Location-based Services Flashcards
Which is the largest source of revenues in the communication market and why
Added-value services because many mobile applications use positioning information
List 5 application areas for LBS in the private sector
- Navigation systems and services
- Fleet and vehicle tracking
- Tourist guide systems
- Nearby points of interest
- Indoor guidance systems
List 4 application areas for LBS in the public sector
- Emergency Management
- Road tax
- Smart cities
- public transportation
What functionality is provided by Google Latitude (Commercial Application) now embedded in Google Maps
Functionality:
- See friends location in real time
- Control who sees your location for how long
- Initiate communication (call, message)
Give an example for an object location service.
- Object includes Bluetooth / RFID for identification
- People move around with their phones as sensors
- Global query service where users can ask for a lost item
Describe the different dimensions of location
Location is a hierarchical construct.
Location
Information | Reference System
Phyiscal | Symbolic Absolute |Relative
GPS Postal address GPS proximity
Describe what the principles of a LBS
- Uses location information to enhance the functionalities provided to the final user
- Relies on automatic obtention of location data
Name three ways to represent a location
- Maps
- Descriptive (left of the gas station)
- Coordination (latitude/longitude)
How is a location automatically obtained
- With positioning
What is positioning and what are its components?
- A process to obtain the spatial position of a target
- parameters observed by measurements
- positioning method
- reference system
- infrastructure & protocols (to coordinate the process)
Name five positioning methods
- Proximity
- Trilateration
- Triangulation
- Dead Reckoning
- Fingerprinting
Explain the positioning method: proximity
Principle: proximity / distance to one reference is detected
- Relative positioning Proximity can be: - binary (in proximity to X or not) - quantitative (a distance) - in relation to one ore more references
Examples:
- Scan for bluetooth devices
- WLAN
- GSM cell ID
(Simplistic way of fingerprinting)
How to obtain a absolute position from proximity
- Device connects to Access Points with BSSID
- DataBases store Location related to a BSSID
- LBS query for location with BSSID as parameter
What is the tradeoff for proximity with the GSM cell ID example
- Size of location area and number of messages send to update location (smaller location areas more messages - always when a area is left sends a message)
How can distance be measured?
- Direct -> measuring tape
- Attenuation -> reduction of signal amplitude
- Time of Flight
- ToA -> Time of Arrival of a signal
- TDoA -> Time Difference of Arrival between 2
signals
- Indirect -> Based on other primary observation
Signals:
- Ultrasound
- Radio signal
- Infrared
Explain how to measure a distance with Ultrasound + ToA
- Transmittor sends ultrasound signal
- Receives responds with ultrasound signal
- Distance is calculated
d = (((T3-T0)-(T2-T1)) X V)/2
What is the principle of Lateration and Trilateration
Principle: Determine the position based on N distance measurement to known points
- Measure distance relative to known points
- N = 3 -> Trilateration (can identify one point at the intersection of the signals of all three base stations)
Consists of Base stations, Terminals, and Pilot signals
Explain how to measure a distance with Ultrasound & Radio + TDoA
- Combined Transmitter (send simultanously both signals)
- Combined receives calulates distance ( measures time diffrence between incoming signals) the receiver does not respond as in case of ToA
d = (T3 - T1) X Vus
What is the principle of Triangulation
Principle: Angulation
- Measures the angle from the location to a known point (Angle of Arrival of incoming signal)
- using antenna arrays
Triangulation: Having 2 angles to calculate location based on trigonometry
What is the principle of Dead Reckoning
Principle: Estimate position based on a previously known position; information about movement
Movement (obtained by accelerometers, gyroscopes):
- Direction of movement
- Velocity or distance
Measure distance relative to last known point
e.g. Positioning in a tunnel
What is the principle of Pattern Matching / Fingerprinting
Principle: Determine position based on a comparison of the current observation with previously stored patterns of signal strengths of multiple reference points
e.g. Use signal from WiFi Access points to determine location
Explain the phases of Pattern Matching / Fingerprinting
Phase 1: Off-line phase (record phase)
- Store previously known patterns in a database and develop a position model
Phase 2: Real-time phase ( operate phase)
- Use the current observation as an input for the positioning model and obtain the position out of it
How does WiFi Fingerprinting work
Record phase:
- Measure for each reference point the RSS (received signal strength) for each SSID
- Create Radio Map (Position Model)
- May require several measurements (line-of-sight, people blocking the signal)
Operate phase:
- Measure the RSS for each SSId
- Look for the reference point with minimal difference
What are the different approaches of fingerprinting?
