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2-18
2.17 Criteria for Selecting
Targets for Tracking
The FURUNO ARPA video processor de-
tects targets in midst of noise and discrimi-
nates radar echoes on the basis of their
size. Target whose echo measurements
are greater than those of the largest ship
in range or tangential extent are usually
land and are displayed only as normal
radar video. All smaller ship-sized echoes
which are less than this dimension are
further analyzed and regarded as ships
and displayed as small circles superim-
posed over the video echo.
When a target is first displayed, it is shown
as having zero true speed but develops a
course vector as more information is col-
lected. In accordance with the Interna-
tional Marine Organization Automatic
Radar Plotting Aid (IMO ARPA) require-
ments, an indication of the motion trend
should be available in 1 minute and full
vector accuracy in 3 minutes of plotting.
The FURUNO ARPAs comply with these
requirements.
Acquisition and tracking
A target which is hit by 5 consecutive ra-
dar pulses is detected as a radar echo.
Manual acquisition is done by designat-
ing a detected echo with the trackball.
Automatic acquisition is done in the ac-
quisition areas when a target is detected
5-7 times continuously depending upon
the congestion. Tracking is achieved
when the target is clearly distinguishable
on the display for 5 out of 10 consecutive
scans whether acquired automatically or
manually.
Targets not detected in 5 consecutive
scans become "lost targets."
Quantization
The entire picture is converted to a digital
from called "Quantized Video." A sweep
range is divided into small segments and
each range element is "1" if there is radar
echo return above a threshold level, or
"0" if there is no return.
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 I's indicating an echo
presence at the exact same range, a tar-
get "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 acquisi-
tion and tracking of ship sized echoes
beyond the closest coast outline. 5 con-
secutive scans of coastal outline are re-
tained in memory to allow for signal
variation. All smaller echoes are declared
to be ship sized and the middle of the lead-
ing 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 track-
ing and subsequent calculation develop
the relative course and speed of the tar-
get just as a man would do when plotting
the relative course and speed of the tar-
get 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 eas-
ily computed by vector summing of the
relative motion with own ship’s course and
speed. The resulting true or relative vec-
tor is displayed for each of the tracked
targets. This process is updated continu-
ally for each target on every scan of the
radar.