Global Positioning System Reference
In-Depth Information
crossings so the zero-crossings of the reference and incoming signals could be
timed. As the receiver moves the cycle offset will change with distance and repeat
for each wavelength moved. Very accurate positioning can be achieved by this
interference-like technique that has been developed from radio-astronomy. A
more simplistic approach to radio positioning need not involve either the
measurement of timing or angles but rather rely on the absence or presence of a
signal when the receiver and transmitter are within a certain degree of proximity.
Usually it is not strictly the true absence of signal but rather the fact that the power
level of the signal has fallen below a level threshold set in the receiver. Many
receivers contain received signal strength indication (or RSSI) circuits that give
readings dependent on the level of the incoming signals. Sometimes these are
displayed to the user and can aid manual frequency selection or tuning. The
accuracy of the proximity measurement is usually very poor because of the wide
variability of signal strengths in the environment. Nevertheless, it is a simple and
cheap approach and is used widely since it needs no extra equipment for the
positioning function. Usually antennae are used that have low directionality or are
omnidirectional.
6.1.3
The Radio Propagation Environment
One of the most inconvenient aspects of radio does not concern the equipment at
each end of the links but rather what happens to the signals in between. If our
positioning systems were operating in deep space with a vacuum and no material
between or anywhere near the antennae positioning would be idealized. In free
space, the power density (
p)
of the signal falls as the square of the distance (
r
)
between transmitter and receiver
(6.3). It is thus possible to accurately map the
radio-universe with highly directional radio telescopes. Unfortunately terrestrial
signals are subjected to a number of disturbing terrain factors.
2
p
∝
1
r
(6.3)
Although in general signals travel in straight lines, being waves they can diffract
and reflect off surfaces and edges. This can be an advantage for mobile
communications, since in dense urban situations there is very rarely a free line of
sight between transmitter and receiver. Coverage in buildings, for example, is
made up of multiple reflections and refractions. For positioning this is very
unhelpful since the line of sight is the shortest path and it is mostly absent, with a
whole host of signal fragments coming from many directions with differing flight
times. Proximity detection may be the only viable option, in which case a large
number of low-power transmitters are helpful.
