Chapter 4 SPAN Operation
CPT7 and CPT7700 Installation and Operation User Manual v12
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The measurements for the translational offsets should be done as accurately as possible, preferably
to within millimeters especially for RTK operation. Any error in the offsets will translate into an error in
the INS position.
The translational offsets from the IMU to the GNSS antenna are required for all SPAN systems. However,
some SPAN systems may have other sensors or devices integrated into the system for which the SPAN needs
translational offsets. For example, a gimbal mount or a camera or LiDAR connected to an Event Input line. The
offsets to these additional sensors and devices are also entered using the
SETINSTRANSLATION
command.
By default, the translational offsets are entered in the IMU Body Frame. However, in some systems the IMU
Body Frame may not be known precisely. For these cases, the
SETINSTRANSLATION
command has an
option that allows the offsets to be entered relative to the Vehicle Frame. In these cases, the rotation from the
IMU Body Frame to the Vehicle Frame (RBV) is used to rotate offsets into the default IMU Body frame as
required. To achieve the best results, this means that the RBV rotation must be known very precisely.
Typically this is achieved through the SPAN RBV Calibration procedure. See
Body to Vehicle Frame Rotation
on page 91 or
Multi-Line Body to Vehicle Frame Rotation Calibration Routine
for information about this calibration procedure.
4.2.2 Rotational Offsets
The differences in orientation between the IMU and other SPAN components are called rotational offsets.
Rotational offsets are given as the rotation from the IMU Body Frame to the frame of interest.
The order of rotations is Z-X-Y and all rotations are right handed.
Generally, frames of reference are defined as Z up, with Y forward, and X completing the right-handed system.
An example is the vehicle frame, where Z is always considered to be upwards, Y forward through the direction
of travel, and X to the right.
Rotational Offsets - Example
Consider an IMU installed in the following way:
Here, the Euler Angles to rotate from the IMU Body frame to the Vehicle frame (RBV) are:
