## 4 Discussion  
The GNSS-R of the future will feature the combination of GPS, BDS, GLONASS and GALILEO, and high spatiotemporal resolutions. In order to evaluate observation capacity of a multi-GNSS-R LEO constellation 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 this work. And other useful advice and hints on how to design certain criteria for future spaceborne GNSS-R instruments and constellations are given in this paper. Additionally, this research not only  
Figure 11: Mean revisit times in the bins for GPS-R at different latitudes during a single cycle at the GPS L1 (**a**) and L2 (**b**) frequencies.  
Figure 12: Mean revisit times in the bins for the four-system GNSS-R at different latitudes during a single cycle at the GPS L1 (**a**) and L2 (**b**) frequencies.  
advances our understanding of the observation capacity of the proposed four-system GNSS-R but also presents some appropriate simulation methods and results.  
Pervious researchers had also stated some results about the GNSS-R spatial and temporal resolutions, but early simulations were conducted based on GPS and one LEO satellite, and other late results are lack of details [24; 25; 26; 27; 28; 31]. Here, we describe the simulation and analysis methods, as well as the parameters' values, used in this paper, and conduct the experiments using 119 GNSS satellites and 8 LEO satellites. However, the spatial and temporal resolutions of GNSS-R missions vary with several parameters, such as constellation constructions, antennae beamwidths and observation modes. The ones who design GNSS-R missions with different targets can use the methods given in this paper to recompute the observation resolutions.  
Evaluation of the spatial and temporal resolutions of the GNSS-R reflections is very important for different types of GNSS-R missions with different objectives. Among the current applications of the GNSS-R techniques, monitoring of sea surface winds is the most mature. Other potential applications include ocean and ice altimetry. For the investigation of cyclone generation processes, we suggest that one or two of the GNSS-R LEO satellites should be put into orbit at a 20\({}^{\circ}\) inclination because 87% of hurricanes form no farther away than 20\({}^{\circ}\) north or south [42; 43]. However, for ocean sea surface altimetry, the GNSS-R observatories should be inclined at 70\({}^{\circ}\) to cover the high latitude sea surfaces and the number of observatories required will be less than that required for monitoring of the sea surface winds.  
In this study, the results are obtain by using simulated data, including parts of GNSS constellations and the GNSS-R LEO constellation. It is impractical for us to perform an officially sensitivity/uncertainty assessments on the simulated orbits. So, there are inevitable potential errors that might be associated with these constellations.  
Two or three years later, the four main GNSS systems will be fully operational, and their precise orbits will be released. If the precise orbits and other related parameters of a certain GNSS-R LEO constellation are provided, the methods presented in this paper can be used to evaluate the performance of that GNSS-R system with higher accuracy and authoritative.  
In addition, we assess the spatial resolutions of GNSS-R using the numbers of reflections in different 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) bins simply. However, the observed scene for one reflection can be divided into several areas with smaller size [22]. So, the spatial resolutions also depend on the algorithm of GNSS-R data processing. A deeper analysis on the spatial resolutions based on different algorithms can be achieved in future work.