the outputs is used for the automatic frequency
control (AFC) circuits. The other output is routed to
the summing network, which provides a composite
video output to the display indicator.
The display indicator displays the video on the
CRT. The operator can then set the position by
aligning the master and slave pulses and reading the
time delay. This information is then plotted on a chart
to determine aircraft position.
Loran-D uses the same theory of operation,but it
is used in conjunction with the navigational computer.
With this system, the indicator automatically displays
the latitude and longitude of the aircraft. The operator
just has to plot this information on a chart to
The omega navigation system is an outgrowth of
the loran A and loran C systems. It is a worldwide
network of eight transmitting stations that provide a
means of navigation accurate to within 4 nautical
miles anywhere in the world.
Theory of Operation
The AN/ARN-99(V) omega navigation set
provides digital data representative of aircraft phase
displacement to any combination of eight selected
omega ground stations. These eight ground stations
broadcast 10 kW in the VLF band at 10.2 kHz, 11.3
kHz, and 13.6 kHz. These stations are strategically
positioned around the world so their combined
propagation will cover the entire surface of the earth.
Each station transmits burst of the three different
frequencies during a 10-second period, which are
multiplexed so that only one station is on at one time
on one frequency. All signals are transmitted starting
at zero time (omega time), and maintained at the exact
starting time by using atomic clocks at each station.
The omega system in an aircraft must synchronize
itself to this pattern.
Synchronization is done by
analyzing all the signals received in the 10.2-kHz
frequency over one 10-second period. This period is
broken up into 100 intervals of 0.1 second each. The
beginning of each of these 0.1-second intervals is then
considered a possible starting point. The signal levels
are averaged over small intervals during the
remaining 9.9 seconds of the pattern for each of the
100 intervals, and then all are compared with the
Only one start time fits into the predicted pattern.
When this start time is found, the omega system
knows where each frequency is originating from
during each burst.
It can then make the proper
measurements from each station. If the system cannot
synchronize at 10.2 kHz, it will try to synchronize at
11.3 kHz and then at 13.6 kHz.
The omega system uses the great circle distances
to all stations. This is done to ensure that the effects
of modal interference (interference between the
primary wave and the sky wave and/or ground wave)
and wrong way propagation do not bias the
measurements. The stations less than 600 or more
than 7,200 nautical miles from the aircraft are not
used for the measurements. They are deselected and
their strength readings indicate zero. Station range
and bearings are recomputed every 10 seconds in the
burst filter routine, and station selection/reselections
The omega system can use either the hyperbolic
or the circular (RHO-RHO) method to process this
data. The P-3C uses the circular measurement
process, which measures phase from each station
directly. With RHO-RHO processing, a line of
position is generated from each station by direct
measurement of the omega signal received from that
station. Using another station and again generating
another line of position, the position fix is found. The
advantage of this method is that only two stations are
required to establish a geographical fix.
disadvantage of this method is its need to establish the
oscillator error of its receiver before the omega
signals can be used.
Since circular processing measures phase
directly, it must subtract oscillator error from the
measurement to be accurate. The RHO-RHO method
uses a software routine based on many measurements
to solve for this error. The omega on the P-3C is
totally dependent on the central computer for
operation. There are no operating controls or
indicators other than the elapsed time meter and the
power control panel.
The AN/ARN-99(V) consists of three major
components. These components are a control panel,
an antenna coupler, and a receiver-converter.
OMEGA POWER CONTROL PANEL. The
960767 omega power control panel (fig. 2-15)
controls the power to the omega system. When