# A Survey of Precision Formation Relative State Measurement Technology for Distributed Spacecraft
## 4 Relative State Measurement Technology Trends and Optimization  
### Summary of the Relative State Measurement Technology Current Status  
At present, the relative state measurement methods applied in the field of spacecraft precision formation flight can be divided into three groups: non-autonomous measurement technology, autonomous measurement technology, and new composite relative measurement technology. Non-autonomous measurement technology includes GNSS relative measurement technology, and autonomous measurement technology includes laser measurement, infrared measurement, radio frequency measurement, and visible light vision measurement [75; 76]. The specific working range, accuracy, advantages, and disadvantages are shown in Table 10, below. As GPS relative measurements rely on GPS signals, they are only applicable to low-Earth orbits; other relative measurement methods are applicable to arbitrary orbits [77].  
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In this paper, the key parameters of existing international spacecraft precision formation projects are summarized. As shown in Table 6, certain conclusions can be drawn: the number of spacecraft in formation projects is increasing, and the precision requirements are becoming increasingly high. In addition, the trend of precise spacecraft formation is developing toward the ultra-long-distance precise formation and visual short-distance precise  
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formation; orbital distribution has also evolved from low-Earth orbit to deep space, which enables higher precision and autonomy for relative measurement technology. As shown in Table 10, influenced by mission requirements, such as orbit distribution, formation shapes, and accuracy, the focus of relative measurement technology is gradually developing from non-autonomous GNSS measurements and radio measurements to laser measurements for ultra-long-range precision formations, and visual measurements for close-range precision formations. Figure 31 shows the accuracy and valid range of different relative measurements, which can more clearly demonstrate the most advantageous intervals of different relative measurements.  
At the same time, various subsystems influence one another in the actual project. Errors and disturbances will inevitably appear in the relative measurement system, so the robustness and reconstruction speed of the measurement technology are also important considerations. The sources of errors include the inherent properties of integrated circuits [78], space environment radiation [79], space atmospheric noise [80], etc. In addition, large disturbances may occur due to the partial failure of control systems, propulsion systems, and other subsystems, or even hardware single-particle upset [81]. These errors can be compensated by hardware anti-noise and anti-radiation design and algorithm filtering [82; 83], etc. Large disturbances can be reconstructed and measured via robust algorithm control [82; 84]. Different measurement technologies have different anti-interference capabilities and reconstruction times; for example, laser and infrared have strong anti-interference abilities, while vision and radio have rapid reconstruction characteristics. It is critical to select the appropriate measurement method in different application scenarios while improving the robustness of the relative measurement algorithm.