# A Survey of Precision Formation Relative State Measurement Technology for Distributed Spacecraft
## 4 Relative State Measurement Technology Trends and Optimization
### Development Trend  
This paper reviewed the key parameters of the existing international spacecraft precision formation engineering projects. As shown in Table 10, it can be concluded that the accuracy requirements of formation projects are becoming increasingly high, and the number of formation spacecraft is also increasing. In addition, the precise formations of spacecraft have developed according to two aspects: ultra-long-distance precise formation and visual short-range precise formation; the orbit distribution is likewise evolving, from near-Earth orbit to deep space. This poses higher requirements for the precision and autonomy of relative state measurement technology. As shown in Table 10, when affected by orbit distribution, formation configuration, and accuracy, relative state measurement technology has gradually developed from non-autonomous GPS and radio measurement to laser and visual measurements. The development trend of specific relative measurement technology is as follows. Sensor fusion for spacecraft relative state measurement presents a promising development direction [92; 93]. Table 10 shows some of the data for the precision dimension. The mainstream composite relative state measurement techniques can be divided into two approaches: the tight coupling approach and the loose coupling approach. The tight coupling approach usually combines heterogeneous data from different measurement methods at the algorithm level to produce the final measurement result. Wide coupling is used to  
Figure 33: Multidisciplinary optimization structure diagram of the overall design of distributed spacecraft formation flying. select a method with higher measurement accuracy or to combine simple processing as the final measurement result at different range stages. Tight coupling can often improve specific dimensions such as accuracy, real-time characteristics, and autonomy. The results of wide coupling depend on the accuracy and real-time characteristics of each measurement method. The advantage of this approach is that it has better robustness and can make full use of the advantages of each measurement method. The focus of fusion technology is to guarantee the best accuracy while ensuring an optimal spacecraft platform; that is, to establish how to use sensor fusion with the fewest resources and lowest cost consumption to achieve the best accuracy, real-time characteristics, robustness, and other indicators. This can provide more possibilities for formation flying missions, rather than just pursuing accuracy metrics. The intelligence of the relative state measurement of distributed spacecraft formation flying is also a promising development direction. Due to limitations in measuring and controlling the visible arc, there is a limited amount of time in which ground personnel can intervene in decision-making. This makes intelligence regarding the formation system even more important. With the development of artificial intelligence technology, people are no longer satisfied with a fixed, traditional operating mode but instead hope to improve the formation's own system through autonomous learning. For example, [PERSON] completed the allocation of radio resources to satellites via reinforcement learning, and accomplished the assignment of conflicting mission objectives [94]; [PERSON] adopted a learning method to take the task reward and control cost as dual optimization objectives and obtained an optimal reconfiguration strategy for the spacecraft altitude control system [95]; [PERSON] solved the difficulty of the mutual visual recognition of formation satellites by using an unsupervised learning architecture [96]. Combining information data from relative state measurement, communication, mission-planning, etc., enables the formation members to make intelligent decisions.