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equipment, but ATC must be asked
first. It is always subject to traffic.
When asked to squawk ident, your
return becomes temporarily brighter,
so you can be positively identified.
PAR
Precision Approach Radar is primarily
used by the military, which is highdefinition
in nature, because it uses
the 9-10 GHz range and has a 3cm
wavelength, similar to weather radar
(below), so weather clutter can be a
problem (meaning you may be
denied its use when you need it
most!). It can give horizontal and
vertical guidance to aircraft on final
approach up to about 10 nm within
20° of final track and 7° of elevation.
It's supposed to be a landing aid, not
one for sequencing or spacing
aircraft, and it may also be known as
Airport Surveillance Radar (ASR).
The controller will give you the
headings and heights to fly, and in
the final stages you will be told not
to acknowledge the instructions as
things will be happening hard and
fast. Its usefulness lies in the fact
that navaids are not required and
neither is a compass, since the
controller tells you to turn left, right,
etc. Also, since acknowledgements
are not required, you can listen to
instructions over an ADF or VOR if
your radio doesn't work. Frequencies
and Decision Heights are in the AIP
under GEN.
Weather Radar
Although it shares the same name,
this is not a good system for
detecting other aircraft or ground
returns because it is tuned to the
average size of raindrops (it uses the
SHF band at around 9375 MHz, so
the wavelength is about 3 cm). If
used for navigation, it is only a
secondary means.
The antenna (scanner) is kept inside a
radome in the nose of the aircraft, and
there is a box containing the
transmitter/receiver, together with a
scope in the cockpit. The scanner
sweeps through 45-60° either side of
the nose – in weather mode, the
beam is narrow and cone-shaped.
For mapping, wide and fan-shaped,
but for long range mapping, you
should use weather mode.
It detects rainfall to avoid (not
penetrate) severe weather, as large
raindrops in a small area are a dead
giveaway for thunderstorms or,
rather, their activity is - turbulence is
proportional to the rate at which
rainfall increases or decreases over a
given distance. A clear area on the
radar screen doesn’t mean there isn’t
any cloud, as minute cloud droplets,
ice, dry snow and dry hail have low
reflective levels, if at all. It is more
likely to indicate large water droplets,
as they will totally absorb the energy
as they approach the size of the
radar wave. Absence of returns
produces a use for the stray side
302 Canadian Professional Pilot Studies
lobes mentioned above, in that the
downwards one produces a ring on
the screen at the same range as your
height above ground, so you can
check if the equipment is working.
Operation is quite simple, but full
use on the ground should be avoided
(not below 500 feet, in fact).
Naturally, you've got to check the
equipment before departure, but
most sets have an internal procedure
for this. When you do switch it on, it
should be set to Standby for at least
3 minutes first, to allow things to
warm up. Ground testing requires
tilting up in weather mode.
You will have several scan ranges to
choose from, possibly from 250
miles down to 5, but 80 is adequate,
which is about what you would get
with a 10 inch antenna, the usual fit
in small aircraft. The smaller it is, the
wider the beam and the dispersal of
energy, which means that a lot of it
will pass by whatever storm is
around, giving you an indication very
much less than the true hazard. You
would be safe in assuming that
whatever you see is one or two levels
more severe.
Once airborne, there is a tilt
capability which will point the
antenna up or down so you can get
more detail about any approaching
storm cells, but don’t expect to see
the tops of a storm, because the
crystals won’t reflect the energy in
the first place, and your beam
focussing will be too narrow to
include it. In the same way, you will
also get ground echoes, which are
good for detecting the enemy coast
ahead, but only because water will
absorb the echoes and you will see a
big black hole instead. Buildings and
the like won’t reflect properly at all –
you might just see a mass of
　
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