Sea Surface Height and Significant Wave Height Calibration Methodology by a GNSS Buoy Campaign for HY-2A Altimeter

GNSS buoys are innovative ocean instruments, which can simultaneously provide calibrations of the sea surface height (SSH) and significant wave height (SWH) products of satellite altimeters. This paper investigates the SSH and SWH retrieval methods by GNSS buoys. To improve the SSH accuracy, an innovative adaptive Vondrak filter is designed to reduce the ocean-wave noise, and the singular spectrum analysis technology is adopted to reduce the instrument noise. A method of the altimeter products calibration by GNSS buoys is proposed. An ocean calibration campaign was implemented to measure the SSH and SWH at the HY-2A altimeter nadir. A fifth-order polynomial fitting strategy is applied to exploit the potentials of altimeter SSH measurements. A SSH bias of + 4.9 +/- 5.0 cm and a SWH bias of + 3.8 +/- 4.0 cm using four-parameters maximum-likelihood estimation (MLE4) retracking algorithm were acquired, which were well belowthe specifications of the altimetrymeasurement. Besides, the SSH bias and SWHbias were+ 4.2 +/- 5.2 and+ 22.9 +/- 9.6 cm, respectively, based on the HY-2A operational SWH products (using MLE3 retracker), proving that the four-parameters retracking algorithm outperformed the conventional three-parameters retracking algorithm in accuracy and consistency, at least for SWH retrieving.