# Summary of Lunar Constellation Navigation and Orbit Determination Technology
## 2 Current Status of Lunar Relay Navigation Satellite Networks  
In the 21 st century, the strategic significance of lunar exploration activities has become increasingly prominent. Lunar resources include location resources, environmental resources, material resources, etc., which have huge development and utilization value. The world's major aerospace countries and organizations have successively planned several unmanned/manned lunar exploration missions, with the aim of ultimately realizing long-term residence on the Moon and the utilization of resources. China has continued to carry out scientific research and engineering practice in the field of lunar exploration. It has been undertaking a 'lunar exploration project' since 2004. The three-phase mission of 'orbiting, landing and returning' has been a complete success, and the subsequent four-phase lunar exploration mission is in full progress. The south pole of the Moon has the largest and deepest impact basin in the solar system--the Aitken basin. There is also water ice at the north and south poles of the Moon. The poles and back of the Moon may contain rich resources and have greater exploration value. Exploration is the cornerstone of human exploration and research of the entire solar system. In previous lunar exploration missions as well as landing and sampling return missions on the front side of the Moon, the probe's measurement and control communication during each flight phase could rely on ground stations' support, so a dedicated communication and navigation satellite system was not developed. However, as lunar exploration activities progress towards the direction of long-term habitation and development, and the exploration values of the lunar poles, backside and other areas becomes more prominent, the issue of communication and navigation services for the cislunar space has garnered significant attention. In previous lunar exploration missions, the number of probes in a single mission was limited, and measurements and control communications at each flight phase could be supported by ground stations. Therefore, there was no urgent need to develop a dedicated communication and navigation satellite system. As the value of lunar exploration becomes more prominent, the quantity of probes operating in the Earth-lunar space will increase rapidly, and the orbit determination of multiple probes will place a greater load on ground stations. It is also difficult to directly provide navigation support for the lunar south pole or lunar far-side probes due to the propagation characteristics of radio, the influence of lunar tidal locking and the impact of ground stations. On the other hand, future lunar resource development and other tasks will require more real-time high-precision positioning and speed control in the Earth-lunar space. Current orbit determination methods of lunar probes include satellite-to-ground laser measurement, GNSS signal measurement, astronomical navigation, etc. Among them, satellite-to-ground laser ranging is highly accurate, but also imposes high requirements on the Earth's atmospheric environment and limited usage time. GNSS can achieve real-time positioning, but has weak side-lobe signals, poor visibility, and limited positioning accuracy in deep space exploration [16]. Astronomical navigation includes angle measurement, velocity measurement, and ranging. Combined with orbital dynamics, real-time positioning of satellites can be achieved. However, it also faces the problem of limited accuracy and low maturity of high-precision navigation instruments. Therefore, the existing single-satellite orbit determination method for lunar probes cannot meet the needs of future large-scale lunar resource development. Future lunar exploration will be more complex and diversified.