# Design of Mega-Constellations for Global Uniform Coverage with Inter-Satellite Links
###### Abstract
However, there are only a few studies on the design of mega-constellations. [PERSON] [24] proposed a large-scale constellation design optimization framework for CubeSats. It analyzed several framework cases applied to the Internet of Space Things (IoST). [PERSON] [25] studied the constellation optimization problem for the LEO enhanced global navigation satellite system (LeGNSS) with 240 LEO satellites of orbital inclinations at 90\({}^{\circ}\), 60\({}^{\circ}\), and 35\({}^{\circ}\) selected. [PERSON] [26] introduced a hybrid communications architecture and a 5G unifying protocol architecture based on mega-constellations. This work balanced all aspects of system design for such large and complex systems. Based on the Starlink constellation, [PERSON] [27] proposed a framework to navigate LEO satellite signals with differential carrier phase measurements. [PERSON] [28] applied FCs theory to generate sets of LEO slots free from self-conjunctions. He also innovatively proposed the 4D Lattice FCs design method and applied it to the design of mega-constellations [14]. While the constellation is capable of deploying more satellites at the same orbital altitude than the Walker constellation, this comes at the cost of a non-uniform distribution of satellites, preventing the constellation from achieving sustained global coverage. In summary, more and more creative findings are emerging for mega-constellations. However, almost all studies are based on a Walker constellation to explore the application value of mega-constellations. Few have focused on the in-depth research of constellation configuration design. The successful practice of the Walker constellations in GPS [29], Glonass [30], Iridium [31], and other space systems has proven its significant superiority in serving traditional space missions. However, when the number of satellites increased to the order of thousands, the serious unevenness of the Walker constellation's coverage along the latitude zones became prominent. Its sub-satellite points will show a trend of sparse distribution at low latitudes and extremely dense distribution at high latitudes [32]. This drawback not only causes the waste of space resources but more seriously, a large number of satellites accumulate over high latitudes, increasing the risk of collision between satellites and threatening the space security environment. Although this weakness can be mitigated by using a constellation configuration with multiple orbital layers mixed, it will significantly increase the complexity of the inter-satellite links (ISLs) design, since two satellites between different orbital altitudes cannot communicate continuously. Thus, it is relevant to design a constellation configuration that can improve global coverage performance, consider collision avoidance, and be in the same orbital layer. Establishing ISLs [33] within the mega-constellation provides many advantages, such as reduced communication delay, path loss, and inter-satellite data transmission independent of the ground systems [34]. The mega-constellation is a huge space-based information network platform. Each satellite node is not an independent individual but works in concert to pursue the maximum functionality of the overall system. Therefore, inter-satellite communication capability will become a necessary attribute for mega-constellations. It means that ISLs design will also be an integral part of the design of the mega-constellations. Based on previous efforts, this paper is dedicated to studying the design of mega-constellations. We propose a mixed Walker constellation design method that achieves full-time domain and global uniform coverage. It considers the collision-avoidance problem within the constellation, and all satellites orbit at the same altitude. Based on this constellation configuration, we introduce an ISL design scheme that guarantees stable data transmission within the mega-constellation. Firstly, we define a metric that measures the distribution density of the sub-satellite points along the latitudinal zones.