simulated   tracking   and   tactical   situations that resemble   actual   operations.   Data   extracted   during these   simulated   exercises   can   be   used   to   verify   the accuracy  of  operator  tracking  and  system  operation. The  VSS  is  a  computer-controlled  device  capable of  generating  antenna  position  data  and  triggers (sweep   data),   and   radar   video   signals   including simulated   tracks,   IFF/SIF,   receiver   noise,   and   sea clutter.  Only  the  generation  of  video  signals  may  be computer   controlled;   all   other   VSS   functions   are controlled  from  the  VSS  front  panel,  shown  in  figure 3-24. Radar  Sweep  Simulation The   VSS   can   generate   its   own   antenna   position data or use a ship’s radar as a source. If a ship’s radar is  used,  the  LIVE  RADAR  SOURCE  switches  are used  to  select  the  source  radar  and  video  level.  The VSS  then  receives  antenna  position  data  and  triggers from   that   radar’s   RAC   via   the   RDDS.   The   antenna position  data  (digital  azimuth)  and  triggers  are  used  to determine  the  generation  times  for  video  signals. The   VSS   contains   its   own   synchro   assembly, which  generates  synchro  azimuth  and  triggers  to  the VSS  RAC.  The  output  of  the  VSS  RAC  is  in  turn  fed back   to   the   VSS   via   the   RDDS   for   coincidence comparisons. The  VSS  simulated  antenna  rotation (RPM)  and  timing  signals  are  controlled  from  the SIMULATED   RADAR   switches.   To   activate   the servo  assembly,  both  the  E-REF  and  ANTENNA switches  must  be  on. Radar  Video  Simulation The  VSS  receives  video  control  data  from  the computer  in  message  form.  The  output  data  defines the   simulated   video   azimuth   (bearing),   range,   and intensity.  The  VSS  stores  the  output  data  in  its  track storage  and  compares  the  data  with  the  digital  azimuth and  sweep  data  received  from  the  RDDS.  When  the simulated  video  data  and  the  digital  sweep  data  are coincident,  the  VSS  generates  the  ordered  video signals  and  transmits  them  to  the  RDDS.  Console selection  of  the  VSS  radar  and  video  level  allows display  of  VSS  video  and  sweep. 3-20 The  video  output  of  the  VSS  is  made  up  of  three types  of  video:  live  video,  live  and  simulated  (mixed) video,  and  simulated  video.  The  live  video  output  is isolated   from   the   simulator.   The   live   and   simulated output  is  made  up  of  simulated  tracks  intermixed  with live  video  passing  through  the  VSS.  Simulated  video is  made  up  of  computer-generated  video  only. Manual   control   of   the   simulated   video characteristics   is   provided   by   the   SIMULATED VIDEO  switches.  These  switches  allow  the  generated video  to  display  characteristics  that  are  similar  to  live radar  video.  The  VIDEO  DURATION  (µSEC)  switch controls  the  intensity  (pulse  length)  of  the  simulated video  return.  The  sharpness  of  the  return  is  controlled by  the  IF  BANDWIDTH  switch.  The  sector  width  of the return is c o n t r o l l e d    b y    t h e    R A D A R BEAMWIDTH   DEGREES   control. TEST   MESSAGE   GENERATOR   (TMG) The   test   message   generator   (TMG),   shown   in figure  3-25,  is  used  to  generate  simulated  computer output   data   for   testing   and   troubleshooting   display equipments. Normally  contained  in  the  PANG cabinet,  the  TMG  is  removable  for  its  testing  and troubleshooting   role.   The   TMG   maybe   connected   to the  equipment  under  test  by  a  jumper  cable.  This permits  greater  freedom  of  action  for  maintenance personnel  in  troubleshooting  individual  devices,  such as  a  PPI  console. The   TMG   can   simulate   any   desired   computer output  data  message  up  to  four  data  words  in  length. Coding  of  TMG  data  is  controlled  by  word  group switches  on  the  TMG  control  panel. The  TMG  is  capable  of  operating  in  five  different modes. The   mode   selection   allows   for   great flexibility  when  testing  or  troubleshooting  the  display suite   equipment. Free  Run  Mode The  free  run  mode  of  operation  repeats  an operator-selected   four-word   message   approximately every  400  microseconds.  In  this  mode,  the  TMG  may be  used  with  any  display  equipment.