Global Positioning System Reference
In-Depth Information
Inertial systems are beginning to have a major impact on consumer
electronics as the original military technology is adapted now that the basic
components are so cheap. Accelerometers (and other sensors) will be increasingly
finding their way into more and more devices like mobile phones, cameras,
gaming machines, toys, and models, and anything with a human machine interface
(HMI). All these devices will now have the potential to find position by
autonomous inertial methods, and the impact on Whereness will be profound.
7.5.1
Vehicle Positioning Using Wheel Sensing
Vehicle motion has traditionally been sensed by methods based on wheel rotation,
which gives an accurate measure of distance provided there is no wheel spin,
skidding, or sliding. Mechanical systems took a rotational feed from the vehicle
power train (after the gearbox so that that rotation was a constant ratio with
respect to the wheels) and provided the driver with a mechanical (or
electromechanical) speedometer and odometer. Some mapmaking systems have
used vehicles fitted with a fifth wheel centrally mounted at the rear of the
vehicles. They provide more accuracy because they eliminate the effects of
differential wheel rotation when turning. However, the differential effect can be
useful for navigation since it provides turning information. The ancient Chinese
south-pointing carriage [7] was a two-wheeled towed vehicle that used a
differential gear to show when a straight course had deviations. If one wheel
rotated slightly more than the other, then a curve was being followed and the
indicator pointed the need for and direction of a course correction.
The modern equivalent is the use of ABS pulses from each rear wheel. Some
antilocking breaking systems (ABSs) are electronic and use a system where the
rotation of the wheel is sensed magnetically and a series of electronic pulses
proportional to wheel rotation are generated. Differences in pulse rates between
the rear wheels allow a processor to calculate accurate turning information. Some
vehicle navigational systems used these pulses with map matching as an
alternative to GPS positioning (especially before GPS was available). The
advantage of autonomous navigation is that it relies on no external signals, unlike
GPS systems (see Section 6.2.3).
7.5.2
Pedometers
Pedometers are devices that sense and measure footsteps. They are the human
equivalent of vehicle odometers but as legs do not rotate, motion is difficult to
measure accurately. It is not usual to measure the length of a footstep directly so
an indirect method is adopted where the upward and downward motion of the hip
is detected instead.
A simple mechanical pedometer can be made using a small horizontal
pendulum that is spring-biased to be effectively weightless in the vertical plane.