Empirical vs Modeling
Deterministic vs Probabilistic
Explain the empirical fingerprinting approach
Create radio maps from measurements
Disadvantage:
- Time consuming
- Repeat measurements when configuration of access point changes
Explain the modeling fingerprinting approach
Create a radio map from a mathematical model which takes into account:
- position of access points
- transmitted signal strength
- free-space path loss
- obstacles reflecting / scattering signals
–> Tries to mathematically model the real world situation
Disadvantage:
- Complexity and accuracy of the model
Explain the deterministic fingerprinting approach
- Record several RSS samples for each reference position and direction
- create radio map from mean values of these samples
- Online phase: match observed and recorded sample with euclidean distance
- Take the reference position with the smallest distance as current position of the terminal
Explain the probabilistic fingerprinting approach
- Describes variations of signal strength experienced in the offline phase by probability distribution
- Applies the probability distribution of the access points to the observed RSS pattern
- Better accuracy compared to deterministic approach
When to use each approach?
Comparison Criteria:
- Accuracy. What basic resolution can be achieved
- Precision: How often do we get the stated accuracy
- Report accuracy and position
–> Consider the application scenario
What systems are used for indoor / outdoor positions systems and which methods do they use
- Active Badge System (Proximity-based; infrared signal)
- Active Bat & Cricket (Trilateration; Ultrasound, RF)
- iBeacon Bluetooth Positioning (Proximity-based; Bluetooth signal)
- AnyPlace (Fingerprinting; WiFi)
Explain the Active Badge System
- Proximity based infrared location system
- Tracks room levels of people
Basic concept: - Badges emit signal regularly (with person ID)
- Sensors receive signal and attach sensor location
- Map Person ID/ Sensor ID for location dependent apps
- Privacy –> dont wear the badge
Which commands exist in an active badge application
- FIND (Name): Where is name
- WITH (Name): Who is together with name
- LOOK (location): Who is at location
- HISTORY(name): Where was name recently
Sample applications for Active Badge Systems
- Passing phone calls to the right room
- Find people in the building
Explain the iBeacon Bluetooth Positioning
- Uses Bluetooth Low Energy (BLE)
- Broadcast of ID, distances are calculated on the phone using signal strength
- relative positioning via bluetooth signal
+ Many devices exist
Explain the AnyPlace System
- Uses fingerprinting with WiFi
Record phase: - smartphone application to upload floor plans, add POI’s, perform and send WiFi measurements and POIs
–> Crowdsourced dynamic radio map
Operate phase:
- navigate indoor
Name five outdoor location systems
- GPS ( US Global Positioning System)
- DGPS (Differential Global Positioning System)
- A-GPS (Assisted Global Positioning System)
- Galileo (European global navigation satellite system)
- E-OTD (Enhanced observed time difference) GSM Location method
Explain the basic of the GPS
- Satellite-based 3D positioning with trilateration
- In theory three satellites to determine location
- Developed by US Department of Defense
Explain the system architecture of GPS
Three segments:
Space Segment: 31 medium earth orbit satellites
Control Segment: ground-based control stations and antennas
User Segment: GPS receivers
Explain how positioning with GPS works
- Satellites periodically broadcast messages
- GPS receivers combine these messages and compute location
Message content:
- time of transmission
- position
How many satellites are used per measurement ?
At least 4 satellites
- 3 would be often enough
- But: Small clock error would result in large positioning error
- Thus: More satellites to fix time
What is the most promising Indoor positioning system
Positioning with WLAN because its also a cheap approach
What are the most promising outdoor positioning systems
- GPS is everywhere
- GLONASS (Russia) and Galileo (EU) also supported by modern smartphones
