Fixing an incorrect IMU orientation in SpatialExplorer

Fixing an incorrect IMU orientation in SpatialExplorer

Fixing an incorrect IMU orientation in SpatialExplorer

IMU data is relative to the IMU's own coordinate system (specified by the manufacturer), so to get IMU data in vehicle frame, we must know how the IMU is oriented in respect to the vehicle. The IMU orientation is specified in the IMU session in the project window:


The initial IMU orientation comes from the rover profile. Rover profiles are stored on the lidar hardware and contain settings for standard acquisition styles (i.e. mobile, UAV, helicopter). If the installation of the lidar system onto the vehicle does not match the rover profile, the IMU orientation must be modified.

Here's a couple ways you can determine what the correct IMU orientation is:

Use the Axis Mapping Tool

IMU axes are indicated on the lidar system, usually etched into the Navbox. You can associate the IMU axes with a vehicle direction using the axis mapping tool, which will update the IMU orientation values. This is useful when the lidar system is level, as each IMU axis will directly correspond to a vehicle axis. In the case of mobile mapping, where the lidar system is mounted with a pitch (i.e. 30 or 45 degree), the axis mapping tool cannot be used to completely express the IMU orientation. 


Rotate the existing IMU orientation

If you know how your IMU orientation is incorrect, you can try to rotate the IMU orientation accordingly. A great example of this is when the lidar system is mounted 180 degrees incorrectly, such as when the lidar is mounted facing forward, rather than backwards (most rover profiles specify the lidar to face vehicle-rear, and the camera to face vehicle-forward). In this case, you can add 180 degrees to the InertialExplorer Z axis:


Additional considerations 

External Trajectory Processing

If you are using NavLab embedded for trajectory processing, NavLab will use the IMU orientation specified in SE, however if you process your trajectory externally, either via InertialExplorer or NavLab on LiDARMill, you will need to update the orientation there, as well as in SpatialExplorer. In either of those cases, you should input the InertialExplorer Rotation (shown above) into NavLab or InertialExplorer. 

GNSS antenna lever arm offsets

When the IMU orientation is adjusted, GNSS antenna lever arm offsets change. Sometimes the change is relatively minor, depending on the lidar system. Consider estimating lever arm values when modifying the IMU orientation is necessary.