Vector Sensor Reference Manual 25
the unit for one minute at a rate of no more than 15 degrees per second. Then, let it
sit stationary for four minutes. Your Vector’s gyro is now fully calibrated. Since this
setting cannot be saved, this procedure must be performed every time the Vector’s
power is cycled.
1.8.6 Time Constants
The Vector Sensor incorporates user-configurable time constants that can provide a degree of
smoothing to the heading, course over ground, and speed measurements. The following sections
describe how to configure their values.
1.8.6.1 Heading Time Constant
The heading time constant allows you to adjust the level of responsiveness of the true heading
measurement provided in the $HEHDT message. The default value of this constant is 2.0 seconds
of smoothing when the gyro is enabled. The gyro by default is enabled, but can be turned off. By
turning the gyro off, the equivalent default value of the heading time constant would be 0.5
seconds of smoothing. This is not done automatically, and therefore must be entered manually by
the user. Increasing the time constant will increase the level of heading smoothing.
The following command is used to adjust the heading time constant.
$JATT,HTAU,htau<CR><LF>
Where ‘htau’ is the new time constant that falls within the range of 0.0 to 3600.0 seconds.
Depending on the expected dynamics of the vessel, you may wish to adjust this parameter. For
instance, if the vessel is very large and is not able to turn quickly, increasing this time is reasonable.
The resulting heading would have reduced ‘noise’, resulting in consistent values with time.
However, artificially increasing this value such that it does not agree with a more dynamic vessel
could create a lag in the heading measurement with higher rates of turn. A convenient formula for
determining what the level of smoothing follows for when the gyro is in use. If you are unsure on
how to set this value, it’s best to be conservative and leave it at the default setting.
htau (in seconds) = 40 / maximum rate of turn (in °/s) – gyro ON
htau (in seconds) = 10 / maximum rate of turn (in °/s) – gyro OFF
You may query the Vector for the current heading time constant by issuing the same command
without an argument.
$JATT,HTAU<CR><LF>
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