Obtaining the User’s Position
Dr. Miguel A. Labrador
Department of Computer Science & Engineering
labrador@csee.usf.edu
http://www.csee.usf.edu/~labrador
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Outline
• Positioning systems and techniques
• Outdoor
– GPS system
– Cellular-based systems
• Indoor
• Java ME Location API 2.0
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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
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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
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The Java ME Platform
Frame = 5 subframes = 1500 bits at 50 bps = 30 seconds total
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Subframe #1
60
30
TLM
HOW
300
Clock Correction Data, GPS Week Number
600
Subframe #2
TLM
HOW
Ephemeris of transmitting Satellite
900
Subframe #3
TLM
HOW
Ephemeris of Transmitting Satellite
1200
Subframe #4
TLM
HOW
Messages, Ionospheric Data, Coordinated
Universal Time (UTC)
1500
Subframe #5
TLM
HOW
Almanac Data, Health Status, Almanac Reference Time
TLM = Telemetry Word HOW = Hand-Over Word
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GPS System
• Almanac contains coarse orbit and status information 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 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
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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
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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
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The GSM Cellular Network
• Cellular networks play a crucial role in LBIS
– Transport network
– Estimation of the user’s position
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The GPRS Architecture
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Cellular Positioning Technologies
• Cell Identification or Cell ID
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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
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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
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Improved accuracy
Better indoor coverage
Shorter TTFF
Less power consumption
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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
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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
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Skyhook’s Hybrid Positioning System (XPS)
• XPS is a software-based positioning systems that combines WiFi 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
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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
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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
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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
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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
}
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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
}
. . .
}
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The Location API 2.0
• Modifies the following features
– Criteria and LocationProvider
– ProximityListener
– Landmark and LandmarkStore
• Includes the following new features
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Landmark Exchange Formats
Geocoding
Map User Interfaces
Navigation
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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
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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);
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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
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