Seiwa Si-Tex Vector 3D GPS - Manual (Page 25)

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Chapter 3: Operation
will have drifted too far and the 3D begins outputting null fields in the heading output messages. There is no
user control over the timeout period of the gyro.
Calibration, which is set at the factory, is required for the gyro to remove latency from the heading solution
as well as provide backup heading when GPS is blocked. The receiver will calibrate itself after running for a
while but it may be important to follow the manual calibration instructions if you want to guarantee
performance quickly after powering up the receiver.
The gyro initializes itself at powerup and during initialization, or you can calibrate it using $JATT. 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 3D on a high speed,
maneuverable craft, it is essential that when gyro aiding in the 3D 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 GPS 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. Setting an
appropriate HTAU value for the application is further discussion later.
Time Constants
The 3D incorporates user-configurable time constants that can provide a degree of smoothing to the heading,
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. If you are unsure on how to set this value, it is best to be conservative and
leave it at the default setting.
Note: 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 2.0 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. You can use the following formula to determine level of
heading smoothing required when the gyro is in use:
htau (in seconds) = 40 / maximum rate of turn (in /sec).
When the gyro is disabled, the formula is: htau (in seconds) = 10 / maximum rate of turn (in /sec)
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. you can
use the following formula to determine the level of pitch smoothing required: ptau (in seconds) = 10 /
maximum rate of pitch (in /sec)
Heading Rate time constant: Use the $JATT,HRTAU command to adjust the level of responsiveness of the
rate of heading change 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 heading smoothing. You can
use the following formula to determine the level of smoothing: hrtau (in seconds) = 10 / maxiumum rate of
the rate of turn (in /sec).