Sea Surface Height Measurements Using UAV Altimeters with Nadir LiDAR or Low-Cost GNSS Reflectometry

Although UAV height is precisely determined using GNSS, the vertical distance between the UAV and the sea surface should be subtracted to obtain the sea surface height (SSH). This distance can be measured using nadir-looking LiDAR or GNSS reflectometry (GNSS-R); thus, these two methods are examined in this study through three two-minute UAV experimental flights. The measurements of the flight-averaged SSHs made with both approaches were in good agreement with the reference SSH determined from a GNSS buoy, with differences of 0.03 m (LiDAR) and 0.05 m (GNSS-R), although the standard deviation (SD) for GNSS-R (1.72 m) was significantly larger than that for LiDAR (0.20 m). Each 4 Hz GNSS-R measurement was subject to errors caused by surface waves, though over 16 GNSS reflection points within a 70 m diameter footprint were used; these errors were, however, removed in the temporal mean. Extending the footprint diameter to 230 m with stronger data quality controls resulted in a smaller error (0.02 m) and SD (0.79 m). Meanwhile, LiDAR measured the flat-surface SSH at the nadir only, which inherently filtered out slant reflections, resulting in a lower SD. However, this filter reduces data acquisition rates, especially when the UAV attitude tilts.