Location-Sensing and Location Systems 1

A positioning system provides the means to determine location and leaves it to the user device to calculate its position, whereas a tracking system monitors objects without involving them in the computation. A location-sensing system is a hybrid of a positioning system and a tracking system. Location-Sensing and Location Systems 2

An important aspect of mobile computing is to give information that is relevant to the user’s context. In most cases, the user’s location is an important factor in determining the user’s context. For example, if I am in Ankara and I request for a list of restaurants Location-Sensing and Location Systems 3

For the system to deliver location-sensitive information, the system has to be able to detect my location and tailor the information delivered to me to meet my needs. Location-Sensing and Location Systems 4

There are two approaches to implement a positioning system: self positioning and remote positioning. In a self-positioning system, a mobile device uses signals transmitted by gateways or antennas to calculate its own position. In a remote positioning system, the position of a mobile device or a tagged object is determined by measuring signals detected by a set of receivers. Signal measurementsare used to determine the location of the object of interest. Location-Sensing Techniques 5

There are three classifications of location-sensing techniques: triangulation, proximity, scene analysis Location-Sensing Techniques 6

Triangulation is divided into lateration and angulation. Lateration involves using distance measurements. The 2D position of an object is calculated by measuring its distance from three noncollinear (not on the same line) points. “You’re 30 km from Bangi.” You draw a circle on your map to mark your possible location. Triangulation 7

“You’re about 40 km from Kajang.” That is slightly better. You can narrow down your location to the intersection area of the two circles (Figure 9.1b). A third person tells you that you are about 10 km from Serdang. This is how 2D lateration works. Triangulation 8

2D lateration 9

Four non-coplanar points are required to calculate a 3D position (latitude, longitude, and altitude) of an object. A 2D position would be sufficient, for example, in an implementation involving an active badge that tracks the location of users or objects in an office environment. On the other hand, a 3D position would be more helpful when sending a team to rescue climbers trapped in an avalanche. Triangulation 10

There are four approaches to measuring distance: Distance measuring 1) Direct: Involves using a physical action or movement to take a measurement. 2) Time-of-flight: Involves measuring the distance between an object (stationary or moving) to a point P by measuring the time it takes to travel between the object and P at a known velocity. For example, because sound travels at a speed of 344 m/s,. 11

Distance measuring Clock synchronization becomes an issue for the distance measuring. GPS satellites are precisely synchronized with each other and transmit their local time in the signal so that receivers can compute the difference in time-of-flight. A GPS receiver calculates a 3D position using four satellites. 12

Distance measuring 3) Attenuation (loss of intensity): The decrease in signal intensity as the distance from the signal source increases. It is possible to measure the distance between an object and P using a function that correlates attenuation, the distance for a type of emission, and the original strength of the emission. 13

Distance measuring 4) Angulation: Involves using angle or bearing measurements instead of distance. A 2D angulation involves two angle measurements and one length measurement. 3D angulation requires one length measurement, one azimuth measurement, and two angle measurements. 14

Proximity Proximity is a location-sensing technique that involves determining when an object is near a known location. There are three approaches: 1. Detection of physical contact using pressure sensors, touch sensors, and capacitive field detectors. 2. Monitoring wireless cellular Access Points (AP) involves detecting when a mobile device is within the range of one or more APs in a wireless cellular network. 15

Proximity 3. Observing automatic ID systems involves using automatic ID systems such as credit card point-of-sale, telephone records, computer login history, and use of ATM card. For example, the location of a person driving on a highway can be inferred from the last time that person used a Touch-n-Go card because the scanner that reads the card has a static, known location. 16

Scene Analysis Scene analysis uses features of a scene observed from a vantage point to deduce the location of the observer to objects in the scene. There are two types of scene analysis: 1) In static scene analysis, observed features are cross- referenced to a predefined dataset to map them to object locations. 17

Scene Analysis 2) In differential scene analysis, the differences between successive scenes are tracked to estimate location. The differences correspond to movements of the observer. If the features are identified as being at a specific location, the observer can determine its own position relative to them. 18

The Global Positioning System (GPS) is a space-based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver. Location-Sensing Techniques 19

GPS: An Example of a Positioning System The GPS uses 24 satellites orbiting the earth. It was developed by the U.S. military as a military navigation system but has been made available to the public. The satellites are at a distance of 20,000 km above earth’s surface. At any one time, four satellites cover a certain portion of the earth’s surface. A GPS receiver determines its location based on signals it receives from the four satellites. 20

GPS: An Example of a Positioning System A GPS receiver picks up radio wave signals that travel at the speed of light from the satellites. The receiver determines its distance from the satellite based on how long a signal transmitted by the satellite takes to reach it. A satellite transmits a signal, termed a pseudorandom code, at predetermined intervals known by the receivers. When the satellite starts transmitting the code, the receiver starts to run the same code at exactly the same time. 21

GPS: An Example of a Positioning System When the satellite’s signal reaches the receiver, its signal pattern will lag (delay) slightly behind the code the receiver is running. The amount of time the signal lags behind (the delay) is equal to the signal’s travel time. The receiver calculates its distance from the satellite by multiplying the delay by the speed of light (300,000 km/s). This calculation requires a receiver’s clock to be synchronized with the satellite’s clock. 22