RADAR INDICATORS (REPEATERS) The purpose of a radar indicator (repeater) is to analyze radar system echo return video and to display that information at various remote locations. For the repeater to present correct target position data, it must have three specific inputs from the radar selected: video input,  trigger  (timing)  pulses,  and  antenna  information. A video input  from the radar via a video amplifier for each returning echo enables the repeater to display detected targets. Trigger (timing) pulses  from the radar ensure that the sweep on the repeater starts from its point of origin each time the radar transmits. This allows repeaters to display the target at actual  range from the radar based on  the  time  lapse  between  the  instant  of  transmission  and the instant of target echo receipt. Antenna  information  from the radar allows the angular  sweep  position  of  the  repeater  to  be synchronized with the angular position of the radar antenna. This will produce and display the target at its actual bearing (azimuth) from the radar. The three most common types of displays are the A scope   (range-only   indicator),   the   PPI   scope (range-azimuth   indicator),   and   the   RHI   scope (range-height indicator). The A scope, limited by its range-only   capability,   is   normally   considered   an auxiliary display rather than a radar repeater. The PPI scope is by far the most used radar repeater. PLANNED POSITION INDICATOR (PPI) The  PPI  is  a  polar-coordinate  display  of  the surrounding area with the origin of the sweep (normally located at the center of the screen) representing your radar. The PPI uses a radial sweep pivoting about the center of the presentation, resulting in a maplike picture of the area covered by the radar beam. A relatively long-persistence  screen  is  used  so  targets  will  remain visible  until  the  sweep  passes  again. Bearing is indicated by the target’s angular position in relation to an imaginary line extending vertically from the sweep origin to the top of the scope. The top of the scope  represents  either  true  north  (when  the  radar  is operating in true bearing), or ship’s head (when the radar is  operating  in  relative  bearing). To allow a single operator to monitor several tactical data inputs from one location, many radar repeaters are being replaced with multipurpose consoles on Naval Tactical  Data  Systems  (NTDS)  equipped  ships. However, radar repeaters still serve as a back-up to the consoles used on NTDS ships and are irreplaceable on non-NTDS  ships. The most common radar indicator group used in the Navy  is  the  AN/SPA-25G.  This  Radar  Display  and Distribution System usually includes the AN/SPA-25G Indicator, the CV-3989/SP Signal the  SB-4229/SP  Switchboard. AN/SPA-25G Indicator Group Data  Converter,  and The AN/SPA-25G Indicator Group is found on 90 percent of all Navy ships. It meets the diverse mission requirements  of  antiair  warfare,  antisurface  warfare, antisubmarine  warfare,  electronic  warfare,  strike  and amphibious warfare, as well as navigation and bridge requirements  such  as  piloting  and  station  keeping.  The AN/SPA-25G   will   replace   the   AN/SPA-4,   SPA-8, SPA-25,  SPA-33,  SPA-34,  SPA-40,  SPA-41,  and SPA-66. The AN/SPA-50 and SPA-74 radar display system/indicator  groups  are  also  potential  candidates for replacement by the AN/SPA-25G. The  AN/SPA-25G  is  an  advanced,  solid-state (except  the  CRT  display)  radar  indicator  for  both Combat   Information   Center   (CIC)   and   bridge environments. It can receive multiple data inputs, including three radar video signals from the same radar, radar  triggers,  antenna  synchro  data,  external  course  and speed,   off-centering   inputs,   and   dead   reckoning analyzer (DRA) inputs. The various radar inputs, except video that is in analog  form,  are  in  the  Radar  Display  and  Distribution Systems (RADDS) serial 64-bit data stream format. The  data  is  continually  processed  through  five megabits  of  digital  memory.  By  correlating  the  radar data with internally generated graphic symbols, the operator   can   fully   interact   with   the   displayed information   on   the   CRT.   Figure   2-10,   the AN/SPA-25G   top   panel   layout,   shows   all   of   the operational  controls  and  indicators. Some  of  the  significant  design  features  of  the AN/SPA-25G   include: High  Definition  Raster  Scan  Display-enables the  AN/SPA-25G  to  perform  at  maximum  capacity, without a hood, in either the subdued lighting of CIC or the bright daylight on the ship’s bridge. Flicker   Reduction—provides   an   effective display  refresh  rate  that  suppresses  flicker  in  any lighting  environment. 2-19