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power, with the reflective
characteristics of the target will have
their effects. Minimum range is set by
the pulse width.
The Display
This is also called a Plan Position
Indicator (PPI). Its timebase (briefly,
the frequency with which the picture
is repainted) is linked to the aerial, in
that when it passes through North,
so does the beam painted on the
display. As a pulse is fired off, a spot
of light moves from the centre of
the tube to the outside, reaching the
circumference before the next pulse
goes. The effect is a line of light
rotating round the screen, at 60
RPM. When a return is received, the
electron flow is increased and display
intensity increases to a spot which
fades away slowly as the line moves
on. Range markers are displayed
with a saw-tooth wave. By making
the grid (brilliance) very negative, the
fly-back is suppressed, meaning that
you won't see the lines where the
300 Canadian Professional Pilot Studies
pulse flies back from the end of one
line to the beginning of the new line.
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 doublepulse
secondary equipment to
provide more information, hence the
name. Participating aircraft carry a
transponder (for transmitter/responder)
that receives the interrogation pulse
from the transmitter (1030 MHz),
superimposes information on it and
sends it right back on another paired
frequency (1909 MHz). 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 your height
readout, depending on the type of
transponder you have. There are
standard numbers to squawk, when
not otherwise instructed, which 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 according to the clock system,
such as “fast mover at 6 o’clock”,
based on the track seen on the radar.
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. In
other words, it is for identification.
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.
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,
Electricity & Radio 301
like Saskatoon or Moncton (again,
full list is in the CFS).
It is the spacing between pulses that
determines the mode of operation.
They are 8 microseconds apart for
A, and 21 apart for C.
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
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