Positioning MAR MARK K ESS ESSIEN IEN Seminar Location-based Services (S19567) Instructor: Prof. Dr. Agnès Voisard 11.11.2009 1
Table of Contents Table of Contents • Introduction 1 A quick overview of positioning systems • Satellite Based Positioning 2 Positioning with GPS • Network Based Positioning 3 GSM and Wireless LAN based positioning techniques • Indoor Positioning 4 Infrared, Radio, Ultrasound, and Video based techniques • Conclusion and Future Perspectives 5 2
INTRODUCTION TO POSITIONING 3
What are positioning systems? These are systems that allow us detect the location of a person or object. There are lots of different such systems in different areas. Here we will talk about the systems we need for Location Based Services. 4
Such as: Indoor Positioning Network Based Satellite Systems Positioning Positioning Systems Systems 5
How do we do get locations? Sensor Network tracks Example: the device Infrared Tracking Example: RFID Location Techniques Example: GPS Positioning Example: WIPS Device discovers its own position 6
Basic techniques to discover locations Cell of Origin 7
Basic techniques to discover locations Time of Arrival 8
Basic techniques to discover locations Angle of Arrival 9
Basic techniques to discover locations Measuring Strength 10
Basic techniques to discover locations Processing Video Data 11
Satellite Positioning 2 12
Advantages • Can be used anywhere on earth • Not disturbed by enviromental conditions • Precise Disadvantages • Expensive to launch satellite • Cannot position indoors 13
Basic Mechanism of Satellite Navigation The user knows the distance of the satellite to him, as well as the position of the satellite So he can calculate a radius is somewhere on. But he does not know where on the radius he is. 14
Basic Mechanism of Satellite Navigation By looking at the intersection of the coverage radius of at least 3 satellites, he can discover his exact position 15
How does the device know the position of the satellite? Satellites are on fixed, known orbits. Additionally, the position of all satellites is updated by sending a so-called almanac with currrent position information to the device. (By the way, if you don’t use your GPS device for a long time, it needs to download this almanac, which is why it takes much longer to start) 16
What about the distance r from the satellite? r = c * t r = Distance from Satellite c = Speed of Light t = Time it took signal to reach device 17
Problems • The speed of light is very high. An error of 1 μ s leads to a 300m inaccuracy • Satellites have atomic clocks, and so their times are very accurate, but the devices do not. The clocks are hence not synchronised. • To correct the unknown factor, a fourth satellite signal is drawn in, and equations that factor in the time offset built • These non-linear equations can be calculated using Kalman filters or Taylor series 18
Global Positioning System • A system of 21 to 30 satellites in orbit around the earth and providing positioning Information. • Conceived in 1970, satellites launched in 1984, 12 working satellites by 1990 • Full operational capability in July, 1995 • Made up of 3 segments 19
Space Segment / The GPS Satellite • Weighs between 1.5 and 2 tons • Energy supplied by solar cells • Central computer is a 16 Mhz CPU • Expected lifetime of 7.5 years • 12 hours for an orbit • 60 days to launch after failure • Programmed in ADA • About 25.000 lines of code. In comparison, Microsoft Office has 30 million lines of code. - 20
User Segment / The GPS receiver 21
Control Segment /GPS Master Control Station Schriever Air Force Base, near Colorado Springs, U.S.A 22
GPS Properties • SPS (Standard Positioning Service): Available for civilian users – Less Accurate (100m horizontal) • PPS (Precise Positioning Service): 22m horizontal. For military use • Data channel with 50bps 23
GPS Accuracy Accuracy affected by clock errors, fluctuation in satellite orbit, disturbances of the atmosphere/ionosphere and multipath errors 24
Improving GPS accuracy with DGPS DGPS involves a system of base stations with fixed, known positions that broadcast correctional information to the devices directly. 25
Correction Stations in Australia AMSA's Differential Global Positioning System provides a network of radio beacons that improve the accuracy and integrity of the Global Positioning System (GPS) around selected areas of Australia's coast. 26
Improving GPS accuracy with WAAS • Base stations calculate correction data, then transmit it to geostationary satellites, who then pass it on to the devices 27
Selective Availability • Artificial Distortion of GPS signal by U.S government • Switched off in 2000 • New Satellites being launched 2009 do not have this capability any more 28
Other Satellite Based Navigation Systems • Abandoned in 2007 by industry, taken • GLONASS – Russian alternative to GPS • Launched in 1996 over by EU • Slated for operation start by 2013 • Financial problems, only 10 satellites • Bases in Germany and Italy by 2000 • Partners include China, Israel, Ukraine, • Partnership with India, 12 satellites Morroco, South Korea added by 2008 29
Network Based Positioning 3 30
Network Based Positioning Using existing networks for positioning Wireless LAN networks The GSM network 31
Overview • Already covers a wide area • User knows his position already, based off the cell he is in • However, cell accuracy from 1km to 35km, so very inaccurate 32
Improving Accuracy Sony Ericsson developed the MPS (Mobile Positioning System) that improves the accuracy. It does so using multiple methods. – Detect Cell – Detect the segment antennae user is, allowing an angle of antennae to user – Use Timing Advance to determine distance. Accuracy is circa 555m – Signal runtimes to 4 base stations 33
WLAN: Overview • Position can be detected by measuring signal strength of all wireless LAN access points • Requires a training phase where the locations are mapped to signal strengths • Realized as prototype by Microsoft, as well as with the Nibble system • Outdoor variant in use in the iPhone 2G, using a service by the company SkyHook 34
Skyhook • Used in iPhone 2G to discover the location of users even without a GPS radio • Reasonable accurate • Company drives around in metro areas and maps available wireless LAN devices and signal strengths to the GPS location 35
Coverage (Skyhook) 36
Indoor Positioning 4 37
Indoor Positioning Indoor positioning does not work with satellites because the radio signals do not penetrate the walls. So other systems are required. The indoor positioning devices can be split up into categories based on the technology in use: Infrared Radio Ultrasound Video 38
1. Infrared Beacons Infrared devices are cheap and easy to get, and so are often used for indoor positioning. Two sample system are the Active Badge System and the Wireless Indoor Positioning System (WIPS). 39
The Active Badge System • Every user carries a transmitter in the shape of a card • Infrared signal with pulse length 0.1s is sent every 15 seconds • Signal identifies user with unique code • Low cost with a long battery life 40
Active Badge: Technology • Signals do not penetrate walls, but rather reflect of walls, so can receive signals even without line of sight • Signals flood room, so positioning is accurate to room level • Long battery life because the pulse duration very short compared to idle time • Signals do not collide because of low differences in pulse duration 41
2. Radio Beacons Using radio, time of arrival as well as signal strength methods can be used to compute positions in buildings (in a manner similar to satellites). Positioning in 3 dimensions becomes possible if transmitters are on multiple floors. 42
The SpotON System • Uses the strength of the signals at the spot from where the measurement occurs • The signal strength is assumed to decrease with the square of the distance. This is not always the case, however, as there may be obstacles. • Accuracy of 3m can be achieved with this 43
RFID • Small systems with processor, memory, antennae, but without power supply • Use the energy from the radio waves • Distance of 1 meter • Often used to track objects – can be used to find out if an object has passed a certain point 44
3. Ultrasound Ultrasound devices use ultra sound transmitters to transmit the position of a user. Sensors are placed in the building, and the transmitter sends ultra sound signals on request. 45
Recommend
More recommend
