What is the "rate-of-turn" parameter?

Class A vessels periodically report the rate at which they are rotating (if at all) within AIS Messages 1, 2 & 3. This parameter is called the rate-of-turn of the vessel.

How is "rate-of-turn" measured?

Vessels reporting this parameter generally have an external "rate-of-turn indicator" (TI) onboard that feeds the measured value from the sensor into the AIS transceiver, where it then gets converted into the "raw" value stored in the AIS messages.

What is the formula to convert the "raw" rate-of-turn into degrees per minute (°/min)?

The formula is as follows:

  • ROT_ais = 4.733 * √(ROT_sensor) 

Where:

  • ROT_ais : "raw" rate-of-turn value stored in the AIS message
  • ROT_sensor : rate-of-turn in °/min as reported by the TI onboard the vessel

For instance, a value of +126 for ROT_ais translates into a value of +708 °/min for ROT_sensor.

Note:
Be aware that without special handling, converting a negative ROT_ais value to the ROT_sensor value will cause it to lose its negative (-) sign in the process. This means that the vessel's rotation would get reported as turning right/clockwise when it is actually turning left/anti-clockwise.

Range of Possible Values

  • 0 to +126 : Vessel is turning right/clockwise at up to 708 °/min or higher
  • 0 to -126 : Vessel is turning left/anti-clockwise at up to 708 °/min or higher
  • +127 : Vessel is turning right/clockwise at more than 5° per 30 sec; TI is unavailable
  • -127 : Vessel is turning left/anti-clockwise at more than per 30 sec; TI is unavailable
  • -128 : No turning information is available from the vessel

References & Other Resources

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