A Study of the Technology Used to Distinguish Sea Ice and Seawater on the Haiyang-2A/B (HY-2A/B) Altimeter Data  
[PERSON]  
1 School of Marine Science and Technology, Tianjin University, Tianjin 300072, China; [EMAIL_ADDRESS] (C.J. ); [EMAIL_ADDRESS] (H.W. )1 School of Marine Science and Technology, Tianjin University, Tianjin 300072, China; [EMAIL_ADDRESS] (C.J.) classification samples or for verification purposes, since they are able to provide point metadata with high spatial resolution. [PERSON] [3] published the first analysis of quantified sea ice and seawater using the altimeter waveforms from Geodynamics Experimental Ocean Satellite 3 (GEOS-3). He found that the values of the Automatic Gain Control (AGC) on the surfaces of the ice formations were larger than the sea ice. At the same time, he defined an index which combined AGC and Average Attitude/Specular Gate (AASG) which presented different values on different surfaces. [PERSON] [4] observed the AGC values would display steep rises from the seawater to sea ice along the satellite tracks. [PERSON] [5] mapped the ice distributions of the Antarctic using the AGC values. In 1994, he presented the typical waveforms of diffuse and quasi-specular surfaces (Figure 1). [PERSON] [8] used a KNN classifier combined with Cryostat-2 data in operational ice charting processes. [PERSON] [9] adopted six different methods (convolutional neural network, Bayesian, K nearest-neighbor, support vector machine, random forest, and back propagation neural network) to classify sea ice types using the data of Cryostat-2. It was found that in the aforementioned studies, [PERSON] and [PERSON] had achieved accurate rates. The data obtained by altimeters can also be used to extract ice concentration data [6; 10; 11] for the purpose of estimating the thicknesses of sea ice in polar regions [12; 13; 14; 15]. On 15 August 2011 (UTC), the HY-2A satellite was launched into space [16]. Then, approximately eight years later, the China National Space Administration (CNSA) launched the HY-2B on 25 October 2018 by [PERSON] 4 (CZ meaning \"Long March\") at Taiyuan. The HY-2 spacecraft have been operated by the National Satellite Ocean Application Service (NSOAS). The primary objectives of the HY-2 missions are to measure marine dynamics and environmental parameters in the microwave region [17]. The HY-2B is the second satellite of Chinese Marine Dynamic Environment Satellite series. These satellites form China's first steps in its marine dynamic satellite constellation program, with follow-up missions (HY-2C and HY-2D) planned for approximately 2020. The subsequent missions will greatly improve the observation capabilities of the global marine dynamic environment elements. The four main payloads onboard the HY-2A/B satellites include the following: Radar altimeter: Microwave scatterometer; scanning microwave radiometer; and three-frequency microwave radiometer [18]. In addition, the Haiyang-2B is equipped with global ship Automatic Identification System (AIS) and marine buoy measurement Data Collection System (DCS) capabilities. The HY-2A/B altimeter (ALT) measure the sea ice in the polar regions while observing the marine environments, similar to the European Remote Sensing satellite-1/2(ERS-1/2), European Space Agency Environmental Satellite (ENVISAT), and Sentinel 3 a/b systems. The HY-2 satellites support long duration series datasets for the research studies of polar regions. Additionally, the HY-2A/B use a new tracker for ocean and land compatibility, the Model Compatible Tracker (MCT), in order to prevent unlocked tracking. For the OCOG and SMLE, the two different track algorithms are cooperated in parallel. The original intention of the design was to handle distinct signals using different track algorithms automatically [19]. However, the track algorithms had not consistently detected the OCOG on the sea ice surfaces. As a result, it was found that the sea ice and seawater could not be accurately distinguished using only the different track flags.