Global Positioning System Reference
In-Depth Information
The main disadvantages of all compasses are due to the erratic nature of the
field. Iron structures, electrical plant, and northerly latitudes (where the horizontal
field component is weak) all cause problems. A further longer-term concern is the
gradual weakening of the field over the past decades, which may be a precursor to
a field reversal during which time the field is likely to be unusable. Each year the
magnetic poles move, and this is another factor that has to be taken into account.
Compasses will not disappear and the trend may well start to increase when
new applications are exploited as basic positioning creeps downmarket and starts
to be included in new generations of consumer devices, especially toys and games.
7.6.2
Electric Field Proximity
Positioning of objects by close physical proximity, using electric fields, is a
technique that is increasing. For example, a touch pad or touch screen can detect
the close proximity of a hand or finger. In a vehicle, an infant seat can detect the
presence of a baby. Oscillating electric field devices are used to detect the changes
in capacitance that occur when many objects are moved within a short distance of
a sensor, which is usually a simple pair of conductive plates.
The advantage of these techniques is that no moving parts are needed since
actual touch is not needed. The latest device to exploit this technique is the screen
of the Apple iPhone. It is likely that the use of active surfaces will increase and
together with active flooring will be part of smart buildings and furniture of the
future.
7.6.3
Oscillating Magnetic Field Positioning
Digitizing tablets are used as peripheral devices to input graphical and map
information into computer applications (see Chapter 8). These tablets are flat
surfaces with embedded magnetic field windings (i.e., coils of wire) below the
surface. A moving puck or pen is used by the operator and it is also fitted with an
embedded winding. They operate using a magnetic field that is generated by the
windings connected to an electronic oscillator. As the changing field propagates
across the surface, interaction with the moveable coil (in the pointing device) is
detected because the subsurface embedded coil acts as both a transmitter and
receiver of magnetic energy. The moving coil can either be passive or active
depending on the exact design, and a degree of orientation detection is also
possible. Cartesian position is outputted by the sensing system and precision is
very high (this is the most accurate positioning system discussed in this topic).
Room-sized experimental versions of this technology have been used for
detailed indoor positioning experiments. The ideal situation is to use coils that are
located at the edge of the space, but the problem is that the power needed to
generate a field over extended distances is large and the coils inconveniently
large. It is also inconvenient to use a mesh of coils spread out under the floor.
Unlike the pressure-sensitive floor, the subject being tracked must carry a coil that
