-2.2-
2.2 Criteria of Tracking
A target measuring 800 m or more in the radial or circumferential direction is regarded as a
landmass and not acquired or tracked. Echoes smaller than 800 m are regarded as targets to
be tracked.
The FURUNO ARPA ATA video processor detects targets in the midst of noise and
discriminates 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
superimposed 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 collected. In accordance with the International Marine
Organization Automatic Radar Plotting Aid (IMO ARPA) requirements, an indication of the
motion trend should be available within 20 scans of antenna and full vector accuracy within 60
scans. The FURUNO ARPAs/ATAs comply with these requirements.
Acquisition and tracking
A target which is hit by 5 consecutive radar pulses is detected as a radar echo. Auto acquisition is
not defined in paints but in time, which should be less than 3 seconds of initial stage.
Manual acquisition is done by designating a detected echo with the trackball. Automatic acquisition
is done in the acquisition 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
consecutive or alternate paints out of 10 consecutive scans whether acquired automatically or
manually. Required tracking facilities are available within 0.1-32 nm on range scales including 3, 6,
12 nm; full plotting information is available within one scan when the range scale has been
changed. Targets not detected in 5 consecutive scans become “lost targets”.
Quantization
The entire picture is converted to a digital from called “Quantified 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 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.
