Furuno FR-2125V - Manual (Page 107)

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5.18 Criteria for Selecting Targets for Tracking
The FURUNO ARPA video processor detects targets in 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 dis-
played 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 Ma-
rine 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 comply with these requirements.
5.18.1 Acquisition and tracking
A target which is hit by five consecutive radar pulses is detected as a radar echo. Manual
acquisition is done by designating a detected echo with the trackball. Automatic acquisi-
tion 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 distin-
guishable on the display for 5 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 five consecutive scans become “lost targets.”
5.18.2 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 an-
tenna scans, if there are five consecutive radar pulses with 1’s indicating an echo pres-
ence 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 indi-
cates 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