## 1 Introduction  
Accurate location services in harsh environments are one of the main obstacles to automatic driving, unmanned driving and even search and rescue services. Therefore, high-precision real-time location services in harsh environments have become a research topic of interest in recent years. Reference [1] discussed how to use a low-earth-orbit (LEO) constellation for enhanced navigation and analyzed a series of advantages in the LEO constellation, but did not give a specific design plan for navigation and positioning based on LEO constellations. Reference [2] studied, at the signal level, and proposed an algorithm for navigation based on the differential carrier phase measurement of the LEO satellite signal, but the algorithm did not consider that the time for the LEO satellite to pass the zenith is relatively short and requires frequent switching. Reference [3] used the highly dynamic characteristics of LEO satellites and used the Doppler frequency shift as a measurement value, introducing a Doppler-based positioning algorithm; similarly, the algorithm did not consider the issue of satellite switching. Reference [4], on the basis of a dual-satellite positioning system and a synchronous navigation satellite, a 3-satellite positioning system was proposed. However, it also needed radar altimeter or barometer information to provide complete location services. For 3-satellite positioning technology, References [5; 6; 7] provided an algorithm for combining 3-satellite positioning and a strapdown inertial navigation system (SINS), but it also required the help of elevation information to complete the positioning. Based on radio navigation satellite system/radio determination satellite service (RNSS/RDSS) combined service pseudorange observations, Reference [8] proposed an integrated method that could be connected to a variety of navigation sensors, such as global navigation satellite systems (GNSSs), sensors and INSs, which improved the positioning accuracy in challenging environments, but the use of multiple sensors undoubtedly increased the cost and complexity of the system. For land vehicle navigation, when various signals block the GPS, especially when the number of visible satellites is not complete, References [9; 10] gave a loosely coupled navigation scheme, but this method required an odometer to provide velocity information and multiple sensors were also necessary. Reference [11] combined GPS and vision-based measurements to explore the feasibility of navigation in harsh environments, but the monocular camera used was limited by the weather, so this program could not provide services in increment weather or at night. Reference [12] gave a regional positioning system solution based on satellite communication to establish a complete system with 3 GEO (Geosynchronous Earth Orbit communication satellite, GEO) + 3 DGEO (Decommissioned Geostationary Orbit communication satellite, DGEO) and 3 IGSO (Geostationary Orbit communication satellite, IGSO). The disadvantage of this solution was that it did not incorporate the three LEO solutions. Reference [13] discussed the feasibility of using a high-precision integrated chip-level atomic clock to assist GPS receivers using three satellites for navigation in urban canyons; however, the price of high-precision atomic clocks was difficult to accept by the general public, thus, limiting the application of the program. To address the weak GPS environment, for a situation with only three visible satellites, the combination of Doppler measurement and an inertial navigation system (INS) was used to achieve 3-dimensional attitude determination, but the premise was that continuous observation is required [14]. Reference [15] gave an integrated navigation scheme that uses frequency-modulated continuous wave radar (FMCW-Radar) for automatic positioning in harsh environments.