# Evaluation of HY-2 Series Satellites Mapping Capability on Mesoscale Eddies
## 1 Introduction  
Satellite altimetry is an unprecedented observing system for a large number of applications that arose in the 1970s [1, 2]. It allows sea surface height (SSH) to be systematically identified on a global scale for the first time [3, 4]. While a satellite measures SSH along ground tracks, it produces 6-7 km along-track sampling but leaves 200-300 km cross-track gaps [5]. In the case of multiple satellites in orbit, data from different altimeter satellites can be fused by an optimal interpolation (OI) analysis [6]. The synthetic product of two satellites can achieve a spatial resolution of 150 km [7]. While a good representation of mesoscale variability requires at least two altimeter satellites [8, 9], near-real-time studies require more than three to four satellites [10, 11].  
Thus, in order to satisfy the requirement of having at least two altimeter satellites work in orbit simultaneously, a series of altimeter missions have been implemented. Since the launch of the first high-precision altimetry satellite (Topex/Poseidon) in 1992 [12], the National Satellite Ocean Application Service (NSOAS) also launched HY-2 series satellites and constructed the first batch of HY-2 constellation dedicated to the ocean dynamic environment on 19 May 2021 [13]. The first batch of HY-2 constellation consists of HY-2B [14], HY-2C [15] and HY-2D [13], integrating active and passive microwave sensorsto acquire oceanic environmental information [16]. These altimetry satellites are roughly equipped with similar detection devices but operate in different orbits, with HY-2B in a sun-synchronous orbit but HY-2C/D operating in \(\pm\)66\({}^{\circ}\) inclined orbits [17]. This orbital design permits oceanic and polar observations and facilitates in-orbit calibration and validation, making HY-2 an independent, controllable and highly detectable satellite constellation [18].  
However, studies of HY-2 series satellites have mainly focused on the precise orbit determination and data quality on radar altimeters [19; 20], and the oceanic observations for single, dual satellites or in combination with other satellites [21; 22; 23], lacking an evaluation of the ocean phenomena of HY-2 constellation. Mesoscale eddies [4] are typical mesoscale phenomena that exist almost everywhere in the ocean [24]. They can trap and transport water with heat, mass and energy, playing an essential role in multiple ocean processes [25; 26]. Thus, the analysis of the detection performance on mesoscale eddies could provide a potential approach to evaluating HY-2 series satellites.  
In this paper, we selected three oceanic regions at different latitudes as interest regions and constructed pseudo-observations based on the geographical and temporal positions of altimetry satellite along ground tracks, using the outputs from the HYCOM model. Two altimetry satellite combinations were selected, namely, the reference group in the Jason-3+Sentinel-3A/B configuration and the evaluation group in the Jason-3+Sentinel-3A/B+HY-2B/C/D configuration. After generating synthetic SSH maps by an OI procedure, we applied the eddy identification algorithm to the original model outputs and the gridded products and then compared the eddy fields characteristics. The remainder of this paper is organized as follows: Section 2 describes the materials and methods used in the paper. In Section 3, the improvement of HY-2 series satellites on along-track coverage and synthetic gridded products is assessed, and the impact of HY-2 series satellites on the eddy identification capability is examined. Section 4 is a discussion, in which eddy identification in models and gridded products are presented. Section 5 is the conclusions.