## 1 Introduction  
At present, the global navigation satellite system's (GNSS's) high-precision navigation, positioning and timing functions make it widely used in military, traffic, remote sensing and mapping fields [1, 2, 3]. It has many advantages such as global positioning under all-weather conditions, short observation time, and no requirement for intervisibility between observation stations. However, it also has some imperfections, for example, it is easily affected by physical space environmental factors. Moreover, due to the limited number of satellites, communication in harsh environments such as urban blocks, bridges, tunnels, and canyons leads to insufficient GNSS signal strength [4]. In addition, in practical applications, the continuous positioning of the GNSS receiver can not be guaranteed due to satellite failure and carrier attitude change, which leaves the navigation and positioning in an incomplete state. Moreover, to recover from this incomplete state and relocate also consumes a certain amount of time, which is fatal for some real-time applications. Compared with the traditional medium earth orbit (MEO) constellation and highly elliptical orbit (HEO) constellation navigation system, the low earth orbit (LEO) constellation satellite has the advantages of fast movement, closer proximity to the earth and low manufacturing cost, such that the LEO constellation has fast positioning convergence, small propagation delay, and rapid deployment. On the one hand, it can be used as a global high-speed data broadcasting channel; since low-orbit communication satellites have larger signal bandwidth and higher information rate, they can be used as the broadcast channel for satellite navigation basic messages and differential correction messages. Since the low orbit constellation is networked around the world, the broadcast range can cover the whole world; when broadcasting basic navigation messages, it can shorten the time to first fix (TTFF) of the receiver cold start, which plays a role similar to assisted global positioning system (AGPS). Additionally, when broadcasting differential correction messages, it can achieve wide-area accuracy enhancement and serve as a precise point positioning (PPP) system. On the other hand, due to the low orbit and fast motion of LEO satellites, the correlation of the observation equations between adjacent epochs is weaker than that of MEO and HEO satellites, so it can also be combined with other types of constellations for fast PPP [5]. In addition, when LEO satellites function as communication satellites, they can eliminate the clock bias between LEO satellites and user receivers through a communication method similar to full-duplex (FD); the accuracy requirements for user clocks are relatively lower, therefore, the preferred constellation scheme is more suitable for the integration of communication and navigation (ICN) [6]. _Exploring the navigation enhancement application of LEO constellation_. Several studies have discussed the applicability of navigation in low earth orbit (LEO), described the satellites in today's LEO, their role in navigation, the possible LEO constellation configuration in the future, and provided the required mathematical background [7; 8; 9]. At the level of constellation design, to overcome the shortcomings of traditional LEO constellations that use monopolar or near-polar orbits for global coverage, which leads to fewer visible satellites at low latitudes, Reference [10] used genetic algorithms to optimize several hybrid LEO enhanced constellations, the algorithm can provide the possibility of 100% global coverage through one to three LEO satellites used for broadband internet access. A new algorithm is used that is based on the LEO constellation, using a differential carrier phase measurement architecture which can effectively solve the integer ambiguity solution [11], but the algorithm is similar to the MEO constellation-based integer ambiguity solution algorithm.