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AIS Receiver–processor / User manual
1.3. OPERATION
1.3.1. Collision avoidance data processing
The A.I.S Receiver/Processor continually processes the signals received on the 2 A.I.S.
frequencies. For each A.I.S. message received, it will check the safety zone and generate
alarms according to the level activated:
Alarm if CPA < 6 nautical miles and TCPA < 30 minutes
Alarm if CPA < 4 nautical miles and TCPA < 20 minutes
Alarm if CPA < 2 nautical miles and TCPA < 15 minutes
CPA, TCPA, bearing and distance of the target are calculated and sent to the Gyrographic.
Alarm principle
The CPA and TCPA values are chosen with the rotating button to define the safety range. The
unit will generate an alarm if a target is detected and its CPA and TCPA are both below the
level defined.
In the example (figure 2) the alarm level selected is « CPA < 4 Nm and TCPA < 20 minutes ».
When the value of the CPA calculated is below 4 nautical miles and the TCPA calculated is
less than 20 minutes, an alarm message will be sent to the Gyrographic. For CPA below 4
nautical miles with a TCPA greater than 20 minutes, no alarm will be sent.
Target
TCPA 20 mn
TCPA 10 mn
TCPA
4 nauticals milles
Re
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co
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se
CP
A
Be
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ing
/
dis
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Figure 2: Alarm range CPA<4 Mn and TCPA<15 minutes
