WI-FI BASED POSITIONING SYSTEM

Presentation by: RAVITHEJA.H.G 1KS08EC075

Today’s Topic

Positioning Technology Global Positioning System Mobile Positioning System Wi-Fi Positioning System History of WLAN positioning Wi-Fi Positioning Techniques Cell ID based Positioning Triangulation Trilateration Wi-Fi Fingerprinting Applications of WPS Hybrid Positioning System

Positioning system is a navigation system that provides location information in all weather, anywhere within the coverage of the network.

Positioning Technology

Positioning technology types :

• Global Positioning System (GPS)

• Mobile Positioning Systems CDMA Network / CDMA2000 GSM Network

• Wireless Positioning Systems Wi-Fi Positioning Bluetooth Sensor Networks & RFID

• Hybrid Positioning Systems

Global Positioning system (GPS)

• The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites.

• A GPS receiver measures the distances from 4 or more satellites and utilize a mathematical method, called “Trilateration” to calculate the actual position.

Limitations of GPS

Signal Propagation Errors :

Blind Spot Problem: signals from the satellites would be blocked – buildings (indoor), urban canyons, tunnels, flyovers, garage…etc

Multipath Problem: Non line-of-sight problem - signals reflected by buildings.

Signal Delay caused by the Ionosphere and Troposphere.

Satellite errors : Although the clock used in GPS satellites are of high quality, they are not perfect. The clock error and satellite ephemeris error leads to few meters (tens of feet) of inaccuracy.

Receiver errors : These errors are due to GPS receiver clock mismatch and internal noise.

Mobile Positioning Systems (MPS)

• Mobile Positioning or Mobile phone tracking refers to the attaining of the current position of a mobile phone, stationary or moving. Localization is determined via multilateration of radio signals between (several) radio towers of the network and the phone.

• Mobile positioning, which includes location based service that discloses the actual coordinates of a mobile phone bearer, is a technology used by telecommunication companies to approximate where a mobile phone, and thereby also its user (bearer), temporarily resides.

Transmitter

Estimated mobile location

r1

r2r3

Applications and limitations of MPS

Applications :

It is widely used in mobile phone navigation systems. The maps in mobile phone system basically use MPS technology.

It even used by Police Department to track a lost mobile phone.

Limitations :

Though MPS is known for its accuracy in tracking it has some limitations when it comes to localized tracking.

For example consider a localized area such as a university campus or an office. Navigation system using MPS in such a small area is expensive and complex.

Wi-Fi Positing System

Wi-Fi-based positioning system (WPS) emerged as an idea that can solve

the positioning in certain situations (like indoors), taking advantage of the rapid

growth of wireless access points in urban areas.

Technology for both Indoor / Outdoor is required

1m 10m 100m

Estimation Accuracy

Env

ironm

ent

PHS

GPS

Outdoors

Indoors

CellPhone

Ultra Sonic

RFID

UWB

Wireless LAN

now

2000:RADAR(Microsoft)

2000:Ekahau

2005/7:Locky.jp(Nagoya Univ)

2006/7:PlaceEngine(Sony CSL)

History of WLAN positioningStart around 2000 (MS RADER)

2006/3:Loki(Skyhook Wireless)

2003:AirLocation(Hitachi)

2003:PlaceLab(Intel)

Service/Activities

Products

Technology

2007/9:Digial Camera(Sony)

2008/1:iPhone (Apple)

Positioning methods using WLAN

Most popular techniques used in Wi-Fi positioning :

Cell ID (Proximity)

Triangulation

Trilateration

Fingerprinting

Proximity (Cell ID Based Positioning)

Cell ID positioning or Proximity sensing is a positioning technique based on the unique ID/address of an Access Point.

Each AP has a unique ID/address known as Media Access Control address (MAC address).

Depending on the signal strength, the location of the Client/User is determined by reporting back the position of the closest AP.

Wi-Fi positioning - Cell ID

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points. 

In Triangulation the angles (α and β in figure ) of two sites A and B between the location in question are measured. With these angles it is possible to calculate the position of the desired location when the distance of the two points is known.

γ

A

B

αβ

γ

Client/User

d

e f

Known Distance

Angle γ, distance e and distance f are calculated using properties of Triangle

Therefore the Client/User location is estimated with respect to AP’s A and B

Access Point

Access Point

Angles α and β are measured

Trilateration

In geometry, trilateration is the process of determining absolute or relative locations of points by measurement of distances, using the geometry of circles, spheres or triangles.

The same geometric concept is implemented in Wi-Fi positioning system.

The distance between the Access Point (AP) and Client/user is estimated by using the time taken to send a packet from the client to the AP or the strength of the signal received by the AP from the Client/User (RSSI –Received Signal Strength Indication). Wi-Fi positioning - Trilateration

equations for the three spheres

Solving the equations, we get

Station

Home

Restaurant

Airport

RSSIUniversity

Company

Wi-Fi Fingerprinting

Wi-Fi Fingerprinting is one of the wide used Wi-Fi positioning technology based on RSSI .

Wi-Fi Fingerprinting creates a radio map of a given area based on the RSSI data from several access points.

Live RSSI values are then compared to the fingerprint to find the closest match and generate a predicted (x,y) location.

The Fingerprinting technology works in two phases Offline or Training phase Online or Positioning phase

The objective of the training phase is to build a fingerprint database. In order to generate the database, reference points (RP) must first be selected.

The Signal Strength (SS) of all the Access Points at the reference point is tabulated in the database in the form of a vector. This process is repeated at all the chosen reference points.

Offline or Training phase

Online or Positioning phase

Online or Positioning phase is the process of estimating the location of the Client/User.

In the online phase, the Client/User unit measures the signal strength(SS) from all AP’s and this SS’s vector is compared with the reference vectors in the database.

‘Nearest Neighbour’ (NN) algorithm is used to estimate the location of the user/client.

Applications

Turn by turn navigation to any address.

Locating people on a map displayed on the mobile phone.

Receiving alerts, such as warning of a traffic jam.

Location-based mobile advertising.

Hybrid positioning system

Hybrid positioning systems are systems for finding the location of a mobile device using several different positioning technologies.

These systems are specifically designed to overcome the limitations of GPS, which is very exact in open areas, but works poorly indoors or between tall buildings. By comparison, cell tower signals are not hindered by buildings or bad weather, but usually provide less precise positioning. Wi-Fi signals may give very exact positioning, but only in urban areas with high Wi-Fi density.