1. 11 Obtaining the User’s Position Dr. Miguel A. Labrador Department of Computer Science & Engineering labrador@csee.usf.edu http://www.csee.usf.edu/~labrador

2. 2 Copyright© Dr. Miguel A. Labrador 2 2 Outline Positioning systems and techniques Outdoor –GPS system –Cellular-based systems Indoor Java ME Location API 2.0

3. 3 Copyright© Dr. Miguel A. Labrador 3 3 Global Positioning System (GPS) Three major components –Space segment –Control segment –User segment Space segment consists of the orbiting satellites –24 satellites in six orbital planes centered on the Earth are needed so at least six satellites can be detected from almost anywhere 6 more have been added to provide redundant signals, improve precision, improve reliability and availability of the system Control segment consists of several ground stations used to track and monitor the space segment –Main control station in Colorado Springs, Colorado –Updates the atomic clocks on board of all satellites and the ephemerides or table with the exact position of the satellites in the sky

4. 4 Copyright© Dr. Miguel A. Labrador 4 4 Global Positioning System (GPS) –Ephemerides are later broadcast by the satellites and used by GPS receivers along with the signal’s elapsed time to calculate their own position User segment is made up of all GPS receivers GPS satellites continuously broadcast a navigation message –1500 bits broken down in 5 subframes 300 bits long, 10 words 30 bits long each –Words 1 and 2 always contain the same information Telemetry Word (TLM): used by the receiver for synchronization Hand-Over Word (HOW): also for synchronization; enables the receiver to identify the subframe –Words 3 to 10 contain the rest of the NM

5. 5 Copyright© Dr. Miguel A. Labrador 5 5 The Java ME Platform TLM HOW Almanac Data, Health Status, Almanac Reference Time 1500 Subframe #5 TLM HOW Messages, Ionospheric Data, Coordinated Universal Time (UTC) 1200 Subframe #4 TLM HOW Ephemeris of Transmitting Satellite 900 Subframe #3 TLM HOW Ephemeris of transmitting Satellite 600 Subframe #2 TLM HOW Clock Correction Data, GPS Week Number 300 Subframe #1 TLM = Telemetry Word HOW = Hand-Over Word 0 30 60 Frame = 5 subframes = 1500 bits at 50 bps = 30 seconds total

6. 6 Copyright© Dr. Miguel A. Labrador 6 6 GPS System Almanac contains coarse orbit and status information about every satellite in the constellation Each NM contains 1/25 th of the almanac –Receiver needs 12.5 minutes to receive the entire almanac from a particular satellite –Almanac is very important because it helps GPS receivers to locate satellites at power up Satellites transmit NM at a very low transmission rate of 50 bps –Transmit a NM every 30 secs Main responsible for the time delay to obtain the first GPS fix –Time To First Fix (TTFF) Satellites use CDMA technology to transmit the NM –Same two frequencies of 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal) –Encoding codes are known to all GPS receivers

7. 7 Copyright© Dr. Miguel A. Labrador 7 7 Lateration Process of calculating the user’s position using distances between entities Finding the position of the user consists of finding the distance between the GPS receiver and the satellite and solving a systems of equations using Pythagora’s theorem

8. 8 Copyright© Dr. Miguel A. Labrador 8 8 Lateration Problem of this method is in the calculation of the distance between GPS receiver and the satellites Calculated measuring the time it takes the satellite signal to reach the receiver and multiplying it by the speed of light The NM contains the exact time at which the signal was sent In order to make GPS receivers affordable, clocks are not very precise –Synchronization problem introduce errors in distance calculations 1 microsecond error introduces an error of 300 meters! To eliminate this error, it is included in the calculations as an additional unknown variable –Four equations with four unknowns A fourth satellite is needed –Satellites are needed to be far from the user and separated from each other Dilution of Precision (DOP) used to select most appropriate satellites

9. 9 Copyright© Dr. Miguel A. Labrador 9 9 The GSM Cellular Network Cellular networks play a crucial role in LBIS –Transport network –Estimation of the user’s position

10. 10 Copyright© Dr. Miguel A. Labrador 10 Copyright© Dr. Miguel A. Labrador 10 The GPRS Architecture

