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3. TARGET TRACKING (TT)
3-23
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.
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 is
changed. Targets not detected in nine consecutive scans become "lost targets."
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 tracked target is easily computed
by vector summing of the relative motion with own ship's course and speed. The re-
sulting 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.
The REF point for tracked target calculation can be the radar antenna position or the
CCRP. For CCRP, the displayed value includes the distance between the CCRP and
the antenna position. Therefore, when switching the REF point, the range, bearing,
CPA and TCPA of the tracked targets change.
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), then all tracked targets 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.