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Schuler tuning makes the platform
follow it correctly by oscillating at
84.4 minutes (probably an exam
question, although there are no
gimbals in a strapdown system,
therefore there are no Schuler
errors). Otherwise, the system has to
cope with aircraft movement and
transport wander. Also, since the
accelerometers only sense aircraft
acceleration, it has to filter out
centripetal force and coriolis effect
(bias correction).
292 Canadian Professional Pilot Studies
The gyros drive the torque motor to
keep the platform level and aligned.
Gyro corrections from platform
control include aircraft motion,
transport and Earth rotation. The
gyro pick-off feeds the torquer if there
is precession. Gimbal pickoffs provide
the VNAV indications (that is,
heading, pitch and roll) – the North
reference for VNAV is True North.
The resolver synchro on the inner
gimbal is there to decide whether to
correct for pitch or roll, according to
whether you are flying N/S or E/W.
Self-correction limits are 10° for
base latitude and 30° for base
longitude.
Doppler
Doppler radar is another selfcontained
system. It is based on the
fact that waves compress when
directed to a station and elongate
when going away (in other words, as
two objects get closer together, the
frequency between them will
increase). A good example comes
from listening to the change in
quality of the noise of an aircraft
approaching you, and passing by.
The pitch is higher than normal at
first, and reduces to one lower than
normal when it has passed. This is
because the forward speed means
that the sound waves have to fit into
a smaller space, and therefore have
shorter wavelengths, and a higher
pitch. The opposite is true after
passing – they have more room to fit
into, and the wavelength becomes
longer, hence the lower sound.
Apply the principles to radar, and
you have a good navigation system.
Radar waves (using the Janus array)
are beamed into four quadrantals, 2
forward, left and right, and the same
for the rear, each depressed about
60-70° from the horizontal (the
beams are used in opposite pairs,
such as front right with rear left).
The differences in the returned
wavelengths are noted, and a
computer number-crunches the drift
and groundspeed readings, and
rotates the aerial until the frequency
shifts are equal and aligned with the
track. The difference between the
aerial's alignment with the
longitudinal axis is noted, and the
drift angle calculated. Errors are
cumulative over time, so the system
has to be periodically realigned with
a position fix, and the magnetic
compass may give wrong readings.
The Janus array also reduces errors
resulting from vertical speed and
pitch. Whenever readings are taken
over water, however, true
groundspeed will under-read by
about 1%, so a sea bias must be
applied with the land/sea switch (the
maximum return now comes from
the rear of the beam).
Modern Doppler systems use
continuous wave signals, as opposed to
the original pulsed systems, which
means using two aerials instead of
one, but they are smaller as a unit
than before. They will also register
very low ground speeds – pulsed
systems unlock below 70 kts. In
addition, pulsed systems have a
minimum range (see Radar), which
means a minimum height at which
the equipment can be used –
continuous wave can go down to
ground level, which makes it
especially useful for helicopters
(vertical speed shift is also tied into
the VSI for more accuracy and speed
in the hover).
Electricity & Radio 293
The formula for Doppler Shift is:
fD = fT x V
C
where V is aircraft speed in
metres/second.
LORAN
This system was originally designed
for marine use on frequencies
between 90-100 KHz, but later got
extended to the whole of the USA. It
uses ground waves (although sky
waves will be present as well), so it is
only approved for secondary use in
aircraft, because the receiver may
lock on to them. Pulsed signals are
detected from a Master and at least
two out of up to six secondary
stations. LORAN-C is the third
version out of four since WW II.
Transmitters are grouped into chains
of 4-6 stations, one of which is
designated the Master, and the others
known as Secondary Stations. Each
chain, or group, transmits pulses at a
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