TRACK-RECEIVE   AXIS   COLLIMATION In  track-receive  (TR)  axis  collimation,  the  angle- error   null   method   of   measurement   is   used   to   deter- mine  the  error  between  the  track  receive  axis  and  the borescope   axis   of   the   AN/SPG-51C/D   radar.   At   the shore  tower,  the  Range  Calibrator  Set  AN/SPM-6  is connected  to  transmission  line  C,  which  connects  to antenna  C  on  the  tower  array.  Aboard  ship,  the  AN/ SPG-51  radar  director  is  manually  pointed  toward  the shore   tower. The  director  is  not  energized  during  any  AN/SPG- 51C/D  radar  collimation  checks.  When  the  director  is in  the  required  position,  the  AN/SPG-51C/D  radar track  transmitter  is  set  to  RADIATE,  thus  triggering the  AN/SPM-6  range  calibrating  set  at  the  shore tower.   When   triggered   by   the   radar   signal,   the   AN/ SPM-6  begins  transmitting  back  to  the  radar  set  RF pulses  identical  to  the  pulses  being  received,  which, when  received  by  the  radar,  appear  as  target  video  on the  A-scope  of  the  radar  operator’s  console.  Only  three to  five  of  these  video  pulses  are  observable  on  the A-scope   because   of   the   high   pulse-repetition   fre- quency  (PRF)  of  the  radar  set. The  radar  is  placed  in  range  track  when  video pulses  are  observed  on  the  A-scope.  This  is  accom- plished  by  gating  one  of  the  returned  pulses.  A  Dop- pler   target   is   required   to   ensure   that   an   angle-error output   voltage   is   generated   whenever   the   antenna (director)  is  manually  moved  off  target.  This  require- ment  is  accomplished  by  setting  the  CLUTTER  RE- JECT  switch  on  the  operator’s  console  to  the  0  KT position.  The  traverse-  and  elevation  angle-error  volt- ages  generated  from  the  angle-error  detector  module of   the   AN/SPG-51C/D   data   converter   are   monitored by  two  voltmeters. With  the  radar  in  track,  the  director  is  held  sta- tionary  on  target  (brakes  set)  in  one  axis  (TRAIN  or ELEVATION)   and   rocked   through   the   angle   error null  point  (as  indicated  by  the  variable  time  voltmeter [VTVM])  in  the  other  axis.  The  mark  method  is  used to   correlate   angle-error   null   voltage   readings   with borescope  readings.  The  borescope  readings  (in  roils) are   taken   with   respect   to   the   AN/SPG-51C/D   track radar  optical  target  6  or  10  on  the  tower  array.  A minimum  of  20  nulls  and  the  borescope  readings  are taken  to  determine  the  collimation  error  between  the TR  axis  and  the  borescope  axis.  This  procedure  is then  repeated  for  the  other  axis  (traverse  or  elevation). Before  actually  determining  the  TR  axis  error  by the  angle-error  null  method,  checks  should  be  made  to determine  that  an  angle  error  of  proper  sensitivity  (1 voltage   per   millimeter   [v/mil])   can   be   generated   in each   quadrant   and   that   conical-scan-on-receive-only (COSRO)   phasing   is   correct   (minimum   crosstalk)   to preclude  offset  of  the  angle-error  null. TRACK-TRANSMIT   AXIS   COLLIMATION In  track-transmit  (TX)  axis  collimation,  the  beam- pattern  plot  method  of  measurement  is  used  to  deter- mine  the  error  between  the  track-transmit  axis  and  the borescope  axis  of  the  AN/SPG-51C/D  radar.  There  is no  requirement  for  test  equipment  aboard  ship  during the   performance   of   this   test.   The   test   setup   aboard ship   consists   of   placing   the   track   transmitter   in   the RADIATE  mode,  pointing  the  radar  antenna  (director) toward   the   shore   tower,   and   centering   the   AN/SPG- 51C/D  track  radar  optical  target  6  or  10  in  the  bore- scope. In  the  tower,  the  average  power  meter  is  con- nected   through   the   calibrated   attenuator   (C-band)   to the  waveguide  line  connected  to  antenna  C  on  the array.  Attenuation  is  adjusted  to  allow  the  track  trans- mitter   power   to   be   read   conveniently   on   the   power meter   (in   decibels   or   milliwatts,   as   preferred).   After establishing   a   reference   point   (reading   of   maximum power)  by  coaching  the  director  operators  aboard  ship via   sound-powered   telephone,   the   track   transmitter output  power  at  the  tower  as  the  antenna  (director)  is held   stationary   in   one   axis   (traverse   or   elevation) and  moved  off  target  in  the  other  axis  is  plotted.  The offset   readings   in   roils   are   given   by   the   borescope observer   aboard   ship. Usually,  a  30-mil  excursion  on  each  side  of  the maximum  power  point  is  sufficient.  After  plotting  the power  readings  to  offset  in  mils  on  a  graph,  a  horizon- tal  line  connecting  the  half-power  points  (3  decibels down)  is  drawn.  By  bisecting  this  line  and  observing where  the  bisecting  line  intercepts  the  mils  axis,  you 4-7