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unique Group Repetition Interval (GRI),
and each secondary has a unique
delay with reference to the Master
before generating its own pulses
inside the GRI. You can therefore
identify a transmitter by the GRI
(for the group) and its relationship
to the Master by the delay. A delay
of 0.1 microseconds is roughly
equivalent to about 100 feet, so it is
converted to distance and your
position found by triangulation from
at least three stations. Change of
position can also be calculated and
displayed on suitable equipment in
the cockpit.
Secondaries radiate pulses in groups
of eight, but the Master has nine. A
typical GRI would be 99, 600
microseconds, or 9960 for short.
Lines between Masters and
Secondaries are called baselines,
typically between 600-1,000 nautical
miles long. Accuracy is within about
½ nm over 120-1000 nm. Near the
baseline it is .1-.2 nm. Signals are
best over salt water at night.
Selecting the best transmitters of the
network, without tracking Masters, is
called Masterless Cross-Chaining.
To use the system properly, you
have to tell the machine in your
cockpit your approximate Lat &
Long, or the GRI you wish to use.
A Signal Blink is an alert signal for
errors, arising from a distinctive
change in the eight pulses normally
transmitted by a station. The
airborne receiver will have a way of
detecting and displaying them. To
eliminate interference from signals
near the authorised operating range,
internal (or notch) filters are used in
the receiving equipment.
Omega/VLF
Omega operations were terminated
(in the USA anyway) in 1997. For
exam purposes, though, it uses eight
purpose-built transmitters around
the world, over four frequencies,
namely 10.2, 11.05, 11.3 and 13.6
KHz (one at a time), using pulsed
signals. It can process them in direct
or hyperbolic mode to produce an
accuracy within 2 nm by night and 1
nm by day, although this does
depend on propagation patterns.
There is no Master station, and a
two-line hyperbolic fix needs
information from at least three
stations, that is, a hyperbola each
from Stations A and B and Stations
B and C, for example. The receiver
will automatically sort out the best
294 Canadian Professional Pilot Studies
signals, even from different stations
as you move around. In direct mode,
you only need two stations, and a fix
is obtained from each one, using
lane boundaries that arise out of
phase differences, based on atomic
clock timing. Three stations are still
better, though. The reason why the
unit needs UTC and the date when
you turn it on is so it can adjust for
diurnal variations in the height of the
ionosphere (as well as faster wave
movement over water). At the same
time, latitude error is also computed,
as the Earth is not round.
VLF uses 9 US Navy long range
communications stations, working
between 15-30 KHz in direct
ranging mode (see above). It has not
been approved by Transport Canada
as a primary aid (a bit late now
anyway), although both can be used
in NAT MNPS or CMNPS airspace.
Some receivers can switch to VLF if
Omega signals are poor.
The VLF signals used by both
systems have a significantly longer
wavelength, and can reach distances
of over 10,000 nm with little
attenuation as ground waves (with
the help of a process similar to
ducting), so you could theoretically
get a signal from at least two stations
anywhere in the world (and
confusion, when both direct and
reverse path signals are received over
7,500 nm away from a station, and
sky and ground waves combine
below 600 nm. This is sorted out
with automatic station deselection).
The above systems are sophisticated
and cheaper than inertial systems.
GPS
The Global Positioning System was
originally set up by the US military,
using 24 satellites orbiting every 12
hours to give extreme accuracy at a
very much reduced cost compared
to, say, INS. Although they have
guaranteed to keep the system
running for the foreseeable future, in
National Emergencies it may be
unavailable, which is why it is still
not acceptable as a sole means of
navigation without traditional
systems as backup (that is, it can be
used as a primary aid in some areas).
It will give altitude information as
well as location, but 3D readouts
require 4 satellites. Selective
Availability, where there was a
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