The AIS provides the Rate of Turn (ROT) information to other
ships in order to early detect ships manoeuvres. There are two possible
parameters indicating turning of a ship derived from two different
sensors (see Figure 3: ROT sensor input):
- the heading from a GYRO or THD and
- the rotation rate itself from a Rate of Turn-indicator.
If a Rate of Turn Indicator according to resolution A.526(13)
is connected, the AIS should use this information to broadcast both
direction and value of turn on the VDL.
If valid ROT or
HDG data is available from other external sources (Gyro, INS,...),
the AIS should use this information to broadcast the direction of
turn on the VDL, if greater than 5° in 30 s (might also be implemented
as 2.5° in 15 s by configuration); the AIS may also derive ROT
information from HDG internally for that purpose.
If
no ROT information is available, the AIS should transmit default values
indicating "not available". ROT data should not be derived from COG
information.
If a ship is not required to
carry Turn-Indicator or if external sensor fails, the AIS should react
according to following priorities:
Table A ROT sensor fallback
conditions
|
Priority
|
Affected data in msg 1, 2, 3 ⇒
|
contents of ROT field
|
|
Position Sensor status
|
| 1.
|
Rate of Turn Indicator
in usefootnote
|
0..+ 126 = turning right at up to 708°
per minute or higher;
|
| 0..- 126 = turning left at up to 708°
per minute or higher
|
| Values between 0 and 708°/min should
be coded by
|
|
|
where  is the Rate of Turn as input by the external Rate of Turn
Indicator (TI).
|
| Values of 709° per minute and above
should be limited to 708° per min.
|
| 2.
|
other ROT source in use
footnote
|
+ 127 = turning right at more than
5°/30s (No TI available)
|
| 0 no turn
|
| - 127 =
turning Left at more than 5°/30s (No TI available)
|
| 3.
|
no valid ROT information
available
|
-128 (80 hex) indicates no turn
information available (default)
|
Figure A