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the opposite end, you will be using a
backcourse, with the needles reversing
their sense.
Category I
This caters for a 200-foot Decision
Height and RVR 2600', with a high
probability of success. If RVR is not
available, substitute ½ mile visibility.
Category II
This takes you further down, to as
low as 100 feet DH and RVR 1200.
Not surprisingly, you need special
training for this. You also need the
following components to be up and
running:
·  Lighting; approach and
threshold, touchdown zone,
centreline, runway edge and end
lights.
·  ILS; localiser, glidepath and
middle marker.
·  RVR; two transmissometers,
one at the threshold, and one at
the mid-point.
·  Power; airport emergency power
as the primary source for
essential elements, with
commercial power available
within one second.
VDF
ATC can get a bearing for you to
steer (QDM) to get to their location
from your transmissions, when using
VHF Direction Finding equipment.
Being based on VHF, it is subject to
the usual limitations (see above), so
the higher you are, the better the
results you will get. You need to
transmit for a few seconds for a
bright line to spread from the centre
of a screen to the outside which is
marked with compass bearings.
Older equipment will use a cathode
ray tube on which the line appears
(rather like a radar sweep) pointing
to where your transmission is
coming from. More modern digital
equipment uses a circle of LEDs at
10-degree intervals, which will show
the same information, with a digital
readout in the centre. The controller
can store the last transmission, in
case of being busy with something
else at the time. Radar can help by
giving distance information as well.
Accuracy comes in three classes, A,
B or C. Alpha is ± 2°, B is ±°5 and
C is ±10°. You will only be told the
classification if the bearing is B or C.
Radar
The use of radar improves aircraft
spacing and improves safety - the
word stands for Radio Direction and
Ranging, without which, it is arguable
whether the Allies would have won
World War II (see Bibliography). It
was called RDF (Radio Direction
Finding) until 1943, when the name
was changed to harmonise with the
Americans. It works on the basis
that radio pulses will reflect off
objects and a proportion of the
energy will return to the transmitter.
Electricity & Radio 299
You can calculate the distance
between them because the speed of
the radio wave is known, and the
direction the aerial was pointing in at
the time will supply the bearing. The
“blips” representing the objects are
displayed on a Cathode Ray Tube (TV
screen) and an air traffic controller
can see the relative positions of
aircraft reflecting pulses:
The word pulses, mentioned above, is
the operative word – short bursts of
electromagnetic energy are mixed
with relatively long periods of silence
(in electronic terms, this means
somewhat less then a thousandth of
a second).
This is known as primary radar, and it
has a few limitations. First of all,
radio signals weaken over distance
and, since they have to make two
journeys, the range of a target is
necessarily limited. Secondly, the blip
is quite large, and aircraft very close
together cannot be distinguished.
Finally, radio waves can be bent by
the atmosphere or screened by
objects, such as mountains or
buildings, and different aircraft
return signals differently, in terms of
shape or surface.
A parabolic dish is used for an
antenna, for a narrowly focussed
beam, but it will spread out with
distance, and some energy escapes as
side lobes. The larger the dish, the
narrower the beam width; for example,
a 30" dish produces one 3° wide, and
12" gives 7°, assuming the same
frequencies. Advantages are
obtaining bearings more easily,
greater concentration of energy and
more range, plus better target
definition.
The range detected will be a slant
range, as per DME, and maximum
range will be determined firstly by
the Pulse Repetition Frequency (PRF),
because pulses have to return to the
transmitter before the next ones are
sent, and the Pulse Interval (PI).
Radar is also limited to line of sight,
because it uses UHF, and transmitter
　
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