Consistency of HY-2B/C/D Altimeter Sea State Products in Chinese Coastal Seas: Wave Height, Wind Speed, and Parameter-Dependent Inter-Satellite Bias

The HY-2B/C/D constellation delivers routine altimeter significant wave height (SWH) and sea-surface wind speed over the Chinese coastal seas through four product streams (GDR, IDR, ODR, and SDR), yet whether the three satellites are mutually consistent, and whether that consistency is the same for both parameters, has not been established. We assess three years (2023–2025) of 1 Hz Ku-band retrievals against 30 quality-controlled coastal buoys, using a deduplicated matchup database, a common-buoy and common-period protocol with bootstrap confidence intervals, and triple collocation against ERA5. Within each satellite, the product streams are consistent for both parameters: the reprocessed GDR shifts the operational SWH by a sea-state-independent $-0.157$ m and the wind streams coincide to rounding. Across satellites, consistency is parameter-dependent. For SWH, the biases on common buoys span 0.18 m; HY-2B has the lowest random error, significantly lower than HY-2D, and HY-2C cannot be separated from either in random error. For wind speed, HY-2C reads about 1 m s${}^{-1}$ low in all four Chinese seas and in every wind band once the bands are defined by an independent reference, so that most of the offset is removable by a constant. The near-shore degradation of SWH inside 50 km largely reflects the sub-0.5 m sea states that dominate the near-shore buoys, and wind speed shows no comparable coastal signal. We translate these findings into applicability bounds and a parameter-aware recipe for merging the constellation.