## 1 Introduction  
Ocean wave, as an environment dynamic variable, plays an important role in various fields, such as air-sea interaction, numerical weather prediction, oceanographic engineering and shipping. Significant wave height (SWH) is a representative wave parameter that can describe the total energy of a wave system; thus, it is widely used in these fields. At present, spaceborne radars measuring the SWH include traditional altimeters, ocean wave spectrometers (OWSs), and synthetic aperture radars (SARs). These radars have special detecting features for retrieving the SWH using different methods. Currently, the most commonly used method for SWH remote sensing is the traditional altimeter approach, which has made an important contribution to global SWH products. In this approach, the SWH is directly estimated from the leading edge slope of a radar echo, and the slope is obtained by fitting the measured waveform to a physical-based echo model [1, 2, 3]. The first spaceborne OWS unit is on board the China-France Oceanography SATellite (CFOSAT), named Surface Waves Investigation and Monitoring (SWIM) [4]. The main feature of SWIM is the use of a single-tilt modulation and the azimuth integration of radar returns for achieving high directional resolution. Owing to these features, the wave spectra can be linearly retrieved from the modulation spectra, which are extracted from SWIM measurements. Then, the SWH can be statistically estimated by integrating the wave spectra [4; 5; 6; 7]. The primary advantages of SARs are their high spatial resolution and wide-swath coverage [8]. They can be used to extract the SWH using two main approaches. The first is by a theoretical transfer relation [9; 10; 11], and the second approach uses an empirical model [12; 13; 14; 15]. In the first approach, the wave spectra are first retrieved from image spectra through a nonlinear transfer relation. Then, the SWH can be estimated by integrating the retrieved wave spectra. However, the nonlinear and azimuth cutoff effects in the transfer relation make this approach only suitable for long waves. To address this limitation, in the second approach, an empirical model was used to directly estimate the SWH by establishing the relationship between the SWH and some variables, such as normalized variance, normalized radar cross-section (NRCS), and wind speed. The latter achieved a satisfactory SWH retrieval accuracy [12; 13; 14; 15]. In addition to the common SWH sensors mentioned above, a new spaceborne radar, the interferometric imaging radar altimeter (InIRA), has also preliminarily exhibited the ability to measure waves using captured wave-like stripes [16]. The InIRA is aboard the Chinese Tiangong-2 space laboratory, which was launched on 15 September 2016. It is the first Ku-band spaceborne imaging radar for low incidence angles. An important objective of designing the InIRA was to test and validate the mechanism of wide-swath sea surface height (SSH) measurement through interferometry with a short baseline. For this purpose, the InIRA uses a similar configuration as an interferometric SAR [17; 18], but with low incidence angles and high-capacity solid-state power amplifier. These features guarantee accurate measurements of the SSH with high spatial resolution in both the range and azimuth directions [19; 20; 21]. The finding concerning the InIRA wave-like stripes in [16] just benefited from the high-resolution performance. Thus far, several oceanic studies have been conducted using InIRA data. For instance, InIRA wind speed retrieval has been preliminarily carried out [22; 23]. An empirical model named KuMOD2 was developed using the Tropical Rainfall Measuring Mission precipitation radar data at low incidence angles. The InIRA wind speeds are retrieved from KuMOD2 with a root mean square error (RMSE) within 2 m/s. The InIRA interferometric processing algorithm for the SSH has been preliminarily developed and validated by collocated traditional altimeter SSH data [24; 25]. In addition, InIRA ship waves are determined and analyzed.