Criteria For Tracking Target Selection - Furuno FAR-2218 Operator's Manual

3. TARGET TRACKING (TT)
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Criteria for Tracking Target Selection

The FURUNO TT 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
displayed only as normal radar video. All smaller ship-sized echoes that are less than
this dimension, are further analyzed and regarded as ships and displayed as small cir-
cles 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 TT) requirements, an indica-
tion of the motion trend should be available within 20 scans of antenna and full vector
accuracy within 60 scans. The FURUNO TTs comply with these requirements.
Acquisition and tracking criteria
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.Automat-
ic acquisition is done in the acquisition areas when a target is detected 5-7 times con-
tinuously depending upon the congestion. Tracking is achieved the target is clearly
distinguishable on the display for 5 out of 10 consecutive scans, whether acquired au-
tomatically 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".
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 an-
tenna scans, if there are five 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 discrimi-
nator indicates the measured maximum range extent and total angular extent sub-
tended 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. Five consecutive 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 leading edge is used to provide precise range and
bearing co-ordinates 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 deter-
mined 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.
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