Aviation Electronics Technician 1 - Historic Naval Ships Association

the outputs is used for the automatic frequencycontrol (AFC) circuits. The other output is routed tothe summing network, which provides a compositevideo output to the display indicator.The display indicator displays the video on theCRT. The operator can then set the position byaligning the master and slave pulses and reading thetime delay. This information is then plotted on a chartto determine aircraft position.Loran-D uses the same theory of operation,but itis used in conjunction with the navigational computer.With this system, the indicator automatically displaysthe latitude and longitude of the aircraft. The operatorjust has to plot this information on a chart todetermine position.OMEGAThe omega navigation system is an outgrowth ofthe loran A and loran C systems. It is a worldwidenetwork of eight transmitting stations that provide ameans of navigation accurate to within 4 nauticalmiles anywhere in the world.Theory of OperationThe AN/ARN-99(V) omega navigation setprovides digital data representative of aircraft phasedisplacement to any combination of eight selectedomega ground stations. These eight ground stationsbroadcast 10 kW in the VLF band at 10.2 kHz, 11.3kHz, and 13.6 kHz. These stations are strategicallypositioned around the world so their combinedpropagation will cover the entire surface of the earth.Each station transmits burst of the three differentfrequencies during a 10-second period, which aremultiplexed so that only one station is on at one timeon one frequency. All signals are transmitted startingat zero time (omega time), and maintained at the exactstarting time by using atomic clocks at each station.The omega system in an aircraft must synchronizeitself to this pattern. Synchronization is done byanalyzing all the signals received in the 10.2-kHzfrequency over one 10-second period. This period isbroken up into 100 intervals of 0.1 second each. Thebeginning of each of these 0.1-second intervals is thenconsidered a possible starting point. The signal levelsare averaged over small intervals during theremaining 9.9 seconds of the pattern for each of the100 intervals, and then all are compared with thepredicted levels.Only one start time fits into the predicted pattern.When this start time is found, the omega systemknows where each frequency is originating fromduring each burst. It can then make the propermeasurements from each station. If the system cannotsynchronize at 10.2 kHz, it will try to synchronize at11.3 kHz and then at 13.6 kHz.The omega system uses the great circle distancesto all stations. This is done to ensure that the effectsof modal interference (interference between theprimary wave and the sky wave and/or ground wave)and wrong way propagation do not bias themeasurements. The stations less than 600 or morethan 7,200 nautical miles from the aircraft are notused for the measurements. They are deselected andtheir strength readings indicate zero. Station rangeand bearings are recomputed every 10 seconds in theburst filter routine, and station selection/reselectionscorrespondingly made.The omega system can use either the hyperbolicor the circular (RHO-RHO) method to process thisdata. The P-3C uses the circular measurementprocess, which measures phase from each stationdirectly. With RHO-RHO processing, a line ofposition is generated from each station by directmeasurement of the omega signal received from thatstation. Using another station and again generatinganother line of position, the position fix is found. Theadvantage of this method is that only two stations arerequired to establish a geographical fix. Thedisadvantage of this method is its need to establish theoscillator error of its receiver before the omegasignals can be used.Since circular processing measures phasedirectly, it must subtract oscillator error from themeasurement to be accurate. The RHO-RHO methoduses a software routine based on many measurementsto solve for this error. The omega on the P-3C istotally dependent on the central computer foroperation. There are no operating controls orindicators other than the elapsed time meter and thepower control panel.ComponentsThe AN/ARN-99(V) consists of three majorcomponents. These components are a control panel,an antenna coupler, and a receiver-converter.OMEGA POWER CONTROL PANEL.— The960767 omega power control panel (fig. 2-15)controls the power to the omega system. When2-16