Review for “A new global marine gravity model NSOAS24 derived from multi-satellite sea surface slopes ”

The manuscript presents the development of a new global marine gravity model, NSOAS24, which utilizes updated data from multiple nadir satellite altimetry missions, including HY-2. The manuscripts address limitations in a previous model, NSOAS22, also developed by the lead author. The authors proposed several technical improvements, including boundary inconsistencies, dataset filtering, near-shore area processing, and re-designing step sizes. The NSOAS24 model has comparable performance compared to the Sandwell & Smith model and DTU model. The work is relevant and aligns well with the ongoing need for high-resolution marine gravity models for applications in geophysics, oceanography, and remote sensing. My primary concern is that the authors compare NSOAS24 against the Sandwell & Smith V32.1 and DTU21 models, which use a shorter timespan of nadir satellite radar altimetry data. Including an explanation of this discrepancy in the manuscript would enhance clarity. Overall, I see no major technical issues and recommend a minor revision.

Detailed comments:

1. Need to introduce DTU21, SS V32.1 in the abstract.
2. In Figure 1e, what are the stripes? Given CryoSat-2’s inclination of 92°, one would expect N-S aligned stripes rather than the NW-SE pattern depicted.
3. Figure 2 caption: in dashed rectangular  in dashed rectangle.
4. Equation 1: Ensure that every parameter in Equation 1, including 'N,' is defined within the text to avoid ambiguity.
5. Line 276: what are the wavelengths used in the Parks-McClellan filter? Need to provide the wavelength range used for this study.
6. Line 281: what is sea surface topography? Is it mean dynamic topography? If so authors should to use this standardized term.
7. Figure 8 is very hard to read. Authors could possibly consider using histogram to compare the noise level and noise points alternatively.
8. Color bars (figures 7,8,10,11,14): The current colorbar make it challenging to distinguish between red and dark pink. Could consider color-blind-friendly colormaps such as turbo, or blue to red when showing differences in Figures 10 and 11.

The manuscript presents a global gravity modal which includes latest unique HY-2 satellite data, besides the conventional altimetry missions, which were used for the development of DTU21 and Sandwell&Smith V32.1. From the analysis and comparison, it is clear that that the NSOAS24 model is an improvement against the predecessor model NSOAS22. The authors revised and optimized the data preprocessing, data editing, high-frequency noise reduction by mission-specific filter, and gridding procedure. All these steps lead to an improved Global marine gravity model compared to the predecessor model NSOAS22. The latest model NSOAS24 show comparable accuracy to DTU21 and Sandwell&Smith V32.1 model, which are well-known and internationally recognized. It is worth to note that in both NSOAS24 and V32.1 models, the marine gravity is inverted from the along track slope and Deflection of the Verticals. The model presented in this paper nearly outperform the V32.1 model globally and in the test regions as shown by the cross validation with ship borne gravity measurements. In general, the work presented is valuable for optimizing data processing approaches and improving the accuracy of marine gravity models. The manuscript can be accepted with a minor revision.

Detailed comments:

1. Line 102: As a model developed in 2024, why the Cryosat-2 data is not processed up to year 2024? 5 years of data missing there.
2. Line 120: why in Figure 1 a,b,f,g visually look so much blurred/white?
3. Line 293: Matric singularity.
4. Line 293: remove ”By the way”, replace with “It is worth to mention”
5. Line 319: Section 4.3 Figure 5. It is good that the authors noticed the edge effect, boundary problems, however, it is not completely eliminated yet, as shown in Figure 5b. What could be done to revise your approaches? Observe latitude 15 deg.
6. Line 376: That is not the only reason. The Area 3 is a reion with high ocean dynamics in the Southern Oceans. Even the north-south DOV components degrade a lot, even though there are many many near Polar altimetry missions.
7. Line 396: Would the author ever considered filling the Land with DOV values from EGM2008 or other high resolution models like XGM2019, so that the boundary effects are mitigated?
8. Line 458: The reasoning for keeping Geosat data is addressed here. Geosat only cover up to latitude 72 deg, so it is not that high compared to others (SARAL, Cryosat-2 and HY2). The most important contribution is for the east-west components. In addition, Geosat GM data is only for 1.5 years, it may be dropped due to the lack of high accuracy. Do the authors have a gravity model predicted without Geosat data? How was the accuracy?
9. Line 506: “with a decline” replace with “with a reduction 0f 0.7 mGal”