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34 SPAN-IGM User Manual Rev 2
Chapter 3 SPAN Operation
7. When the uncertainties of the offsets are low enough to be used for a kinematic alignment, the
calibration stops and the VEHICLEBODYROTATION log is overwritten with the solved values. To
monitor the progress of the calibration, log VEHICLEBODYROTATION using the ONCHANGED
trigger.
To save a calibrated rotation for subsequent start ups, issue the SAVECONFIG command after
calibration is complete. Each time the SPAN-IGM is re-mounted this calibration should be performed
again. See also Kinematic Alignment on page 31 for details on kinematic alignment.
3.2.5 SPAN Wheel Sensor Messages
The SPAN-IGM supports wheel sensor inputs. The SPAN-IGM accepts TTL level input pulses from a
wheel sensor through the AUX connector. See Appendix A, Technical Specifications on page 52 for
specifications on the wheel sensor interface.
3.2.5.1 Measurement Timing and Frequency
Typical wheel sensor hardware generates wheel ticks constantly as the wheel rotates. The SPAN-IGM
interface is configured to accumulate wheel sensor tick counts at a rate of 1 Hz.
3.2.5.2 Wheel Sensor Update Logic
Wheel sensor data is available through the TIMEDWHEELDATA log. The TIMEDWHEELDATA log can
be used for applying wheel sensor updates in post-processing.
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.
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 an 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-IGM and vehicle frames. The offsets are used when aligning the system
while in motion (see System Start-Up and Alignment Techniques on page 30). The angular
offset values are not applied to the attitude output, unless the
APPLYVEHICLEBODYROTATION command is enabled.