Global Positioning System Reference
In-Depth Information
thousands of base stations would be necessary so infrastructure costs can be
prohibitive.
2.5.5
Optical Video Cameras
Video surveillance cameras are useful to identify and track people and objects
within view. Image recognition software is becoming increasingly powerful at
dynamically extracting features from a video stream and following their
movements within the scene. By using scene analysis it is thus possible to infer
location. The approach is referred to as “outside in” video positioning. In contrast,
“inside out” positioning uses mobile cameras looking out at scenes. For example,
if a particular skyline or landscape was captured, it can then be compared with a
stored library and then the location of the mobile camera can be determined. It is
also possible to infer orientation. As mobile cameras are being incorporated into
more portable devices, this approach is likely to become more important.
Cinematic applications use a form of video surveillance for motion capture
with extremely high accuracy. Highly reflective dots may be attached to a target
(perhaps an actor) whose exact positions can be recorded, digitized, and translated
to support animation effects. This is perhaps the ultimate positioning technology
in terms of accuracy but can only be made to work in a highly controlled and
narrow-ranged environment.
2.5.6
Magnetic Fields
Oscillating magnetic fields have been used experimentally for indoor positioning.
Graphic tablets use this technique, but if the “tablet” is made as large as a room,
then it is possible to know the position of the sensing apparatus (usually an
inductor coil) by the relative timing of the changing fields.
Magnetic compasses are also useful in mobile navigation systems since the
Earth's magnetic field is almost always available except when distorted by ferrous
metals and near the poles where there are little horizontal components. Thin film
magnetic sensor technology is available cheaply so that solid, very robust state
compasses are now common.
2.5.7
Mechanical and Inertial Systems
There are a wide range of mechanical systems that can be used to track people and
objects. Active floors have pressure sensors that can monitor footfall as targets
walk across them. Although some other system must be used to identify a target, it
can then be tracked uniquely (assuming the sensor density is high enough).
Mechanical motion sensors are being integrated into portable electronic
consumer equipment. Pendulum pedometers that are used for simple step counting
are being replaced by more sophisticated and highly miniaturized solid-state chip
accelerometer devices (using microelectromechanical systems, or MEMS) that are
