# Significant Wave Height Estimation from Space-Borne Cyclone-GNSS Reflectometry
## 1 Introduction  
The ocean is one of the main factors in affecting Earth's climate change. The significant wave height of the sea surface is an important parameter of the \"digital ocean\", and plays an important role in ocean environmental prediction and safety. Traditional detection technologies like ships and buoys have been used to measure sea surface waves and seawater salinity. The buoys can collect information on various elements of the marine environment in a long-term, automatic, fixed-point, timed and all-weather manner, but have smaller coverage and higher cost, also affected by sea surface factors [1]. Significant wave height (SWH) observations based on numerical models and satellite radar altimeters began in the 1970s. The satellite altimeter provides global coverage and climatological scale SWH data. After the 1990s, represented by ERS-1, 2/SAR and Envisat/ASAR launched by ESA, the space-borne SAR provided strong support for ocean wave research and forecasting. [PERSON] et al. [2] proposed the theory and method of estimating the SWH by using the signal-to-noise ratio (SNR) of thesynthetic aperture radar echo signal and found that there was a linear relationship between the SWH and the root-mean-square (RMS) of the signal-to-noise ratio (SNR). Then, the method was extended to the X-band marine radar image to estimate the SWH [3]. [PERSON] et al. [4] also extracted RMS wave heights from high frequency ocean back-scatter radar spectra through the ratio of second- to first-order energies. However, existing satellite altimeter SWH products have limited spatial resolution. The Global Navigation Satellite System (GNSS) has undergone more than 40 years of development and has gradually become an important part of the national spatial information infrastructure. Since different reflected signals carry different surface information, GNSS-Reflectometry (GNSS-R) technology can be applied not only to ocean measurement but also to sea ice measurement, surface soil moisture, and frozen soil measurement [5; 6]. [PERSON] et al. [7] monitored the snow height and surface temperature changes in Greenland from GPS reflection signals, and then used the BeiDou Navigation Satellite System-Reflectometry (BDS-R) to conduct and evaluate sea level changes for the first time [8]. [PERSON] et al. [9] used the space-borne GNSS bistatic radar receivers to sense changes in soil moisture from the low Earth orbiting UK TechDemoSat-1 satellite (TDS-1). Compared with traditional remote sensing technology, GNSS-R adopts a bistatic configuration to receive and process information with low cost, global coverage and long-term continuous operation. [PERSON] et al. [10] proposed the concept of measuring sea level by using GNSS sea surface reflection signals for the first time. Interference Pattern Technique (IPT) and Interferometric Complex Field (ICF) method have been proposed to estimate the SWH [11; 12]. Later, non-coherent navigation detection was applied to the detection of sea conditions by using SNR to estimate the SWH [13] and proved the possibility of SNR-based GNSS-R inversion of significant wave height. Due to the influence of sea surface roughness on the reflected signal, [PERSON] et al. [14] demonstrated the possibility using the delayed Doppler map (DDM) to invert sea surface winds and waves. Then, [PERSON] et al. [15] used Doppler velocity to estimate the significant wave height of the sea surface, but this method, similar to the IPT method, requires the receiver to be stationary. The Cyclone-GNSS (CYGNSS) satellites were launched in December 2016 [16]. This mission provided DDM information and gave us the first chance to estimate near global SWH using space-based CYGNSS data. This technology can make up for the shortcomings of conventional real-time marine meteorological observation data and provide important technical support for ocean shipping development, marine resource development, marine environmental protection, marine disaster warning and forecasting, regulatory emergency, and cruise rescue.