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30 SPAN-CPT User Manual Rev 8
Chapter 3 SPAN-CPT Operation
The rotation parameters can be saved in NVM for use on startup in case a fast alignment is required
in the future. Each time the SPAN-CPT is re-mounted, this calibration should be performed again.
See System Start-Up and Alignment Techniques on page 27 for details on alignment.
3.4.4 SPAN-CPT Wheel Sensor
The SPAN-CPT system supports wheel sensor inputs, integrated via the SPAN-CPT. The SPAN-CPT
accepts TTL-level input pulses from a wheel sensor through the multi-pin connector. See Appendix A,
Technical Specifications on page 48 for specifications on the wheel sensor interface.
3.4.4.1 Wheel Sensor Update Logic
The wheel sensor information is sent to the SPAN-CPT along with the raw IMU data.
The SPAN filter uses sequential TIMEDWHEELDATA logs to compute a distance traveled between
update intervals (1 Hz). This information is used to constrain free inertial drift during times of poor GNSS
visibility. The filter also contains a state for modeling the circumference of the wheel as it may change
due to hardware changes or environmental conditions.
The modeled wheel circumference is available in the WHEELSIZE log. Information on how the wheel
sensor updates are being used is available in the INSUPDATE log.
3.5 Azimuth Sources on a SPAN System
The SPAN system use three different methods to calculate the azimuth.
• Course Over Ground
• Inertial Azimuth
• Align Azimuth
3.5.1 Course Over Ground
The course over ground azimuth is determined using the position delta between two position solutions
computed by the GNSS receiver. This is the simplest way to compute an azimuth and is done using
either the GNSS solution or the INS solution. This method does not work when the vehicle is stationary
as any position difference is due to position error and the computed azimuth is meaningless.
Course over ground azimuth is of greatest advantage in aerial or marine environments where the actual
direction of travel may not match the forward axis of the aircraft/boat due to winds or currents. This effect
is known as the crab angle. Course over ground azimuth is a great way to compute the offset if another
means of computing the vehicle azimuth are available.
Course over ground azimuths are available in several different velocity logs. See Table 5, Logs with
Azimuth data on page 31.
After the RVBCALIBRATE ENABLE command is entered, there are no vehicle body rotation
parameters present and a kinematic alignment is NOT possible. Therefore this command
should only be entered after the system has performed either a static or kinematic
alignment and has a valid INS solution.
The solved rotation values are used only for a rough estimate of the angular offsets
between the SPAN-CPT and vehicle frames. The offsets are used when aligning the system
while in motion (see System Start-Up and Alignment Techniques on page 27). The angular
offset values are not applied to the attitude output, unless the
APPLYVEHICLEBODYROTATION command is enabled.