11. 11 Copyright© Dr. Miguel A. Labrador 11 Copyright© Dr. Miguel A. Labrador 11 Cellular Positioning Technologies Cell Identification or Cell ID –Simplest localization method –HLR contains enough information to locate a user in the GSM network –Cell ID returns position of the BTS serving the user –Accuracy depends on cell size –Although fast and useful for some applications, not very accurate for many other Enhanced Cell ID –BTS measures RTT and estimates distance Reduces the radio of the circle only Enhanced Observed Time Difference (E-OTD) –BTSs periodically send beacon signals that MS use to measure distance to anchors and apply lateration Terminal-based positioning mechanism

12. 12 Copyright© Dr. Miguel A. Labrador 12 Copyright© Dr. Miguel A. Labrador 12 Cellular Positioning Technologies Uplink-Time Difference of Arrival (U-TDoA) –Similar to E-OTD but more complicated Calculations are performed by the BTSs based on signals transmitted by the MS –MS is not transmitting all the time –Only one BTS is serving the MS –Location Measurement Units (LMU) are included in the network to compile measurements and perform calculations Assisted GPS (A-GPS) –Easier and cheaper to implement in a GSM network –GPS-enabled phone and cellular network collaborate Improved accuracy Better indoor coverage Shorter TTFF Less power consumption

13. 13 Copyright© Dr. Miguel A. Labrador 13 Copyright© Dr. Miguel A. Labrador 13 Cellular Positioning Technologies Assisted GPS (A-GPS) –Relies on assistant servers located in several parts of the GSM network –Servers either provide information that the MS needs to perform the calculations, or perform the calculations using information provided by the MS –Server provides MS with information it cannot obtain Almanac, more accurate clock information, accurate coordinates of the server or BTS –Almanac allows the MS to lock to the GPS satellites faster –Server can receive partial information from MS and use its computational power and good satellite signals to compute position on behalf of the MS

14. 14 Copyright© Dr. Miguel A. Labrador 14 Copyright© Dr. Miguel A. Labrador 14 Indoor Positioning Systems Outdoor positioning systems are difficult to use indoors –Indoor positioning systems have been developed to fill this need –The smooth integration of them is still and active area of research Wireless Local Area Networks (WLANs) –WLAN access points transmit beacon signals like BTSs in cellular networks Proximity sensing adopts the position of the closest AP Lateration techniques can also be used Fingerprinting –Based on off-line measurements of the signal strength in specific reference points within the space of interest Stored in a database and utilized by the system to find the MS’s location Ultrasound-based systems –Use RF and ultrasound signals to estimate distances –Need US hardware and restricted to very few meters

15. 15 Copyright© Dr. Miguel A. Labrador 15 Copyright© Dr. Miguel A. Labrador 15 Skyhook’s Hybrid Positioning System (XPS) XPS is a software-based positioning systems that combines Wi- Fi AP locations, GPS data, and cellular tower locations to provide 10-20 meter accuracy positions in indoor and outdoor environments Mobile Location Client (MLC) and XPS Location Server (XLS) MLC can perform all calculations –Mobile-based location provider model MLC can also off load the calculations on the XLS –MLC send GPS, Wi-Fi AP, and Cell ID data to XLS –XLS has access to a huge DB with AP and cell tower locations and powerful algorithms to make the calculations –Location-provider model –iPhone and iPod use Skyhook’s system http://www.skyhookwireless.com

16. 16 Copyright© Dr. Miguel A. Labrador 16 Copyright© Dr. Miguel A. Labrador 16 The Location API 2.0 JSR 293 recently approved (October 2008) Improves certain features and include new ones with respect to JSR 179 (Location API version 1.0) Two major packages –javax.microedition.location Improvements to classes needed to request and obtain a location –Location, LocationListerner, LocationProvider, ProximityListener, GeographicArea –javax.microedition.location.services New classes and interfaces related to LBS, such as geocoding, map, and navigation

17. 17 Copyright© Dr. Miguel A. Labrador 17 Copyright© Dr. Miguel A. Labrador 17 The Location API 1.0 Most important objects included are the LocationProvider, Criteria, and Location objects –LocationProvider is the provider of location data All interactions with the underlying positioning technology are handled through this object Since there may be several positioning technologies, several LocationProviders may exist –Criteria contains the requirements of the application Accuracy, speed, and course of the MS is needed –Location is the object that contains the location data

