A general evaluation criterion for the coverage performance of LEO constellations

A general evaluation criterion for the coverage performance of LEO constellations that can be used for various constellation configurations is proposed. The method transforms the minimum elevation angle characteristics at the specified latitude limits into the standard number of satellites with the same coverage performance, and the ratio with respect to the actual number of satellites in a constellation is defined as the index of coverage performance. Because this index is actually the average coverage level of each satellite, it is suitable for different configurations. Therefore, the optimal configuration can be selected for global coverage or coverage of specific latitudes. Using the evaluation method, the coverage index for three typical Walker-δ constellations was computed; these indexes were: Globalstar, 0.51; Celestri, 0.46; and NeLS, 0.45. Using the general evaluation method, the optimum Walker-δ constellation suitable for China is determined. The final optimum configuration was selected as 46°:42/7/0, or an inclination angle of 46°, a total of 42 satellites in 7 orbital planes, and a phasing factor of 0. The coverage performance index is as high as 0.566, providing coverage with elevation angles greater than 13° in the targeted area at an orbital altitude of 1400 km. This configuration can continuously cover China with a minimum elevation of 20°, and the coverage range includes the latitudes from 24.5° to 53°, which is definitely suitable for China.