IMU Alignment

There are two alignment routines that can be used:

• Static Alignment

• Kinematic Alignment

Rovers with IMUs of tactical grade or higher are capable of static alignment. Rovers with MEMS IMUs, or any IMU with a gyro bias larger than the Earth rate of 15 deg/hr at the equator, are not capable of deriving a reliable heading from gyro measurements alone, therefore they must rely on kinematic alignment.

Systems equipped with dual antenna will align statically, regardless of the IMU. Once both antennas achieve sufficient satellite tracking, a dual antenna heading can be computed and the navigation system should report an AlignmentComplete status.

Static Alignment

Static alignment is the default alignment routine for IMUs with gyro biases much less than the Earth rate of 15 deg/hr at the equator. At the time of writing, the IMUs supported by Phoenix LiDAR Systems that allow for static alignment are the IMU-32, IMU-33, IMU-34, IMU-41, IMU-44, IMU-52, and IMU-60.

Static alignment begins as soon as a quality GNSS solution is available, the receiver has computed fine time and the IMU is connected and configured. During static alignment, accelerometer and gyro measurements are averaged over a period of time to measure the Earth’s rotation and gravity. Using the averaged measurements obtained, initial estimates of roll, pitch and heading are computed. The attitude estimates solved by the alignment are larger than the system specified attitude accuracy and vary upon the characteristics of the sensor and the geographic latitude of the system. Attitude accuracy converges with motion after the coarse alignment is complete.

In order to obtain a reliable static alignment, the IMU must be completely static (e.g. the engine should be shut off so as to avoid vibrations) BEFORE and AFTER a scan for a period of about 5-10 minutes. Because static alignment uses the average of the sensor output, it is imperative that the vehicle remains completely stationary during the duration of the alignment. Furthermore, we recommend travelling in a straight line for a period of at least 15 seconds, at a minimum velocity of 5 m/s (18 km/h) at the beginning and end of your data collection.

Leaving the IMU static before and after a scan will allow the IMU to collect at least 5-10 minutes of static data. Typically the first 2 minutes of data are used for static alignment followed by the remainder of fine alignment, in which GNSS updates enable the IMU to provide attitude information consistent with the accuracy level achievable by the accelerometer/gyro triad.

Kinematic Alignment

IMUs with gyro biases larger than the Earth rate of 15 deg/hr at the equator are not capable of deriving a reliable heading from gyro measurements alone, therefore they cannot perform static alignments. For these IMUs, the default alignment routine is kinematic alignment. At the time of writing, the IMUs supported by Phoenix LiDAR Systems that require kinematic alignment are the IMU14, IMU-25, IMU-27, IMU-29, and IMU-30.

Kinematic alignment requires kinematic movement. It performs alignment by estimating the attitude from the GNSS velocity vector. To obtain a successful kinematic alignment during operation, we recommend travelling in a straight line for a period of at least 10 seconds, at a minimum velocity of 5 m/s (18 km/h) at the beginning and end of your data collection.

During this time, it is imperative that you ensure the vehicle is moving as straight and as level as possible with an unobstructive clear line of sight. When monitoring your rover in SpatialExplorer, during kinematic alignment, the INS status will remain as Aligning until any significant kinematic movement is performed. Once the heading has been determined with sufficient certainty, the INS Status field will change to AlignmentComplete.