## 3 Quality Assessment  
### Crossover Analysis  
The quality of altimetry measurements is commonly evaluated and validated by calibration based on a statistical analysis at crossover points. Smaller crossover discrepancies indicate better stability and internal conforming accuracy for onboard instruments. A certain premise is to determine the position of crossover points precisely. To do this, we introduced a quadratic equation and a linear interpolation to calculate the initial location and precise location, respectively. This accuracy assessment was similarly executed in the six selected regions for the 4 HY-2 missions, while along-track SSH measurements were  
Figure 3: Standard deviation of retracked height with respect to EGM2008 for HY-2 sample passes. Left figures: statistics for individual points. Right figures: medians over 0.5 m SWH intervals (red: height from sensor geophysical data record; green: height from first step of two-pass retracking; blue: height from second step of two-pass retracking). obtained based on procedures mentioned in Section 2.1. These study areas are shown in blue rectangles in Figure 4. The typical ground tracks inside these rectangles were used for along-track directional experiments during the steps described in Sections 3.2 and 4.2. To keep the amount of crossover points consistent, we designed an experiment of accuracy assessment on the basis of one complete HY-2A GM cycle and eight complete ERM cycles from HY-2B, HY-2C and HY-2D. All the crossovers were defined as positions where each mission crossed its own ground position. The time difference between the measurements of ascending and descending passes might somehow have been related to the corresponding statistical values. Hence, we calculated the standard deviations (SDs) of crossover discrepancies with time limits of 1, 5, 14 and 168 days, respectively. Meanwhile, gross errors were detected and removed according to the criteria of triple-standard deviation. The specific results are listed in Table 2.  
Generally, the SD values ranged from 5 to 20 cm for HY-2 missions, representing a reliable accuracy level compared with other altimetry missions [24]. Moreover, the measurements within the shortest time limits had the best coincidence in each situation. While the time limit was the longest (168 days), the SD values were within a range of 9-19 cm. The HY-2C had the smallest SD, with an average value of 12.4 cm, and HY-2A had the largest value of 16.71 cm. HY-2B and HY-2D had slightly larger average SDs at 13.71 cm and 12.8 cm, respectively. Noticing that the time spans of eight complete ERM cycles were ~112, ~80 and ~80 days, respectively, the SD might increase if the time span extended to 168 days. Hence, the first case proved that all four HY-2 altimetry missions have a similar and reliable accuracy level. When the time limit narrowed to 14 days in the second case, the average SD values improved 22.37%, 23.63%, 16.26% and 17.15% for HY-2A, HY-2B, HY-2C and HY-2D, respectively. In addition, the average SD values declined along with the transition to the third case within five days' time limit, but the improvement percentage was relatively slower. When the time limit was less than 1 day, the SD shrank to 5-12 cm and its average values were within 6-9 cm. This represented a centimeter accuracy level of HY-2 measurements when the time-varying effect is relatively insignificant in the fourth case with the shortest time limit. All these results proved that these HY-2 missions provided reliable and high-quality SSH measurements. Considering that the characteristics of spatial distribution were relatively complementary between HY-2 and the other missions, more stable computations might be expected if HY-2 series data were incorporated into a multi-satellite dataset to recover global marine gravity models.