4. AIS OPERATION
4-19
4.12 AIS Collision Alarm (CPA, TCPA)
The AIS continuously monitors the predicted range at the Closest Point of
Approach (CPA) and predicted time to CPA (TCPA) of each AIS target. When the
predicted CPA of an AIS target becomes smaller than a preset CPA range and its
predicted TCPA less than a preset TCPA limit, the audio alarm sounds and AIS
COLLISION appears (in red) in the Alert Box. In addition, the symbol of the
offending AIS target is red and flashes together with its vector.
CPA/TCPA alarm ranges must be set up properly taking into consideration the
size, tonnage, speed, turning performance and other characteristics of own ship.
The reference point for CPA, TCPA calculation may be selected from antenna
position or conning position. For further details, see section 1.50.
4.12.1 Setting the CPA and TCPA ranges
To set the CPA and TCPA ranges, do the following:
1. Left-click CPA LIMIT box to show the CPA and TCPA boxes. (Not necessary if
the CPA and TCPA boxes are already shown.)
CPA LIMIT 0.5NM 10MIN
TCPA Box
CPA LIMIT Box CPA Box
CPA LIMIT, CPA and TCPA boxes
2. Select the CPA Box.
3. Push the left button or scrollwheel to select CPA value.
Left button 0.5, 1, 1.5, 2, 3, 4, 5, 6(nm)
Scrollwheel 0.1-20(nm), 0-10 nm in 0.1 nm increments, 10 nm
increments thereafter
4. Select the TCPA box.
5. Push the left button or scrollwheel to select TCPA value.
Left button 1, 2, 3, 4, 5, 6, 12, 15(minutes)
Scrollwheel 1-60 minutes, 1-minute increment
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