1、Obtaining the Users Position,Dr. Miguel A. Labrador Department of Computer Science & Engineering labradorcsee.usf.edu http:/www.csee.usf.edu/labrador,Outline,Positioning systems and techniques Outdoor GPS system Cellular-based systems Indoor Java ME Location API 2.0,Global Positioning System (GPS),T
2、hree 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,
3、 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
4、with the exact position of the satellites in the sky,Global Positioning System (GPS),Ephemerides are later broadcast by the satellites and used by GPS receivers along with the signals elapsed time to calculate their own position User segment is made up of all GPS receiversGPS satellites continuously
5、 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 iden
6、tify the subframe Words 3 to 10 contain the rest of the NM,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
7、#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,GPS System,Almanac contains coarse orbit and status information
8、 about every satellite in the constellation Each NM contains 1/25th 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 tran
9、smission 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 GP
10、S receivers,Lateration,Process of calculating the users 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 Pythagoras theorem,Lateration,Problem of this method
11、 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 afforda
12、ble, 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
13、 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,The GSM Cellular Network,Cellular networks play a crucial role in LBIS Transport network Estimation of the users position,The GPRS Architectur
14、e,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 v
15、ery 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 mechani
16、sm,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
17、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,Cellular Positioning Technolo
18、gies,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 Al
19、manac, 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,Indoor Positi
20、oning 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 cel
21、lular 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 MSs
22、 location Ultrasound-based systems Use RF and ultrasound signals to estimate distances Need US hardware and restricted to very few meters,Skyhooks Hybrid Positioning System (XPS),XPS is a software-based positioning systems that combines Wi-Fi AP locations, GPS data, and cellular tower locations to p
23、rovide 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
24、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 Skyhooks system http:/,The Location API 2.0,JSR 293 recently approved (October 2008) Improves certain features and include new ones with respect t
25、o 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
26、 to LBS, such as geocoding, map, and navigation,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
27、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,The Location API 1.0,Location object includes Qualifie
28、dCoordinates 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 getLocati
29、on method to obtain the location one time LocationListener method to obtain the location at predefined intervals,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 met
30、ers 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.getQualifiedCoordin
31、ates(); if (coord != null) / Include code that uses coordinates here / . catch (LocationException e) / Could not retrieve location catch (InterruptedException e) / Location retrieval interrupted ,Location Listener Class,public class LocListener implements LocationListener LocListener locListener = n
32、ew 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); . . . publi
33、c void locationUpdated(LocationProvider provider, Location location) / This code will be triggered with updated / location data at the defined interval . . . ,The Location API 2.0,Modifies the following features Criteria and LocationProvider ProximityListener Landmark and LandmarkStore Includes the
34、following new features Landmark Exchange Formats Geocoding Map User Interfaces Navigation,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
35、 (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
36、 use cell signal-based positioning, and if cell signal-based positioning is not available use Cell ID, and so forth,Criteria and LocationProvider,int preferredLocationMethods = new int3;/First preference of positioning technology preferredLocationMethods0 = MTE_SATELLITE;/Second preference preferred
37、LocationMethods1 = MTE_TIMEDIFFERENCE; /Third preference preferredLocationMethods2 = MTE_CELLID; /Get the LocationProvider for preferred location technologies LocationProvider lp = LocationProvider.getInstance(preferredLocationMethods, parameters);,ProximityListener,Proximity detection has been grea
38、tly 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 c
39、an 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,
