I Know Where You Are

An important aspect of pervasive computing is the ability to provide context-aware services. Location plays an important role in providing context-aware services. For example, what you do at the lab varies greatly from what you do in your living room at home. Likewise, the type of activities you do at home differ from the activities you do at the gym. Therefore, knowing where you are helps in deciding the kind of information and services that you would find useful.

There are a few techniques to pinpoint a location. Time-of-flight measures the distance between an object to a point P by measuring the time it takes to travel between the object and P at a known velocity. For example, sound travels at a speed of 344 m/s. Therefore, an ultrasound pulse transmitted by an object that arrives at P 14.5 ms later means that the object is 5 m away from P.

Attenuation is 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 emission. For example, a radio signal is attenuated by a factor proportional to 1/r², where r is the distance from the source emitting the signal. It is not as accurate as using time-of-flight due to obstructions in an indoor environment, reflection, refraction, and multipath propagation.

Angulation uses angle or bearing measurements instead of distance. A 2D angulation involves two angle measurements and one length. A 3D angulation requires one length measurement, one azimuth measurement and two angle measurements. Azimuth is a horizontal angle measured clockwise from a north baseline. North is 0^o/360^o azimuth; east is 90^o, south is 180^o.

Example: Phased antenna arrays comprising of multiple antennas with known separation measure the arrival of a signal. The angle from the source of signal emission is calculated based on the differences in arrival times and the geometry of the receiving array.

The Global Positioning System (GPS) is one way to identify location. GPS is a constellation of 24 satellites orbiting the earth. At any one time, 4 satellites cover a certain portion of the earth’s surface. A GPS receiver determines its location based on signals received from the 4 satellites.

A GPS receiver uses 3D lateration to calculate its position – latitude, longitude, and altitude. A GPS receiver usually uses signals from 4 satellites to improve accuracy and provide altitude info — position accuracy of 10-meter radius.

How a GPS receiver determines its location:

• A GPS receiver picks up radio wave signals that travel at the speed of light from the satellites.
• It 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 receiver.
• When the satellite starts transmitting the code, the receiver starts to run the same code at exactly the same time.
• When the satellite’s signal reaches the receiver, its signal pattern will lag 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).
• The calculation requires the receiver’s clock to be synchronised with the satellite clock.

A positioning system provides the means to determine location and leave it up to users to compute their actual position, e.g., GPS. There are 2 approaches:

1. Self-positioning: a MS uses signals transmitted by gateways/antennas to calculate its own position,
2. Remote positioning system: the position of the MS or tagged object is determined by measuring signals detected by a set of receivers.

A tracking system monitor objects in their purview without involving the tracked objects in the computation.

How GPS Works