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
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 targets 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 ships 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
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
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
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 ships bridge.
Flicker Reductionprovides an effective
display refresh rate that suppresses flicker in any