RADAR An In Building RF based User Location and Tracking System Paramvir Bahl and Venkata N Padmanabhan Microsoft Research bahl padmanab microsoft com Abstract The proliferation of mobile computing devices and local area wireless networks has fostered a growing interest in location aware systems and services In this paper we present RADAR a radio frequency RF based system for locating and tracking users inside buildings RADAR operates by recording and processing signal strength information at multiple base stations positioned to provide overlapping coverage in the area of interest It combines empirical measurements with signal propagation modeling to determine user location and thereby enable locationaware services and applications We present experimental results that demonstrate the ability of RADAR to estimate user location with a high degree of accuracy Keywords location aware services user location and tracking wireless LAN radio frequency wireless network 1 Introduction The proliferation of mobile computing devices and local area wireless networks has fostered a growing interest in location aware systems and services A key distinguishing feature of such systems is that the application information and or interface presented to the user is in general a function of his or her physical location The granularity of location information needed could vary from one application to another For example locating a nearby printer requires fairly coarse grained location information whereas locating a book in a library would require fine grained information While much research has focussed on developing services architectures for location aware systems e g Maa97 Nel98 less attention has been paid to the fundamental and challenging problem of locating and tracking mobile users especially in in building environments The few efforts that have addressed this problem have typically done so in the context of infrared IR wireless networks The limited range of an IR network which facilitates user location is a handicap in providing ubiquitous coverage Also the IR network is often deployed for the sole purpose of locating people and does not provide traditional data networking services To avoid these limitations we focus on RF wireless networks in our research Our goal is to complement the data networking capabilities of RF wireless LANs with accurate user location and tracking capabilities thereby enhancing the value of such networks In this paper we present RADAR an RF based system for locating and tracking users inside buildings RADAR uses signal strength information gathered at multiple receiver locations to triangulate the user s coordinates Triangulation is done using both empirically determined and theoreticallycomputed signal strength information Our experimental results are quite encouraging With high probability RADAR is able to estimate a user s location to within a few meters of his her actual location This suggests that a large class of location aware services can be built over an RF local area wireless data network The remainder of this paper is organized as follows In Section 2 we survey related work in location determination technologies In Section 3 we discuss our research methodology Section 4 contains the core of the paper where we present and analyze the empirical and the signal propagation modeling methods A discussion of extensions to the base RADAR system appears in Section 5 Finally we present our conclusions in Section 6 2 Related Work Related work in the area of user location and tracking falls into the following broad categories 1 in building IR networks 2 wide area cellular networks based on RF and 3 Global Positioning System GPS The Active Badge system Wan92 Har94 was an early and significant contribution to the field of location aware systems In this system a badge worn by a person emits a unique IR signal every 10 seconds Sensors placed at known positions within a building pick up the unique identifiers and relay these to the location manager software While this system provides accurate location information it suffers from several drawbacks a it scales poorly due to the limited range of IR b it incurs significant installation and maintenance costs and c it performs poorly in the presence of direct sunlight which is likely to be a problem in rooms with windows Another system based on IR technology is described in Azu93 IR transmitters are attached to the ceiling at known positions in the building An optical sensor on a headmounted unit senses the IR beacons which enables the system software to determine the user s location This system suffers from similar drawbacks as the Active Badge system The system described in ATC97 is based on pulsed DC magnetic fields Multiple sensors are placed on bodymounted peripherals such as data gloves and their output is processed to determine a person s location and orientation with a high degree of precision This technology is used extensively in the computer animation industry It is however quite expensive and like IR severely range limited hence unsuitable for large scale deployment Recently several location systems have been proposed for wide area cellular systems Tek98 The technological alternatives for locating cellular telephones involve measuring the signal attenuation the angle of arrival AOA and or the time difference of arrival TDOA While these systems have been found to be promising in outdoor environments their effectiveness in indoor environments is limited by the multiple reflections suffered by the RF signal and the inability of off the shelf and inexpensive hardware to provide fine grain time synchronization Systems based on the Global Positioning System GPS99 while very useful outdoors are ineffective indoors because buildings block GPS transmissions The Daedalus project Hod97 developed a system for coarse grained user location Base stations transmit beacons augmented with their physical coordinates A mobile host estimates its location to be the same as that of the base station to which it is attached Consequently the accuracy of the system is limited by the possibly large cell size Our work differs from previous work in that we tackle the problem of user location and tracking on a widely available radio frequency based wireless network in an inbuilding environment RF networks offer a significant advantage over IR networks in terms of range scalability deployment and maintenance With speeds of up to 11 Mbps these