ALIGN rotation calibration routine

The ALIGN frame (or dual antenna baseline) is defined by the vector originating at the Primary antenna phase centre and terminating at the Secondary antenna phase centre. The ALIGN rotation represents the angular offset between the dual antenna baseline and the IMU frame’s forward axis. It is a right-handed rotation from the IMU body frame, as defined by the IMU label, to the dual antenna ALIGN frame.

In a dual antenna enabled SPAN system, the ALIGN rotation is typically calculated automatically using the lever arm offsets of the primary and secondary antennas provided by the user. This approach is more practical than directly measuring the angular offset between the IMU and the dual antenna baseline.

For detailed instructions on setting up a dual antenna use case, please refer to the Configuring dual antenna ALIGN with SPAN.

Why calibrate ALIGN rotation?

ALIGN rotation calibration is required if the ALIGN rotation offset uncertainties exceed the accuracy requirements of the application. Calibration ensures the INS filter can apply dual antenna ALIGN updates with greater confidence and accuracy, reducing azimuth error growth in low-dynamic conditions.

How to calibrate ALIGN rotation

Use the following steps to calibrate the ALIGN offset:

  1. Power on the receiver and IMU.

  2. Configure the SPAN system by following the instructions in SPAN configuration.

  3. Ensure that accurate RBV and lever arm values (primary and secondary antenna) have been entered into the system manually.

    Optionally, perform an RBV calibration to refine the RBV value. Refer to the Multi-line body to vehicle frame rotation calibration routine.

  4. Perform initial system alignment using one of the methods described in System start-up and alignment techniques.

    Ensure the system converges the INS solution by driving in an open-sky environment with good vehicle dynamics, as outlined in the Convergence Phase section of APN-109: SPAN Overview and Integration Guide.

  5. Enable ALIGN calibration using the INSCALIBRATE command, with the optional standard deviation target omitted:

    INSCALIBRATE ALIGN NEW

  6. Drive the vehicle with good dynamics in an open-sky environment, same as mentioned in step 4 above.

  7. To monitor the calibration status, log the INSCALSTATUS log using the ONCHANGED trigger.

    The calibration is complete when INSCALSTATUS reports a source status of CALIBRATED.

  8. The calibrated offset value can be found in the INSCALSTATUS log or INSCONFIG log, and is automatically entered into the system.

  9. To save the calibrated rotation for future/subsequent startups, issue the SAVECONFIG command.

If the IMU or antennas are re-mounted, this calibration process must be repeated.