Lever Arm Calibration Routine

Each time the system is re-mounted on a vehicle, or the IMU or antenna is moved on the vehicle, the lever arm must be redefined either through manual measurement or through calibration.

We recommend that you measure the lever arm using survey methodology and equipment, for example, a total station. Only use calibrations when precise measurement of the lever arm is not possible.

Initial estimates and uncertainties for the lever arm may be entered using the SETINSTRANSLATION ANT1 command. The calibration routine uses these values as the starting point for the lever arm computation.

The Lever Arm Calibration routine is not available for the OEM-HG1930, OEM-IMU-ADIS-16488, OEM-IMU-EG320N, OEM-IMU-EG370N, OEM-IMU-HG4930, OEM-IMU-STIM300, IMU-CPT, IMU‑IGM-A1, IMU-IGM-S1, PwrPak7-E1, PwrPak7D-E1, PwrPak7-E2, PwrPak7D-E2, CPT7 or CPT7700.

The steps involved in the calibration are:

  1. Apply power to the receiver and the IMU.

  2. Configure the SPAN system, see SPAN Configuration.

  3. Enter the initial estimate for the lever arm using the SETINSTRANSLATION ANT1 command. See the SETINSTRANSLATION command.

    Ensure the standard deviation values entered for the antenna offset are not overly optimistic (i.e., err on the side of a larger standard deviation).

  4. Perform an initial system alignment using one of the methods described in System Start-Up and Alignment Techniques. Ensure the system reaches Solution Good.

  5. Enable calibration using the INSCALIBRATE command with an optional standard deviation target. For example, choose a standard deviation target of 0.05 m.

    INSCALIBRATE ANT1 NEW 0.05

  6. To monitor the calibration, log INSCALSTATUS using the ONCHANGED trigger. See the INSCALSTATUS log.

  7. Start to move the system. The lever arm is not observable while the system is stationary. Immediately, drive a series of maneuvers such as figure eights. The turns should alternate between directions, and you should make an equal number of turns in each direction. Some height variation in the route is also useful for providing observability in the Z-axis.

  8. When the calibration is complete, either because the accuracy requirement has been met, or the calibration has been halted by user command, the calibrated offset value can be viewed in either the INSCALSTATUS log or the INSCONFIG log.

To save a calibrated lever arm for subsequent start ups, issue the SAVECONFIG command after calibration is complete. If the IMU or GNSS antenna are re-mounted, re-run the calibration routine to compute an accurate lever arm.

An accurate Lever Arm calibration depends on observability in all axes. Often, it can be very challenging to achieve sufficient dynamics during a calibration (e.g. a land vehicle can not normally achieve the same dynamics vertically as horizontally). To overcome this limitation, complete multiple calibrations and average the results (see also INSCALIBRATE ADD or do so manually). The calibrated values for axes with poor observability will likely have a large range of results not reflected in the final standard deviation value. Another method that may obtain a better result for these axes is to set the standard deviation target very low (i.e. 0.01) and run the calibration for a long time (at least 15 minutes). In this case, the calibration may not converge (reach CALIBRATED), but the convergence can be observed and a final value chosen appropriately.