Information Technology Reference
In-Depth Information
The GPS is not suitable for indoor positioning; the signal is too weak to
penetrate inside a building. If a GPS-like system has to be used within large
spaces, there can be used a local 'constellation' of transmitters (pseudolites,
i.e. pseudo-satellites), to keep use the same receivers outdoors. As for the
GPS, a minimum of four pseudolites must be received by the navigation
system for suitable indoor GPS-based positioning.
4.2
Active Bat
The AT&T researchers developed the Active Bat positioning system using
the lateration technique by means of ultrasonic time-of-flight [4].
In this system, users and objects to be located are equipped with small
plates with radio-controlled ultrasonic pulse transmitters (the Bats). A local
controller sends a short-range radio signal to the Bat. Once received the RF
signal, the Bat sends back an ultrasonic pulse that is received by a grid of
receivers mounted on the ceiling.
At the same moment in which the local controller sends the radio signal,
the sensors on the ceiling start measuring the time interval between the radio
signal and the received ultrasonic pulse, thus allowing the estimation of the
Bat position. This task is carried out by a central controller by means of the
trilateration technique. Of course, at last three sensors must be in range to
carry on the position estimation task.
The nominal accuracy of Active Bat is 9 cm with a precision of 95%. The
systems are suitable for identification purposes too, since each Bat has a
unique identifier to be recognized.
The use of time-of-flight approach with ultrasonic fixed sensors requires a
large infrastructure installed throughout the ceiling. Furthermore, the
infrastructure must be placed in a suitable layout.
Discussions above show that Active Bat is a good positioning system
concerning accuracy and precision, but it has short range, high infrastructural
cost and difficulty in actual deployment.
4.3
RADAR
In early 2000s a research group at Microsoft has developed RADAR [9], a
tracking system for large buildings based on the wireless technology W-LAN
IEEE 802.11 which makes use of scene analysis technique.
RADAR is based on the principle that the received signal strength
depends on the distance between the receiver and the transmitter. Actually
RADAR uses both the received signal strength and the signal-to-noise (S/N)
ratio to estimate the 2D position of a mobile device within a building.
