Pervasive Computing > Location-based Services > Flashcards

Location-based Services Flashcards

(42 cards)

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1
Q

Which is the largest source of revenues in the communication market and why

A

Added-value services because many mobile applications use positioning information

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2
Q

List 5 application areas for LBS in the private sector

A
  • Navigation systems and services
  • Fleet and vehicle tracking
  • Tourist guide systems
  • Nearby points of interest
  • Indoor guidance systems
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3
Q

List 4 application areas for LBS in the public sector

A
  • Emergency Management
  • Road tax
  • Smart cities
  • public transportation
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4
Q

What functionality is provided by Google Latitude (Commercial Application) now embedded in Google Maps

A

Functionality:

  • See friends location in real time
  • Control who sees your location for how long
  • Initiate communication (call, message)
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5
Q

Give an example for an object location service.

A
  • 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
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6
Q

Describe the different dimensions of location

A

Location is a hierarchical construct.
Location
Information | Reference System
Phyiscal | Symbolic Absolute |Relative
GPS Postal address GPS proximity

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7
Q

Describe what the principles of a LBS

A
  • Uses location information to enhance the functionalities provided to the final user
  • Relies on automatic obtention of location data
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8
Q

Name three ways to represent a location

A
  • Maps
  • Descriptive (left of the gas station)
  • Coordination (latitude/longitude)
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9
Q

How is a location automatically obtained

A
  • With positioning
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10
Q

What is positioning and what are its components?

A
  • A process to obtain the spatial position of a target
  • parameters observed by measurements
  • positioning method
  • reference system
  • infrastructure & protocols (to coordinate the process)
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11
Q

Name five positioning methods

A
  • Proximity
  • Trilateration
  • Triangulation
  • Dead Reckoning
  • Fingerprinting
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12
Q

Explain the positioning method: proximity

A

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)

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13
Q

How to obtain a absolute position from proximity

A
  • Device connects to Access Points with BSSID
  • DataBases store Location related to a BSSID
  • LBS query for location with BSSID as parameter
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14
Q

What is the tradeoff for proximity with the GSM cell ID example

A
  • 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)
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15
Q

How can distance be measured?

A
  • 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
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16
Q

Explain how to measure a distance with Ultrasound + ToA

A
  1. Transmittor sends ultrasound signal
  2. Receives responds with ultrasound signal
  3. Distance is calculated

d = (((T3-T0)-(T2-T1)) X V)/2

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17
Q

What is the principle of Lateration and Trilateration

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A

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

18
Q

Explain how to measure a distance with Ultrasound & Radio + TDoA

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A
  1. Combined Transmitter (send simultanously both signals)
  2. 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

19
Q

What is the principle of Triangulation

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A

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

20
Q

What is the principle of Dead Reckoning

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A

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

21
Q

What is the principle of Pattern Matching / Fingerprinting

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A

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

22
Q

Explain the phases of Pattern Matching / Fingerprinting

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A

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

23
Q

How does WiFi Fingerprinting work

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A

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
24
Q

What are the different approaches of fingerprinting?

Study These Flashcards
A

Empirical vs Modeling

Deterministic vs Probabilistic

25
Explain the empirical fingerprinting approach
Create radio maps from measurements Disadvantage: - Time consuming - Repeat measurements when configuration of access point changes
26
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
27
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
28
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
29
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
30
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)
31
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
32
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
33
Sample applications for Active Badge Systems
- Passing phone calls to the right room | - Find people in the building
34
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
35
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
36
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
37
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
38
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
39
Explain how positioning with GPS works
- Satellites periodically broadcast messages - GPS receivers combine these messages and compute location Message content: - time of transmission - position
40
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
41
What is the most promising Indoor positioning system
Positioning with WLAN because its also a cheap approach
42
What are the most promising outdoor positioning systems
- GPS is everywhere | - GLONASS (Russia) and Galileo (EU) also supported by modern smartphones
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