Fig. 1. (a) Installation status of lidar in the cabin. (b) Schematic diagram of the aircraft roll. The solid line indicates the direction of the laser pulse, and the dotted line indicates the direction perpendicular to the aircraft.

Fig. 2. Sea surface backscatter coefficients at different wind speeds and off-nadir angles. The different colors represent different wind speed conditions.

Fig. 3. (a) Location of the experimental area. (b) Example of flight.

Fig. 4. (a) Flight altitude. (b) Heading: the angle between the aircraft’s body axis projection on the horizontal plane and the geographic north, with positive values indicating a rightward deviation of the aircraft nose. (c) Pitch: the angle between the aircraft’s body axis and the horizontal plane, with positive values indicating an upward tilt of the aircraft nose. (d) Roll: the angle between the aircraft’s body’s lateral axis and the vertical plane through the aircraft’s longitudinal axis, with positive values indicating a rightward roll of the aircraft. (e) Lidar’s original profile data along the flight path.

Fig. 5. Single-profile echo signals. (a) The 486 nm low-gain-channel signal profile for the counterclockwise flight from 0° to 35° on May 17, 2024, with an average flight altitude of H ≈ 415 m and the average sea surface wind speed of v ≈ 7.2 m/s, corresponding to Sea State 2. (b) The 486 nm low-gain-channel signal profile for the clockwise flight from 5° to 35° on May 17, 2024, with an average flight altitude of H ≈ 320 m and the average sea surface wind speed of v ≈ 7.2 m/s, corresponding to Sea State 2. (c) The 486 nm low-gain-channel signal profile for the counterclockwise flight from 0° to 35° on May 18, 2024, with an average flight altitude of H ≈ 510 m and the average sea surface wind speed of v ≈ 2.1 m/s, corresponding to Sea State 1. (d) The 532 nm low-gain-channel signal profile for the counterclockwise flight from 0° to 35° on May 17, 2024, with an average flight altitude of H ≈ 415 m and the average sea surface wind speed of v ≈ 7.2 m/s, corresponding to Sea State 2.

Fig. 6. Average pulse echo of 1000 profiles at varying off-nadir angles.

Fig. 7. Comparison of simulated and airborne measurement results for 0° to 25° off-nadir angles; “mea” represents the measured data from this experiment, “sim” represents the simulation results, and the shaded area indicates the relative error between the simulation results and the measured data. The superscripts (a)–(f) on each subplot denote off-nadir angles ranging from 0° to 25°, with marked error positions corresponding to water depth values of 2, 5, 10, 15, and 20 m, respectively.

Fig. 8. Backscatter coefficient of the sea surface at different off-nadir angles and wind speeds, 6.2 m/s (red), 7.2 m/s (blue), and 8.2 m/s (green), and the number of photons of the sea surface echo signal at different angles (black dotted line).