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 determine  position. OMEGA 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 predicted  levels. 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 correspondingly   made. 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. The 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. Components 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 2-16