Airborne

Mechanical installation

Airborne applications assume a 3D motion. The INS sensor can be located anywhere in the aircraft, considering the following recommendations:

• Sensor is rigidly fixed to the aircraft frame : avoid locations that may be bending like wings
• Sensor is not moving in regard to other equipment (antennas, LIDAR, etc...)
• Sensor is far from strong vibrations sources : avoid placing the sensor directly on the engine mounts.
• Sensor should be protected from high temperature variations.
• When relevant (if magnetometers are used), place the sensor away from magnetic disturbances like strobe lights, high current equipment, radios or moving parts like landing gears.

Note

SBG Systems IMU are designed to handle vibrations without specific care. Nevertheless in case of highly vibrating environment, a mechanical vibration isolation might be required for proper operation. Silicon or wire dampers can be used for that purpose.

Aircraft reference frame

The vehicle coordinate frame is defined as follows :

• X axis points to the front of the aircraft
• Y axis points rightward.
• Z axis points downward.

Note

The sensor can be placed in any orientation in the aircraft. When IMU axes do not match exactly with the aircraft coordinate frame, the rough and fine alignment parameters should be corrected through the configuration interface to realign the IMU and aircraft coordinate frames.

GNSS setup considerations

When installing your INS with a GNSS aiding, you will need to install the GNSS antennas with a clear view of sky, and fixed with respect to the IMU.

The GNSS lever arms shall also be measured, which are the signed distance, expressed in the aircraft coordinate frame, FROM the sensor center of measurements, TO the GNSS antenna.

We usually require these measurements to be precisely performed, within 1cm accuracy.

Note

It's generally not practical to measure with such precision the lever arms, so SBG Systems developed lever arm calibration that enables you to measure rough lever arm estimation (10cm precision) and let the tool refine those measurements.

The GNSS lever arms should be lower than 10m to minimize induced errors.

Single antenna installation

Single antenna installation is possible when regular dynamics are experienced and allow to converge heading angle. Typical heading performance will degrade during straight lines and will re-converge during dynamic phases.

A single antenna installation with GNSS lever arm is shown below:

Dual antenna installation

Dual antenna may be required if dynamics are expected to be low during extended periods of time (eg. navigation with more than 5min of straight lines). When using the INS in a dual antenna setup, the heading will remain stable and precise in all conditions. Heading can also be initialized in static conditions.

Dual antenna systems installation will require special care in order to obtain optimal performance :

• The antennas must be fixed with respect to the to the inertial unit
• Same antenna type should be used
  
• Same cables with identical lengths must be used for both antennas. If Splitter are used make sure that they are adapted and with the same characteristics
• Both antennas must have the same view of sky. Typically avoid placing antennas on each side of the rudder or parts that could mask a significant part of the sky.
  
• If the antenna model does not have integrated ground plane, a 10 cm diameter ground plane must be added for both antennas.
• If antennas are not permanently installed on the aircraft, antennas reference marks (usually the connector position) should be mounted in a repeatable fashion in order to guarantee antennas phase center stability from mount to mount and minimize changes to heading misalignment angle.

Both GNSS antennas lever arms should be measured accordingly.