Navigation Mode
Once the alignment routine has successfully completed, SPAN enters navigation mode.
SPAN computes the solution by accumulating velocity and rotation increments from the IMU to generate position, velocity and attitude. SPAN models system errors by using a filter. The GNSS solution, phase observations and automatic zero velocity updates (ZUPTs) provide updates to the filter. Peripheral updates can also be supplied;
Following the alignment, the attitude is coarsely defined, especially in heading. Vehicle dynamics, specifically turns, stops and starts, allow the system to observe the heading error and allows the heading accuracy to converge. The amount of dynamics required for filter convergence vary by the alignment quality, IMU quality, and maneuvers performed. The INS Status field changes to INS_SOLUTION_GOOD once convergence is complete. If the filter degrades in its system accuracy, the INS Status field changes to INS_HIGH_VARIANCE. When the accuracy converges again, the INS status continues as INS_SOLUTION_GOOD.
While in navigation mode (in firmware versions 7.09.00 or greater), SPAN operates an internal secondary INS filter for built-in redundancy and reliability. When a verified GNSS solution of sufficient quality is available, the primary and secondary filters are validated against the GNSS solution. If divergence between the primary filter and the GNSS solution is detected, SPAN position, velocity and attitude output will fall back to using the secondary filter solution. This allows the system to reinitialize the primary filter if the divergence cannot be recovered, without interruption to the system's position, velocity and attitude output. Once the primary INS filter returns to normal operation (through recovery or reinitialization), the INS solution will revert back to the primary filter.
The system will flag bit 22 of the INS Extended Solution Status (see Table: Extended Solution Status) to indicate whenever the secondary INS solution is in use.