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 localizationThe 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

MultilaterationAssume 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 = 10

6

4

C: 5+6+4 = 15

6

5

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

1

1

1

1

2

2

2

2

2

3

3

3

4

4

A-B: 123 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

Thank you