3. TARGET TRACKING (TT)
3-17
3.12 TT CPA/TCPA Alarm
The TT continuously monitors the predicted range at the Closest Point of Approach
(CPA) and predicted time to CPA (TCPA) of each TT. When the predicted CPA of any
TT becomes smaller than the preset CPA range and its predicted TCPA less than the
preset TCPA limit, the audio alarm sounds and the Alert 526 (ALF format: 191,1) "TT
CPA/TCPA" appears in the [Alert] box. In addition, the symbol of the offending TT is
red and flashes together with its vector.
This feature, when used correctly, helps prevent the risk of collision by alerting you to
threatening targets. It is important that the gain, sea clutter and rain clutter and other
radar controls are properly adjusted.
The accuracy of TT calculations is a function of several variables including target glint,
clutter conditions and signal-to-noise ratio. The design of the TT minimizes the effects
of these variables but you must be aware that such errors will produce inaccuracies in
the derived CPA and TCPA in particular during the initial tracking period.
CPA and TCPA settings must be set up properly taking into consideration the size,
tonnage, speed, turning performance and other characteristics of own ship.
The TT CPA/TCPA alarm should not be solely relied upon to warn you of collision sit-
uations. The operator should check all aids to navigation to monitor possible collision
situations.
3.12.1 How to set the CPA and TCPA limits
To set the CPA and TCPA ranges, do the following:
1. If the values for CPA and TCPA are blank, click [CPA/TCPA] to show them.
2. Click the CPA indication to select desired CPA range (0.1-20 NM).
3. Click the TCPA indication to select desired TCPA time (1-60 minutes.).
3.12.2 How to enable, disable the TT CPA/TCPA alarm
The TT CPA/TCPA alert can be enabled or disabled from the TT/AIS box. Click the
indication shown below to enable or disable the TT CPA/TCPA alarm. The alarm is
disabled when the CPA and TCPA settings are not shown.
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