Global Positioning System Reference
In-Depth Information
6.1.2
How Does Radio Provide Positions?
How does a radio system actually function and how can it provide a position?
Firstly, the electronics in a transmitter generates a rapidly changing RF signal in
the required frequency band and it is modulated with any required information at
the appropriate power. The signal is carried by a “feeder” cable or waveguide (a
hollow pipe used for microwave transmissions) from the transmitter to the antenna
(or array of antennae) that is usually made of metal and shaped in special
configurations. They are often tuned to resonate over a particular frequency band.
The electromagnetic currents that circulate in the antenna cause electromagnetic
radiation to be radiated into free space and the antenna configuration and shape
greatly influence the efficiency (or gain) and directionality of the transmitted
signal, as shown in Figure 6.3. It can be seen for equation (6.2) that the power
gain of a typical parabolic antenna (such as would be used for a RADAR would
be typically 40dB or 10,000 times).
At the receiver the opposite process occurs. The incident wave induces small
electrical currents in the receiver antenna that are then conveyed by a feeder to the
receiver, which is at its heart a powerful amplifier to restore the level of the signal
to useful levels and a filter to select just the required band of frequencies from the
host of unwanted signals usually present. The high directionality of both
transmitter and receiver antennae is put to good use in positioning systems to
measure angles with the highest gain antennae giving the most accurate angle of
arrival readings. There is reciprocity in the way both (passive) transmit and
receive antennae function and in many transceivers a common antenna is used.
Active antennae, however, are used only in receivers and involve amplifier
circuits integrated with the metal receiving conductor and do away with the need
for an expensive low-loss signal feeder. They can be useful since they act over a
very wide set of bandwidths.
Radio waves are also polarized, which is why, for example, some antennae
appear horizontal and others vertical. The electromagnetic waves are thought of as
having two components at right angles to each other: one electric and the other
magnetic. In a conventional vertical dipole, for example, the electric field is
vertical. Circular polarization is also possible, a hybrid of vertical and horizontal
polarization, and is often used in mobile situations where constant alignment is
difficult.
