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221 lines
8.2 KiB
Markdown
221 lines
8.2 KiB
Markdown
---
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title: "07-location-sensors"
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tags:
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- lecture
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- info305
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---
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## Terminology
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- Precision vs. Accuracy
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- How practical are systems that are:
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- Precise but not accurate?
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- Accurate but not precise?
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- Degrees of Freedom (DoF):
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- How many translational and rotational movements are possible?
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- How many degrees of freedom in 3D space?
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- 2 DoF?
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- 3 DoF?
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- 6 DoF?
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- 9 DoF?
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- Unlimited DoF?
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- How many degrees of freedom in 2D space?
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- (Tri)angulation:
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- Based on angles between signals
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- Referred to as angles of arrival
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- Angle to reference point determined by means of special antennas
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- Positions of reference points have to be known
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- 3D-positioning requires three reference points (triangulation)
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> [!INFO] if you have two reference points and the angle towards them, you can get the location
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- (Tri)lateration
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- Based on distances between device and reference points
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- Calculated e.g. by time of arrival (ToA) or signal strength
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- Positions of reference points have to be known
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- 3D-positioning requires three reference points (trilateration)
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> [!INFO] siimilra to triangulation but you need three points and its calculated using distancees not angles. the distances can be calculated by time of arrival of a signal
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## Cell-based
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- Positioning based on GSM cell ID
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- Mobile Positioning System (MPS) by Ericsson to improve accuracy
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- Few changes of infrastructure
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- No changes of user devices
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1. Cell Global Identity (CGI) for identification of the cell
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- Uses GSM cell ID
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- If sector-antennas used position can be narrowed to segment of the circle
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- Angulation
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> [!INFO] compute your position based on which cell tower you are connected to. each tower has a unique ID.
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> used by emergency providers, they know where you last logged in
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2. GSM based on FDMA (Frequency-division multiple access) and TDMA (Time-division multiple access)
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- Exact timing required for the synchronization of uplink and downlink
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- Timing Advance (TA)
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- Device calculates distance based on signal propagation time
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- Can be used for positioning in combination with CGI
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> [!INFO] "towers" use FDMA or TDMA [[21-data-link-layer#multiple access links and protocols]] Timing advance used for multiple access can be used to calculate time
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3. Uplink-Time Difference of Arrival (UL-TOA)
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- Four base stations needed
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- Measurement of signal propagation time at base stations
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- Calculation of position based on lateration
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- Accuracy 50 – 150m
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- Network-based location technology,
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- Can locate any type of mobile phone.
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- Only available to the owner of the sensor network
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> [!INFO]
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- Integrated approach based on existing infrastructure
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- Low costs (for user)
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- Low power draw
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- High availability
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- Works indoor and outdoor (no direct line of sight needed)
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- Information not always available to the user (often only network provider)
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- Used for “Enhanced 911”
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- But relatively low accuracy
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- Other cell-based location approaches: WiFi/WLan-cells
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- Integrated infrastructure approach
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- WiFi/WLAN (cell-id, lateration, fingerprinting)
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- Based on already existing WLAN infrastructure, primarily installed for communication purposes
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- Cell ID requires database with mapping WiFi cell ID to GPS or …. (e.g. created by Apple, Google)
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- Lateration
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- Requires accurate information about access point positions
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- Measurement of signal strength of various access points
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- At least three access points have to be available
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- Measurements are influenced by obstacles like walls
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- Improved approach: Fingerprinting
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- Fingerprinting:
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- 1st Phase: Radio Map Creation (Offline):
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- Measurements of fingerprints at reference points -> stored in fingerprint DB
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- Definition of reference points according to accuracy needs and building structure
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- 2nd Phase: Radio Map Usage (Online):
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- Fingerprint of current position is taken
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- Search for closest matching reference point on radio map Position taken from that reference point or interpolated
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- Fingerprint
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- Location: lat,lon,floor
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- VisibleAccessPoints{
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- Fingerprint DB
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- AP1:
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- SSID: eduroam
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- MAC: 08:17:35:33:5f:80
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- signal strength: -64
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- AP2
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- SSID: VPN/WEB
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- MAC: 08:17:35:33:5f:81
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- signal strength: -61
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- APn o...
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Fingerprinting services using Wifi
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- ~99% of queries use Google location database (Google Maps Geolocation API)
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- https://developers.google.com/maps/documentation/geolocation
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- Access is logged
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- Used to improve DB
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- Privacy issues (e.g. private routers)
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- Alternatives (less coverage & accuracy)
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- OpenCellID
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- Mozilla Location DB
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- Not available when offline, cell DB too large
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- WLAN cells can move/change - frequent update required
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- Cell structure: circular (theory), hexagonal (planning), irregular (reality)
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- Neighbouring cells use different frequencies to minimise interference
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- Some overlap inevitable
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- Typical cell sizes:
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- WiFi: 10m - 100m
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- 3G (UMTS): 100m - 5km
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- 2G (GSM): 100m - 35km
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## Proximity Sensors & Near Field Communication
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- Different technologies for sensing proximity or exchanging data (often dual purpose)
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- Stand-alone infrastructure approaches
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- Not widely accepted due to special hardware requirements infrastructure costs (for tracking)
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- Conceptually often similar to cell-based approaches but require extra infrastructure
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- WIPS (Infrared)
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- Beacons installed in the rooms sending unique ID
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- User’s badges receive signals of local beacons
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- Received beacon ID is sent to location server via WLAN
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- Server maps received beacon ID to semantic location which is sent back to the user
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- Active Badge (Infrared)
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- Users carry badge sending specific user ID
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- IR-receivers in the rooms receive those signals
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- Position of user is tracked by central server
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- Energy-efficient badges (sending short signals of 0,1s each 15s)
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- Bluetooth Low Energy LE (Bluetooth specification 4.0)
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- Not to confuse with “classic” Bluetooth (e.g. used in headsets or for I/O devices)
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- Optimized for battery powered sensors
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- Months or years of battery liftetime
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- Lower data rate (1MBIT/sec)
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- Backwards compatible (same frequency and modulation)
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- Two classes for peripherals:
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- Beacons (pure static broadcasts:
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- iBeacon simply broadcasts UUID
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- Additional data possible on request
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- Sensors (broad/unicast with sensor data)
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- Different profiles temperature, gravity, hear rate, pressure, …
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- Notification possible to avoid polling
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- Bidirectional communication
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- Not primarily focus, mostly for setting parameters
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- Location and Proximity Sensing using Bluetooth LE
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- One beacon (e.g. per room)
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- Pure proximity sensing, looking for strongest signal
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- E.g. Region monitoring to detect Beacon presence
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- Many beacons (e.g. per room)
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- Range calculation using Received Signal Strength Indicator (RSSI) and calibrated transmitter power (txPower = RSSI at 1m)
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- Trilateration using several beacons but not very accurate
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- Range different for beacons but typically between 20-50m (rarely 100m)
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- Different protocols on top of Bluetooth LE
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- iBeacon (Apple)
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- Broadcasts a UUID
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- ID is used with database integrated in the app
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- Further information on request (e.g. range information)
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- Eddystone (Google)
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- Beacons broacasts information about the beacon (telemetry frame e.g. battery or sensor information)
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- Beacons broadcasts and redirects to an URL (physical web)
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