## 5 Conclusions  
The initial motivation of this work was to evaluate the observation capacity of a near-future GNSS-R LEO constellation based on simulated orbits and reflections when all four GNSS systems are available after 2020. In this paper, we propose a strategy for construction of a multi-GNSS-R system that includes 119 GNSS satellites and eight LEO GNSS-R observatories with two nadir GNSS LHCP antennae apiece, along with a three-step method to select the available GNSS reflection signals. In addition, a method for calculation of the revisit time for the GNSS-R specular reflections is presented. The results of these simulations have helped us to analyze and evaluate the performance of future four-system spaceborne GNSS-R missions quantitatively. Detailed comparisons were made between the case where the four GNSS systems were available and the case where only the GPS system was used at the L1 and L2 frequencies.  
In the case of the four-system GNSS-R, a single observatory can track up to 10 and 16 parallel specular reflections at the GPS L1 and L2 frequencies, respectively. However, in the GPS-R case, a given GNSS-R LEO satellite can only track up to four and six parallel specular reflections at the GPS L1 and L2 frequencies, respectively. In addition, 1.2 GPS-only reflections and 4.5 four-system GNSS reflections can be received at L1 on average, while the corresponding values at L2 are 2.3 and 8.7 because of its wider beamwidth. We therefore propose that future spaceborne GNSS-R instruments should have wider beamwidths and the ability to receive more signals from different GNSS systems to allow the system to provide more available reflections.  
Larger numbers of observations would improve the spatiotemporal sampling over that of current spaceborne missions. The number of multi-GNSS-R reflections in a single \(1^{\circ}\times 1^{\circ}\) grid is several times the corresponding number of GPS-only reflections. The spatial density of the specular reflections in the Asia-Pacific region is higher than that in other areas at the same latitude because of the locations of the Chinese BDS GEO and IGSO satellites. Additionally, the numbers of reflections around \(\pm 35^{\circ}\) are two to four times higher than those in other areas because of the orbit inclinations of the LEO GNSS-R observatories. When compared with the GPS-R case, the mean revisit times will be reduced by approximately three times when the satellites from all four GNSS systems are used. Overall, the number of available GNSS illuminators, the distribution of the GNSS satellites and the inclinations of the LEO satellites are closely related to the spatiotemporal resolution of GNSS-R.  
For a specific mission design, the parameters of the instrument and orbits will different from those presented in this paper. In future work, several possible LEO constellations for different targets will be constructed and simulations will be conducted to assess the performances of a planned GNSS-R mission.  
**Acknowledgments:** The authors highly appreciate the valuable comments and critical remarks from the four anonymous reviewers. Their suggestions were of considerable use to the authors in allowing them to produce a much more rigorous and integrated document. The research is jointly financially supported by NSFC (National Natural Science Foundation of China) projects under grant nos. 41604003, 41704017 and 41574013. We thank [PERSON], MSc, from [PERSON], Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.  
**Author Contributions:** [PERSON], [PERSON], [PERSON] and [PERSON] conceived and designed the experiments. [PERSON] and [PERSON] performed the experiments. [PERSON], [PERSON], and [PERSON] analyzed the data. [PERSON] and [PERSON] contributed analytical tools. [PERSON], [PERSON] and [PERSON] wrote the paper. All authors reviewed the manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.