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only a standard omni-directional VHF aerial is required; the beacon’s audio sub-carrier
can also be used to transmit information such as the weather conditions at local airfields.
Although VOR is a line-of-sight system, VOR beacons are given a protected range to
prevent problems of mutual interference with other beacons. Clearly, because the VOR
system is passive (i.e. the aircraft does not transmit), there is no limit to the number of
users of VOR bearing information.
The bearings received from conventional VOR beacons are still susceptible to some
errors mainly through multi-path reflections usually termed “siting errors”. One solution is
to use a different design of beacon known as Doppler VOR. In these beacons, the
rotating loop is replaced by 50 aerials placed on the diameter of about 14 metres.
Transmissions are made sequentially as phase differences so that the Limacon is
reproduced but as a FM signal, and the omni-aerial now transmits an AM reference. The
aircraft receiver cannot tell the difference between conventional and Doppler VOR
beacons and merely compares the two signals as before to produce a bearing. The main
advantages of the DVOR are the large aperture of the aerial which reduces multi-path
effects and the elimination of moving parts giving a higher reliability of the ground beacon.
Typically (with a conventional beacon) the ground beacon will be in error by ± 3° through
multi-path and calibration, and the aircraft receiver contributes another ± 3° through the
inaccuracy of phase difference measurements; this gives a 95% error value of ± 4° for
bearings at the output of the receiver. While this is satisfactory for using to home
overhead a ground beacon, bearing errors of this magnitude are not very satisfactory for
position fixing as the across bearing distance errors increase with increasing range from
the beacon.
VORs use a 50 kHz channelisation. VORs based on an airport (used for short-range
application) tend to use powers in the range of 25-50W; for en-route VORs, this value
increases to 100-200W.
3.4.3.4 L-Band DME
DME (Distance Measuring Equipment) is a system whereby paired pulses at specific
frequencies are sent out from the aircraft (i.e. an airborne interrogator) and received at
the ground station, which then transmits paired pulses back to the aircraft at the same
spacing, but on a different frequency (offset by 63 MHz). The time for the round trip is
measured by the airborne DME unit, and calculated as distance. Only slant range is
displayed, meaning that DME suffers from limitations at short ranges (i.e. when the
aircraft is directly overhead, as slant distance becomes equal to altitude). The pulse
repetition rate (unique to each aircraft) varies from 5-150 per second, allowing up to 100
aircraft to be handled by one DME station.
The Distance Measuring Equipment (DME) developed for military purposes as part of
TACAN was still classified equipment in the late 1940s, and was not made available to
civilian users until the 1950s.
DME operates in the 960 – 1215 MHz band, which means that, like VOR, DME is a lineof-sight
system. DME is an active system; i.e. the aircraft has to transmit to obtain
information. The transmission (or interrogation as it is called) consists of a pair of pulses.
On receipt of such a pulse-pair exceeding a preset amplitude, the beacon responds after
a pre-set delay with an equivalent pulse-pair. In fact, for transmitter efficiency, the ground
beacon transmits continuously producing pulse-pairs mainly as random noise. As more
aircraft interrogate the beacon, its receiver gain is reduced until about 100 aircraft
responses are handled and the beacon is producing replies and no noise; this places a
practical limit on the number of aircraft that can use the beacon.
The aircraft receives its replies amongst many other pulse-pairs (noise and replies to
other aircraft). The aircraft receiver recognises its own replies by integrating over a
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