18. 18 Copyright© Dr. Miguel A. Labrador 18 Copyright© Dr. Miguel A. Labrador 18 The Location API 1.0 Location object includes –QualifiedCoordinates class that contains estimated latitude, longitude, altitude of the current position –Estimated horizontal and vertical accuracies Nice to know how good the estimation is –Speed and course of the MS –Time at which the position was calculated –Positioning method utilized Two methods –getLocation method to obtain the location one time –LocationListener method to obtain the location at predefined intervals

19. 19 Copyright© Dr. Miguel A. Labrador 19 Copyright© Dr. Miguel A. Labrador 19 getLocation Method try { // Create a Criteria object to define desired selection criteria Criteria cr = new Criteria(); cr.setHorizontalAccuracy(20);//Requests an estimated accuracy of 20 meters cr.setSpeedAndCourseRequired(true);//Requests speed and course of MS //Requests a LocationProvider that meets these Criteria LocationProvider lp = LocationProvider.getInstance(cr); // Get the location, 60 seconds timeout Location loc = lp.getLocation(60); Coordinates coord = loc.getQualifiedCoordinates(); if (coord != null) { // Include code that uses coordinates here //... } catch (LocationException e) { // Could not retrieve location } catch (InterruptedException e) { // Location retrieval interrupted }

20. 20 Copyright© Dr. Miguel A. Labrador 20 Copyright© Dr. Miguel A. Labrador 20 Location Listener Class public class LocListener implements LocationListener { LocListener locListener = new LocListener(); int interval = 4;// Interval between location updates is 4 s int timeout = 2;// Timeout after location request is 2 s // Maximum age allowed for a duplicate location value to be returned is 2 s int maxAge = 2; lp.setLocationListener(locListener, interval, timeout, maxAge);... public void locationUpdated(LocationProvider provider, Location location) { // This code will be triggered with updated // location data at the defined interval }... }

21. 21 Copyright© Dr. Miguel A. Labrador 21 Copyright© Dr. Miguel A. Labrador 21 The Location API 2.0 Modifies the following features –Criteria and LocationProvider –ProximityListener –Landmark and LandmarkStore Includes the following new features –Landmark Exchange Formats –Geocoding –Map User Interfaces –Navigation

22. 22 Copyright© Dr. Miguel A. Labrador 22 Copyright© Dr. Miguel A. Labrador 22 Criteria and LocationProvider Eliminates ambiguity when Criteria includes conflicting requirements –Different devices may choose different positioning systems Criteria object now includes priorities, from 1 to N (lowest number, highest priority) An array of prioritized location method constants defined in the Location object can be used to specify the desired fallback order of positioning technologies to be used by the LocationProvider –A tracking application may wish to use GPS, and if GPS is not available use cell signal-based positioning, and if cell signal-based positioning is not available use Cell ID, and so forth

23. 23 Copyright© Dr. Miguel A. Labrador 23 Copyright© Dr. Miguel A. Labrador 23 Criteria and LocationProvider int[] preferredLocationMethods = new int[3]; //First preference of positioning technology preferredLocationMethods[0] = MTE_SATELLITE; //Second preference preferredLocationMethods[1] = MTE_TIMEDIFFERENCE; //Third preference preferredLocationMethods[2] = MTE_CELLID; //Get the LocationProvider for preferred location technologies LocationProvider lp = LocationProvider.getInstance(preferredLocationMethods, parameters);

24. 24 Copyright© Dr. Miguel A. Labrador 24 Copyright© Dr. Miguel A. Labrador 24 ProximityListener Proximity detection has been greatly enhanced in version 2.0 An interval and timeout value can be defined by the application when the ProximityEnterAndExitListener, which has replaced the ProximityListener of version 1.0, is registered –Now, a new locationUpdated() method is called at a particular interval, so that the application can tell how frequently the device is checking proximity to the registered location The specification now supports the detection of departure from a specific area The new specification allows the registration of different types of geographic areas, including circular, rectangle, and polygon geographic areas