • Example • A-GPS • TDOA • Each system has complementary strengths and weakness

  38. Standardization - Satellite Navigation • Recall: GPS and GLONASS remain under the control of their respective militaries • International Civil Aviation Organization (ICAO) • active but limited in its effectiveness • Galileo • SAGA (Standardization Activities for Galileo) • work is currently ongoing

  39. Standardization - 2G Cellular Networks

  40. Standardization - 3G Cellular Networks

  41. Product: Garmin NavTalk

  42. Product: Benefon Esc!

  43. References • E-911 • http://www.fcc.gov/911/enhanced/ • GPS • http://tycho.usno.navy.mil/gpsinfo.html • http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html • GLONASS • http://www.glonass-center.ru/ • Galileo • http://www.esa.int/export/esaSA/GGGMX650NDC_navigation_0.html • EGNOS • http://www.esa.int/export/esaSA/GGG63950NDC_navigation_0.htm • SISNet • http://esamultimedia.esa.int/docs/egnos/estb/sisnet/sisnet.html

  44. The End