## 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