RTK-SLAM GNSS-IMU Lever Arm

The GNSS antenna position in the MID360 IMU frame. It is composed from two pieces of evidence, each pinned by its SHA-256:

• the GNSS-LiDAR lever arm;
• the IMU-LiDAR rotation and translation.

The formula is p_imu = R_lidar_imu^T (p_lidar - t_lidar_imu), with the std propagated from the inputs:

calibrex gnss-imu compose \
  --gnss-lidar rtk_slam_gnss_lidar_stadtgarten_seq2.yaml \
  --imu-lidar-rotation mid360_imu_lidar_rotation_rtk_slam_seq2.yaml \
  --imu-lidar-translation mid360_imu_lidar_translation_rtk_slam_seq2.yaml \
  --reference 0.023 -0.023 0.090 --output gnss_imu.yaml

An axis is estimated only when two conditions hold:

• its propagated std is at most 2 cm;
• no input axis it depends on is unobservable.

The composition never upgrades an input's verdict. The reference is the dataset's CAD value (calib.yaml gnss_antenna_phase_center, from the IMU origin).

Recording	x	y	z	Verdict
stadtgarten_seq2	30.3 ± 11.6 mm (CAD +7.3)	-26.5 ± 7.2 mm (CAD -3.5)	80.8 ± 40.4 mm, unobservable (CAD -9.2)	inconclusive
stadtgarten_seq1	26.8 ± 13.0 mm, unobservable (CAD +3.8)	-31.2 ± 9.0 mm (CAD -8.2)	113.0 ± 13.3 mm, unobservable (CAD +23.0)	inconclusive

• Where the unobservable axes come from. On seq2, z inherits the GNSS-LiDAR z std of 4 cm. On seq1, x inherits the IMU-LiDAR translation's unobservable x, and z inherits the GNSS-LiDAR's unobservable z.
• Agreement with CAD. Where estimated, the axes agree with the CAD value within 1 cm.
• It is a composition, not a direct fit. The std assumes the inputs' errors are independent, and a direct GNSS-IMU fit is not implemented.

Artifacts: docs/assets/rtk_slam_gnss_imu/.

No SOTA claim is made for gnss-imu.