Global Positioning System Reference
In-Depth Information
VHF OMNI RADIO (VOR)
There were two trends in evidence as radio direction finding—RDF—
evolved over the decades of the twentieth century. First, as we have seen,
the task of estimating direction moved from receivers to transmitters,
reducing the complexity and thus the cost and size of receivers and increas-
ing their portability. Second, the transmitter frequencies tended to in-
crease, so that later systems generally transmitted a medium-wave signal
instead of a long-wave signal.
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Shorter wavelength means that a transmit-
ter or receiver of given size can form a sharper beam—a narrower beam-
width. There are a number of advantages to a sharper beam. Power is
concentrated where it is needed, so the system has a longer range; in a
military context the signal is less susceptible to electronic jamming; for
navigators it means that directional beams are more accurate.
VOR is a high-frequency RDF; the letters stand for ''VHF omni radio,''
with VHF meaning ''very high frequency'' (didn't I warn you about TLAs?)
''Omni'' means omnidirectional: in the older NDBs the transmitter sprayed
out a signal equally in all directions. Yet VOR was unlike the NDBs because
the signal that was transmitted depended upon direction. The transmitter
rotated in sync with a modulated carrier signal so that the phase of the
transmitted signal varied in a predictable way with bearing angle. An
aircraft with a VOR receiver could decipher the phase that it was picking up
and thus know the direction to the transmitter. Furthermore, the VOR
receiver did not have to be rotated at all; the moving parts were all in the
transmitter.
VOR became the standard RDF method for civilian aircraft navigation
during the 1950s and 1960s. Most countries adopted a two- (or even three-)
component system, each with its own receivers, and each operating over a
restricted range of aircraft altitudes. Hence, an airliner approaching a
runway would pass from one system to another as it descended. The two or
three VOR systems for civilian aircraft guidance had di√erent ranges and
accuracy requirements, and they operated at di√erent frequencies so that
they did not interfere with one another.
cities—or misdirecting them away from these cities. The original Lorenz had a short range
and was superseded by more powerful and sophisticated aircraft navigation systems that
employed the same principle. See Denny (2007) for a summary.
11. Long-wave radio covers the bandwidth 150-400 kHz, whereas medium-wave radio
covers 520-1720 kHz. Thus LW signals can have wavelengths as long as 2 km whereas MW
signals can have wavelengths as short as 175 m.
