1. A schematic overview of localization in wireless sensor networks
Dave MacCallum
Oct. 22, 2004

2. Need for localization
- applications that need to pinpoint features of the environment (e.g., crack in a bridge)
- geographically based routing
- service allocation (security)
- resource allocation (e.g., energy)
- actuator control
- effective sensor deployment

3. Different environments
Indoor vs outdoor
Fixed vs ad hoc
Stationary vs mobile
Air vs water
2d vs 3d

4. Problem definition
Localization: determination of the location of a sensor by other elements in the network
Location verification: verification by other elements of the network of a location claim made by a sensor

5. Definition of location
physical position vs symbolic location
physical + something -> symbolic
absolute (single shared reference frame) vs relative

6. Computation of location
local (GPS)
infrastructure (RFID)

7. Accuracy and Precision
accuracy (grain size)
precision (probability of accuracy)
Adaptive fidelity: adjust precision in response to dynamic situations such as partial failures or directives to conserve battery power

8. Scale of a location-sensing system
coverage area per unit of infrastructure
number of objects the system can locate per unit of infrastructure per time interval
e.g. RF congestion when passing communication threshold

9. Recognition
needed for action based on location
IDs
system features

10. Other considerations cost: time, space, price
limitations: due to underlying technology - GPS outdoors, RF under water
sensor fusion: use of multiple technologies or location systems simultaneously to form hierarchical and overlapping levels of sensing

11. Location-Sensing Techniques
When attempting to determine a given location, we can choose from three major techniques:
Triangulation
Proximity
Scene analysis

12. Triangulation
lateration, which uses multiple distance measurements between known points
angulation, which measures angle or bearing relative to points with known separation

15. Distance measurements
direct
time of flight
(requires time synchronization)
- electromagnetic (light, RF, microwave)
- sound (acoustic, ultrasound)
attenuation

16. Proximity Proximity measures nearness to a known set of points contact
monitoring wireless cellular access points (Active Badge)
automatic ID systems (e.g. credit card point of sales)

18. Scene analysis Examines a view from a particular vantage point. static
differential

21. Ranging estimation methods
RSSI - RF (attenuation)
ToA, TDoA - RF, acoustic, infrared, ultrasound (time of flight)
AoA - RF

24. Combination methods trilateration triangulation
maximum likelihood estimation

26. AHLoS Ranging estimation: RF/ultrasound ToA
Combination: iterative multilateration
Distributed
Use of beacons

27. Some range-independent schemes:
Dv-hop/Amorphous :
shortest route in number of hops
average hop length (online/offline)
three reference points => location

29. More range-independent schemes
APIT: location within triangle of anchors, based on information from neighbors, use power; estimation based on combination of triangles
- Centroid: locators send out beacons: locator coordinates, crude estimation
- Connectivity based schemes: require centralized computation

31. Range-independent: SeRLoc
locators send out beacons: locator coordinates, angles of antenna boundary lines
sensor:
(1) collects beacons from locators
(2) determines approximate search area based on locator coordinates
(3) computes overlapping antenna sector region (majority vote scheme)
(4) location = center of gravity of overlapping region
security:
(1) encrypt beacons - shared global key and pairwise keys
(2) locator ID authentication
computational efficiency:
symmetric crypto, hash functions

34. References
"Dynamic Fine Grained Localization in Ad-Hoc Sensor Networks," A. Savvides, C.C. Han, and M.B. Srivastava, Proceedings of the Fifth International Conference on Mobile Computing and Networking (Mobicom 2001), July, Rome, Italy, (2001).
“SeRLoc: Secure Range-Independent Localization for Wireless Sensor Networks,” Loukas Lazos and Radha Poovendran, ACM workshop on Wireless security (ACM WiSe 2004), Philadelphia, PA. 
“Location Systems for Ubiquitous Computing”. J. Hightower, G. Borriello. IEEE Computer, Vol. 34, No. 8., pp , August 2001.