# Ocean Surface Topography Altimetry by Large Baseline Cross-Interferometry from Satellite Formation
## 7 Conclusions  
This paper proposed the LB-IRA from satellite formation which utilizes CFS to compensate for large baseline decorrelation. The simulation results indicate that \(\sim\)1 cm relative accuracy can be achieved at 1 km resolution. The LB-IRA is currently a possible solution rather than a specific satellite mission for OST altimetry. Comparing to single satellite altimetry, the advantages of satellite formation are:  
* The CFS method can release the baseline length limitation from baseline decorrelation, also increasing the freedom of selecting other interferometric parameters.
* The altimetric error associated to phase noise is only a few millimeters, which is quite extraordinary owing to much larger baseline length than that of the single platform.
* It poses relatively easier or fewer technical or engineering challenges, such as the rigorous attitude control accuracy, or the mast vibration or dilation error that should be addressed by the SWOT team.
* The relative altimetric accuracy is quite consistent throughout the entire swath, facilitating marine application based on the OST gradient.  
However, one major issue to be addressed is the limited observation coverage of LB-IRA; due to time decorrelation, the suggested altimetry region is within 44.6\({}^{\circ}\) N/S latitude at an orbit inclination of 78\({}^{\circ}\). The possible solution is to adjust the orbit perigee so that the along-track baseline is short at the high-latitude region, as shown in Figure 4c. An alternative solution for the insufficient coverage is to launch two sets of LB-IRAs, however, this is at the cost of a higher budget.  
Another issue is the restricted condition for conducting OST altimetry. The SWH should be smaller than 2 m for higher reliability of the altimetry result, which is the same dilemma faced by any other IRA as well. However, since one of the primary applications for LB-IRA is ocean bathymetry  
Figure 18: The altimetric error determined by an equivalent baseline inclination error. (**a**) The absolute accuracy and (**b**) the relative accuracy.  
inversion (which requires centimetric relative accuracy of OST at least), a large volume of data within a long time range may compensate for the limited observation efficiency.  
Conceptualization, W.K. and B.L.; funding acquisition, B.L.; investigation, W.K. and B.L.; methodology, W.K.; supervision, B.L. and J.S.; writing--original draft, W.K.; writing--review and editing, W.K., B.L., J.S., R.Z. and X.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Qian Xuesen Lab.--DFH Sat. Co Joint Research and Development Fund (Grant No. M-2017-006), the National Defense Science and Technology Innovation Special Zone Project (Grant No. Y-SYS-GZZ-SK-002), and the National Natural Science Foundation of China (Grant No. 42074225).  
We would like to thank the support from the \"National defense Basic research program\" and the \"China Aerospace Science and Technology Corporation Limited independent research and development project\".  
The authors declare no conflict of interest.