LOCALIZATION in Sensor Networking Hamid Karimi

Wireless sensor networks Wireless sensor node  power supply  sensors  embedded processor  wireless link Many, cheap sensors  wireless  easy to install  intelligent  collaboration  low-power  long lifetime

The first Sensor Networks Research started in the 80s by DARPA Early sensor networks were used in the military

Sensor Network Architecture SINK TASK MANAGER Internet/ Satellite

What is Localization? A mechanism for discovering spatial relationships between objects

Modern applications Physical security  Detecting intruders Medical  Patients in a hospital Habitat monitoring  Wildlife, plants Environmental  Tracking forest fires, pollution Smart buildings Air traffic control Required in most applications: Location of the sensor

Reasons for the important of localization The localization problem is even more important in wireless sensor networks for the following reasons: 1. Many WSN protocols and applications simply assume that all nodes in the system are location-aware. 2. If a sensor is reporting a critical event or data, we must know the location of that sensor. 3. If a WSN is using a geographical routing technique, all of the nodes must be aware of their location.(Location Aided routing)

Initialization Initialization: Node has no knowledge of its location.

Localization in Sensor Networks Spatial localization - determining physical location of a sensor node in the network. Localization is an essential tool for:  The development of low-cost sensor networks for use in location-aware applications  Geographic routing

Possible Implementations/ Computation Models

Determining Location Direct approaches  GPS Expensive (cost, size, energy) Only works outdoors, on Earth  Configured manually Expensive Not possible for ad hoc, mobile networks Indirect approaches  Small number of anchor nodes anchor are configured or have GPS  Other nodes determine location based on messages received

GPS The most obvious solution to this localization problem is to simply equip every node with its own GPS device. GPS can work only outdoors. GPS receivers are expensive and not suitable in the construction of small cheap sensor nodes. A third factor is that it cannot work in the presence of any obstruction like dense foliage etc. Most of the proposed localization techniques today,depend on recursive trilateration/multilateration techniques

NASA Mars Tumbleweed Nodes moving, Anchors stationary Nodes and Anchors moving Nodes stationary, Anchors moving Indirect approaches

Steps in Localization Ranging Phase : distances or angles are measured between known points and the object to be located. Localization Phase : distances or angle measurements are combined to produce the location of the object. Refinement Phase : which is an optional step Most localization techniques consist of tree steps or phases :

Ranging Phase Received Signal Strength Indicator (RSSI) Time of Arrival (ToA) Angle of Arrival (AoA) Incremental Stepping of Transmission Power Hybird Some of the prominent techniques for the Ranging phase include:

Received Signal Strength Indicator (RSSI) Problem: Irregular signal propagation characteristics (fading, interference, multi-path etc.) A node receiving a message simply, measures the power of the incoming signal. An inverse relationship between power and distance can be used to estimate the distance between the nodes.

Time difference of arrival (TDOA) Idea: Use time difference between arrival of RF and ultrasound signals. Advantage : No synchronization Accurate Estimate

Incremental Stepping of Transmission Power  the relationship between a device's transmission power and the maximum distance a signal can transmit.  allows one to gradually increment that transmission power. Once a message is "heard", a bound on the maximum distance between the nodes can be inferred.

Localization Phase  Range Base Schemes Trilateration Multilateration Sum-Dist  Range Free Schemes Centroid Localization DV-Hop localization Depending on the method used for ranging, an appropriate localization technique is applied in the second phase. The following localization strategies have been proposed :

Trilateration : unknown coordinates of node Subtracting eq. 3 from 1 & 2: Assuming distances to three points with known location are exactly given : coordinates of anchor point i, Ri distance to anchor i

Trilateration Rearranging terms gives a linear equation in Rewriting as a matrix equation:

Trilateration

Multilateration Assume some nodes can hear at least three anchors (to perform triangulation), but not all  Problem: Errors accumulate

Sum-Dist C A B A: 5 5 B: 6+4 = C: = Nodes  add hop distances  require range measurement

Centroid Localization  Receive beacon from anchor nodes. It is simple and easy to implement Position is estimated in the center of area where circles from which signal is heard overlap.

DV-HOP A B A-B: 12 3 hops avg hop: 4 Anchors  flood network with known position  flood network with avg hop distance Nodes  count #hops to anchors  multiply with avg hop distance

 Local lateration Refinement  Initial estimate A  Receive neighbour positions  Broadcast new position

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