Criteria For Selecting Targets For Tracking - Furuno FCR-21x9-BB Operator's Manual

3. TARGET TRACKING (TT)
3.17

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 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 IMO require-
ments for TT, an indication of the motion trend should be available within 20 scans of
antenna and full vector accuracy within 60 scans. The FURUNO TT complies with
these requirements.
Echo detection (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 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 closest 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 land. This land
echo is not used by TT acquisition and tracking process. All smaller echoes are de-
clared 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 rela-
tive course and speed of the target.
Acquisition
A target that is hit by five consecutive radar pulses is judged to be a radar echo. Man-
ual 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 contin-
uously depending upon the congestion.
Tracking
The range and bearing of an echo are found from the relative distance of the target
from the radar antenna. The distance a tracked target moves between radar antenna
rotations is used to calculate the relative speed of the tracked target. However, be-
cause of calculation error, smoothing is applied to get stable target speed and course
data. When a target changes course, smoothing is reduced in order to quickly follow
target movement.
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