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within ±1° - the system should shut
down automatically if it gets outside
that. Phase comparison error should not
be more than ±3°, and station errors
should be within ±1°. The nominal
accuracy is ±5°.
Problems
Although the VOR is less subject to
static and interference than an NDB,
and it is much more accurate, the
transmissions depend on line of
sight, and there is a zone of
ambiguity at 90° to a radial,
mentioned above. In addition,
certain propeller or rotor RPM
settings can cause fluctuations up to
±6° (change the RPM slightly before
saying the instrument is U/S).
Time to Station
You often need to know the time it
will take to get to a station, which is
simply found by turning 90° from
the inbound radial and noting the
seconds taken to go through a number
of them. To get the time in minutes,
divide the time just noted by the
number of radials (degrees) gone
through. All you need do then is use
Electricity & Radio 127
the groundspeed (or TAS in
emergency) to find your distance.
For time to station, the formula is:
Time (mins) = Mins x 60
Degrees
On the whizzwheel, set the minutes
on the outer scale, and the degrees
on the inner one. Read the answer
on the outer scale opposite the 60
arrow.
For the distance, try:
Distance = Mins x GS
Degrees
ADF/NDB
An Automatic Direction Finder (ADF),
also known as a radio compass, is a
device in an aircraft that picks up
signals broadcast on the Medium
wave band by Non Directional Beacons
(NDBs), so called because they
radiate in all directions.
Transmissions are not dependent on
line of sight, so the system is good
for long distance travel, although it
does have a few problems,
mentioned below. It is possible to
get 1,000 nm range over sea and 300
nm over land if the power is high
enough, but since better systems
have come along, NDBs are now
used as enroute navaids on airways,
homing beacons for instrument
approaches and markers for the
Instrument Landing System (ILS), with a
typical range of about 35 nm.
NDBs should be accurate to within
± 5° by day. If there is no ID, but
the system otherwise appears to
behave normally, the NDB is
undergoing calibration or
maintenance.
The primary function of the ADF
receiver is to determine the bearing
of an incoming NDB signal, which is
vertically polarised. To do this, it
uses a loop aerial. When the loop is
square to the beacon, the signal
reaches both sides of the loop at the
same time and there is no signal
detected. When the loop is sidewayson,
however, the signal reaches one
part of the loop first:
and the second part will be out of
phase, so a current will be generated,
which drives an electric motor to
continually seek the null position. It
is phase sensitive, so it can always
turn the shortest way. Various stages
of magnification inside the receiver
help this along, but that need not
concern us here. The point is that
the detected signal is not actually
used to determine the bearing, but
the null signal point, since the
current flow is slow to build up and
break down, and is a bit on the
woolly side. The null point is much
sharper and easier to find.
Because the current flows in the
opposite direction depending on the
position of the loop antenna, you
also need some way of determining
which end is what, otherwise you
could be 180° out. A single vertical
aerial called a sense antenna helps here
– both signals are combined
algebraically and the magnitude and
128 JAR Private Pilot Studies
polarity of the sense aerial arranged
to be identical to the loop. The result
is a polar diagram called a cardioid,
which has only one null point:
On one side of the loop the polar
diagrams are positive and combine,
but on the other, one is positive and
the other negative. They cancel each
other out, hence the null point on
one side.
The modern (and more stylish)
equivalent is a small housing with
two coils at right angles to each
other, wound on ferrite cores. They
are connected to the stator coils of a
goniometer which points the needle.
Limitations of the system include:
· static.
· station overlap, where NDBs have
the same frequency. Because
this is more pronounced at
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