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Chapter 3: Operation
Time Constants
The V104s incorporates user-configurable time constants that can provide a
degree of smoothing to the heading, pitch, rate of turn (ROT), course over
ground (COG), and speed measurements. You can adjust these parameters
depending on the expected dynamics of the vessel. For example, increasing
the time is reasonable if the vessel is very large and is not able to turn
quickly or would not pitch quickly. The resulting values would have reduced
“noise,” resulting in consistent values with time. However, if the vessel is
quick and nimble, increasing this value can create a lag in measurements.
Formulas for determining the level of smoothing are located in Hemisphere
GNSS’ GPS Technical Reference. If you are unsure on how to set this value, it
is best to be conservative and leave it at the default setting.
Note: For heading and rate of turn there is no lag once the gyro is calibrated
and enabled.
Heading time constant: Use the $JATT,HTAU command to adjust the level
of responsiveness of the true heading measurement provided in the
$GPHDT message. The default value of this constant is 20.0 seconds of
smoothing when the gyro is enabled. The gyro is enabled by default, 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
automatically done and therefore you must manually enter it. Increasing the
time constant increases the level of heading smoothing and increases lag.
Pitch time constant: Use the $JATT,PTAU command to adjust the level of
responsiveness of the pitch measurement provided in the $PSAT,HPR
message. The default value of this constant is 0.5 seconds of smoothing.
Increasing the time constant increases the level of pitch smoothing and
increases lag.
Rate of Turn (ROT) time constant: Use the $JATT,HRTAU command to
adjust the level of responsiveness of the ROT measurement provided in the
$GPROT message. The default value of this constant is 2.0 seconds of
smoothing. Increasing the time constant increases the level of ROT
smoothing.
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