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Pervasive Computing > Location-based Services > Flashcards

Location-based Services Flashcards

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

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

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

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

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

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

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

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?

A

Empirical vs Modeling

Deterministic vs Probabilistic

25
Q

Explain the empirical fingerprinting approach

A

Create radio maps from measurements

Disadvantage:

  • Time consuming
  • Repeat measurements when configuration of access point changes
26
Q

Explain the modeling fingerprinting approach

A

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
Q

Explain the deterministic fingerprinting approach

A
  • 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
Q

Explain the probabilistic fingerprinting approach

A
  • 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
Q

When to use each approach?

A

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
Q

What systems are used for indoor / outdoor positions systems and which methods do they use

A
  • Active Badge System (Proximity-based; infrared signal)
  • Active Bat & Cricket (Trilateration; Ultrasound, RF)
  • iBeacon Bluetooth Positioning (Proximity-based; Bluetooth signal)
  • AnyPlace (Fingerprinting; WiFi)
31
Q

Explain the Active Badge System

A
  • 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
Q

Which commands exist in an active badge application

A
  • 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
Q

Sample applications for Active Badge Systems

A
  • Passing phone calls to the right room

- Find people in the building

34
Q

Explain the iBeacon Bluetooth Positioning

A
  • 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
Q

Explain the AnyPlace System

A
  • 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
Q

Name five outdoor location systems

A
  • 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
Q

Explain the basic of the GPS

A
  • Satellite-based 3D positioning with trilateration
    • In theory three satellites to determine location
  • Developed by US Department of Defense
38
Q

Explain the system architecture of GPS

A

Three segments:
Space Segment: 31 medium earth orbit satellites
Control Segment: ground-based control stations and antennas
User Segment: GPS receivers

39
Q

Explain how positioning with GPS works

A
  • Satellites periodically broadcast messages
  • GPS receivers combine these messages and compute location

Message content:

  • time of transmission
  • position
40
Q

How many satellites are used per measurement ?

A

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
Q

What is the most promising Indoor positioning system

A

Positioning with WLAN because its also a cheap approach

42
Q

What are the most promising outdoor positioning systems

A
  • GPS is everywhere

- GLONASS (Russia) and Galileo (EU) also supported by modern smartphones

Pervasive Computing (11 decks)

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