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4. Tracked Target (TT) Operation
4-26
4.18 Criteria for Selecting Targets for Tracking
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 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 TT) requirements, an indication 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
A target that 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 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 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."
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 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 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. Five 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.
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 TT 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 TTs. This process is updated continually for
each target on every scan of the radar.
Qualitative description of tracking error
The FURUNO TT's accuracy complies with or exceeds 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 TTs that lasts for a minute or two then all TTs 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.