V103/113 Vector User Guide Chapter 3 - Operation Page 30 of 35
The gyro initializes itself at powerup and during initialization, or you can calibrate it as outlined in the Hemisphere
GNSS Technical Reference Guide. When the gyro is first initializing, it is important that the dynamics that the gyro
experiences during this warmup period are similar to the regular operating dynamics. For example, if you use the
V103/113 on a high speed, maneuverable craft, it is recommended that when gyro aiding in the V103/113 is first
turned on, use it in an environment that has high dynamics for the first five to ten minutes instead of sitting
stationary.
With the gyro enabled, the gyro is also used to update the post HTAU smoothed heading output from the moving
base station RTK GNSS heading computation. This means that if the HTAU value is increased while gyro aiding is
enabled, there will be little to no lag in heading output due to vehicle maneuvers. The Hemisphere GNSS Technical
Reference Guide includes information on setting an appropriate HTAU value for the application.
Time Constants
The V103/113 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 the
Hemisphere GNSS Technical Reference Guide. 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 is10.0 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 only if the gyro is disabled.
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.
Course Over Ground (COG) time constant: Use the $JATT,COGTAU command to adjust the level of
responsiveness of the COG measurement provided in the $GPVTG message. The default value of this constant is
0.0 seconds of smoothing. Increasing the time constant increases the level of COG smoothing. COG is computed
using only the primary GNSS antenna and its accuracy depends upon the speed of the vessel (noise is proportional
to 1/speed). This value is invalid when the vessel is stationary, as tiny movements due to calculation inaccuracies
are not representative of a vessel’s movement.
Speed time constant: Use the $JATT,SPDTAU command to adjust the level of responsiveness of the speed
measurement provided in the $GPVTG message. The default value of this constant is 0.0 seconds of smoothing.
Increasing the time constant increases the level of speed measurement smoothing.
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