Global Positioning System Reference
In-Depth Information
ultrasound of increasing intensity until a reliable reflection is received and
distance is calculated.
7.3.2
The Cambridge Bats
Active Bats was the name given by a research team [2] at the University of
Cambridge led by Hopper for an ultrasonic tag system. It was an indoor
positioning system that operated in a way similar to the IR Active Badge systems
but with far greater accuracy. The IR system could position only to within the
confines of a room whereas the Bats system achieved approximately 3-cm
accuracy in three dimensions. The Bats are small tags that contain an ultrasonic
transmitter and a telemetry radio receiver.
To achieve the extreme accuracy, a very dense network of ultrasonic receivers
is needed that are mounted on the ceiling in a grid pattern of average density 0.6
m
-2
. A radio signal is used by the central control system to poll each tag in turn
that then emits an ultrasonic pulse. Each pulse is received by several ceiling
receivers and the flight times calculated. Radio is used only for synchronization
and identification, not for positioning per se.
Calculation of position is by circular multilateration and is performed
centrally. At least three ceiling receivers must have a distance reading from a
given tag for successful positioning to occur (see Figure 7.2). Starting with the
first receiver, it can be assumed that the tag will lie on the surface of a sphere
centered on the receiver with a radius of the distance measured by the flight time.
If a second receiver is then considered, then a second spherical locus centered on
that tag will intersect the first sphere. Position will lie on the points of intersection
that is a vertical circle (since the ceiling is horizontal). The distance measurement
from the third receiver will result in another spherical locus that will intersect the
vertical circle at two points: one above the ceiling (which can be ignored since it
is an impossible position) and one below, which is the true position of the tag. If
full 3D positioning above the ceiling were required, a fourth reading would be
needed.
