Seiwa Si-Tex Vector 3D GPS - Manual (Page 26)

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Vector 3D User Guide
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 GPS 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. You can use the
following formula to determine COGTAU: cogtau (in seconds) = 10 / maximum rate of change of course (in
degree/sec).
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
.
Watchdog
The watchdog is a timer that is controlled by the software that monitors if the heading is lost. The watchdog
software is compliant with IEC 60495.