Preliminary Significant Wave Height Retrieval from Interferometric Imaging Radar Altimeter Aboard the Chinese Tiangong-2 Space Laboratory

Abstract

The interferometric imaging radar altimeter (InIRA) aboard the Chinese Tiangong-2 space laboratory is the first spaceborne imaging radar working at low incidence angles. This study focuses on the retrieval of significant wave heights (SWHs) from InIRA data. The retrieved SWHs can be used for correcting the sea state bias of InIRA-derived sea surface heights and can supplement SWH products from other spaceborne sensors. First, we analyzed tilt, range bunching and velocity bunching wave modulations at low incidence angles, and we found clear dependencies between the SWH and two defined factors, range and azimuth integration, for ocean waves in the range and azimuth directions, respectively. These dependencies were further confirmed using InIRA measurements and collocated WaveWatch III (WW3) data. Then, an empirical orthogonal SWH model using the range and azimuth integration factors as model inputs was proposed. The model was segmented by the incidence angle, and the model coefficients were estimated by fitting the collocation at each incidence angle bin. Finally, the SWHs were retrieved from InIRA data using the proposed model. The retrievals were validated using both WW3 and altimeter (JASON2, JASON3, SARAL, and HY2A) SWHs. The validation with WW3 data shows a root mean square error (RMSE) of 0.43 m, while the average RMSE with all traditional altimeter data is 0.48 m. This indicates that the InIRA can be used to measure SWHs.

Reconstruction of three-dimensional ship wake by Tiangong-2 interferometric imaging radar altimeter

“…It is also a twochannel interferometric radar with a short baseline of 2.3m, so it can acquire interferometric phase very well. At near-nadir incidence, the SNR of the sea surface echo is very high, which is very good for achieving high performance on the measurement of sea surface height with centimetre-level accuracy [9].…”

Preliminary Results of Marine Gravity Recovery by Tiangong-2 Interferometric Imaging Radar Altimeter

“…Benefiting from these new technologies, TG2 InIRA has shown great potential in both ocean observation and terrestrial water measurement. In the past few years, some studies have been conducted based on the observation data of TG2 InIRA, such as significant wave height (SWH) retrieval [40], wind speed retrieval [41,42] and joint retrieval of wind speed and SWH [43], internal wave detection [44] and river-lake water level measurement [45]. In our previous work [46], we presented a very initial result of DOV using the TG2 InIRA SSH data without baseline correction and noise suppression.…”

Ocean Wave Inversion Based on a Ku/Ka Dual-Band Airborne Interferometric Imaging Radar Altimeter

“…Roger Fjørtoft et al, (2014) [15] further conducted the first Ka-band InIRA experiment, which proved the typical characteristics of Ka-band InIRA images, and then assessed the superiority of satellite-based Ka band for detecting ocean waves. In 2016, successful operation of the Tiangong-II InIRA sensor was the first to assess the effectiveness for detecting ocean waves using the Ku band at small incidence angles [16]. In addition, the forthcoming launch of the Surface Water and Ocean Topography (SWOT) satellite was equipped with a Ka-band InIRA to obtain high inversion accuracy of ocean waves.…”