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accuracy better than ½ nm or 3% of
the distance, whichever is the
greater. The reason it's not
completely accurate is because the
distance measured is the slant range
from the station, and not from your
equivalent position on the ground,
although at long distances and lower
altitudes, this will be minimised.
Simple Pythagoras will give you the
real distance:
D = Å(S2 – A2)
D is the ground distance, S is the
readout (i.e. slant range) and A is
your altitude in nautical miles (above
the DME source).
RNAV
Area Navigation is a generic name for
systems that allow navigation over
wide areas – it was originally coined
for a way of electronically moving
navaids, VORs in particular, to other
places enroute. For example, you
could tell the black box the distance
and bearing of your house from the
nearest VOR and it would present all
the signals as if the aid was actually
there. On a direct route with no
specific navaids to aim for, you
could shift all nearby ones to fit on
your direct track so you had a series
of phantom stations in the shape of
instant VORs (called waypoints) to
aim for. Thus, RNAV describes ways
of flying directly without doglegging
all over the place, or where you don't
have to pass directly over a radio fix.
It can be used with VOR, DME,
LORAN and Omega, not forgetting
DECCA (in Europe) and GPS.
In direct ranging systems, the signals
from two stations are enough for a
position. In hyperbolic systems, a
bearing is based on a phased or time
difference between two ground
stations, as used in LORAN or
Omega (or Decca), where three
stations are needed for two bearings
Electricity & Radio 291
(Decca uses three Slaves for each
Master). It is an inexpensive method,
but relies on ground stations being
operational. In this mode, signal
lines are curved – if a map were
constructed based on them being
straight, the land would appear
distorted. A hyperbola joins points
of equal difference of distance between
two points. That is to say, at any
point along a hyperbola, the distance
between stations either side of it is
always the same – if there were a
hyperbola marked +20 nm, for
example, the distance to the station
on the left would always be 20 nm
less than that to the one on the right,
anywhere along the line. A line
marked –20 nm would mean the
station on the right is always 20 nm
less than the one on the left. The
distance along the base line from the
bisector to any hyperbola is always
half the value of the hyperbola.
Accuracy in hyperbolic systems is
always best along the base line, then
around the right bisector, should you
move away from it. Beware of using
hyperbolae too far away from the
right bisector, since the bending
back will give you a choice of two
lines and it's hard to tell one from
the other.
The accepted definition also includes
self-contained methods, such as…..
Inertial Navigation Systems
There are 2 types of INS: stable
platform (accelerometers and rate
integrating gyros) and strapdown (ring
laser gyros and accelerometers), but
both combine input from
accelerometers that detect aircraft
movement in all directions, together
with gyros that stabilise the platform
they are fixed to, all synthesised with
a computer. It is extremely accurate,
but depends on the right
information being inserted in the
first place, which is why it is so
important in the checklist (this is
called gyrocompassing).
A ring laser gyro (strapdown) uses 2
mirrors, a partially silvered mirror
and 2 contra-rotating laser beams. It
suffers from lock-in errors caused by
imperfect mirrors, which are reduced
by wobbling, or vibrating the system
with a piezoelectric motor and a
dither spring.
LNAV sensing is done by the
accelerometers, and VNAV sensing
by the gyros.
The 1st stage integrator produces
velocity, while the 2nd stage integrator
produces distance, going to a latitude
counter for N/S distance and a secant
unit for E/W distance (requires input
from the latitude counter before
entering the longitude counter).
Since outputs from the 1st stage
integrators are velocities, they are
sent to platform control, to be mixed
with Earth radius, rotation rate and
latitude. When the aircraft is not
moving, the platform is still doing so
because of the Earth's rotation.
　
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