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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,
134 Canadian Private Pilot Studies
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.
Secondary Surveillance Radar
This is a development of a system
introduced during the Second World
War called Identification Friend or Foe
(IFF), which was supposed to
distinguish between friendly and
enemy aircraft (friendly aircraft had a
small transmitter that gave a
distinctive periodic elongation to the
blip on the screen).
SSR improves on the primary radar
mentioned above by using
secondary equipment to provide
more information, hence the name.
Participating aircraft carry a
transponder (which stands for
transmitter/responder) that receives the
interrogation pulse from the transmitter,
superimposes information on it and
sends it right back on another paired
frequency. This means, first of all,
that the range of operation can be
doubled immediately, and, secondly,
that the blip on the screen can be
made much smaller, together with
information that makes it more
easily identifiable to ATC, because
the pulses can be coded. Computer
trickery can provide predicted tracks
and collision warnings, amongst
other things.
A controller for example, will give
you a number to squawk, which you
dial up on the transponder in your
aircraft, and which will appear next
to your blip with a height readout,
depending on the transponder you
have. There are standard numbers to
squawk, when not otherwise
instructed, and these are:
·  1200 – at or below 12,000' ASL
·  1400 – above 12,500' ASL
In emergency, squawk:
·  7500 - Hijack
·  7600 – Communications failure
·  7700 - Emergency
You will be given details of other
traffic, on the clock system, such as
“fast mover at 6 o’clock”, based on
the track seen on the radar screen.
When changing squawks as
instructed, take care not to dial up
the emergency ones by mistake, and
do not switch the transponder to
standby during the change to avoid
it, as this will remove your display
from ATC’s screen.
A Mode A transponder is the regular
variety, which just displays the code
you select in the aircraft – you get
this by turning the switch to ON.
A Mode C is directly attached to an
encoding altimeter (or, more
precisely, an altitude digitiser, which
selects a different code to that
selected in the window), but only
Pressure Altitude information is sent
from the aircraft – the conversion to
local pressure, if required, is done
inside the ATC computer. ATC will
not see changes when you move the
subscale (exam question). Mode C is
selected by switching to ALT.
Electricity & Radio 135
You need a Mode C for Class A, B,
C and E airspace between 10-12,500
feet ASL. You also need it in certain
Class D TCAs, such as Halifax or
Winnipeg International (see the CFS
for full list) and Class E aerodromes,
like Saskatoon or Moncton (again,
full list is in the CFS).
If a transponder fails during flight in
a mandatory area, you may go to the
next planned destination, then
complete an itinerary or go to a
repair base, as permitted by ATC.
It is possible to enter controlled
airspace without the required
equipment, but ATC must be asked
first. It is always subject to traffic.
When asked to squawk ident, your
return becomes brighter for a short
time, for positive identification.
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
　
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