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The digital radar signal is then analyzed by a ship-sized echo discriminator. As the antenna
scans, if there are 5 consecutive radar pulses with 1’s indicating an echo presence at the exact
same range, a target "start" is initiated. Since receiver noise is random, it is not three bang
correlated, and it is filtered out and not classified as an echo.
The same is true of radar interference. Electronic circuits track both the closet and most distant
edges of the echo. At the end of the scanning of the echo, the discriminator indicates the
measured maximum range extent and total angular extent subtended by the echo. If the echo is
larger than a ship-sized echo in range extent and/or angular width, adjusted as a function of
range, it is declared to be a coastline and the closet edge is put into memory as a map of the
area. This land outline is used to inhibit further acquisition and tracking of ship sized echoes
beyond the closest coast outline. 5 consecutive scans of coastal outline are retained in memory
to allow for signal variation. All smaller echoes are declared to be ship sized and the middle of
the leading edge is used to provide precise range and bearing coordinates of each echo on
every scan. This range/bearing data is matched to previous data and analyzed from
scan-to-scan for consistency. When it is determined to be as consistent as a real target,
automatic acquisition occurs and tracking is initiated. Continued tracking and subsequent
calculation develop the relative course and speed of the target just as a man would do when
plotting the relative course and speed of the target on the scope with a grease pencil.
The true course and speed of own ship are computed from own ship's gyro and speed inputs,
and the resulting course and speed of each tracked target is easily computed by vector
summing of the relative motion with own ship’s course and speed. The resulting true or
relative vector is displayed for each of the tracked targets. This process is updated continually
for each target on every scan of the radar.
Automatic acquisition areas and suppression lines
Performance of auto-acquisition is enhanced by controlling the limit lines (suppression line) in
the former series of FURUNO ARPAs. In this new series of ARPAs, the automatic
acquisition rings are used instead of the limit lines.
Auto acquisition rings work as suppression lines when viewed from the opposite direction.
They should be placed clear of a landmass or shoreline. The acquisition areas may be a full
360 degree circle or sector of any angles are gyro stabilize.
Qualitative description of tracking error
The FURUNO ARPA accuracy complies with or exceed IMO standards.
Own ship maneuvers
For slow turns there is no effect. For very high turning rates (greater than 150°/ minute,
depending on gyro), there is some influence on all tracked targets which last for a minute or
two and then all tracked targets revert to full accuracy.
Other ship maneuvers
Target ship courses, lag 15 to 30 seconds at high relative speed, or 3 to 6 seconds at low (near
0) relative speed. It is less accurate during a turn due to lag, but accuracy recovers quickly.