# A Survey of Precision Formation Relative State Measurement Technology for Distributed Spacecraft
## 3 Analysis of the Characteristics of Relative State Measurement Technology
### Visible Light Vision Measurement Technology  
Visible light measurement is a passive detection technology, which has the advantages of wide spectral response, good linearity, and high sensitivity [34]. Visible light visual measurement technology is currently widely used in the identification of non-cooperative spacecraft, space junk, and debris [35], and is also one of the key technologies for space cooperation mission rendezvous, as well as for docking and close-range spacecraft formation flying [36]. Vision sensors such as cameras have shorter baselines and are more suitable for close-range pose measurements. Currently, visible light cameras are successfully being applied in the ETS-VII [37] and Orbital Express [38] aerospace projects. [PERSON] used the composite relative measurement technology with multiple visible light and infrared cameras to demonstrate the CubeSat mission, and realized in-orbit verification of the CubeSat's formation flying. Visible light visual measurement technology usually needs to select several feature points on the formation spacecraft to configure visible light-source beacons (usually 4 or more) [39]. The target is imaged using the optical cameras mounted on another formation spacecraft. Combined with the two-dimensional image of the target and the prior information of the optical beacon arrangement, the relative altitude and relative position information of the target spacecraft are obtained. Vision measurement techniques can be classified as monocular vision [40], binocular vision [41], and polycular vision [42] techniques. With monocular vision measurement technology, it is difficult to estimate the depth information of the optical beacon due to the limitation of the sensor, and the combination of the laser rangefinder with the depth information can better measure the space target [43]. In binocular vision measurement, the target depth information can be obtained by matching the left and right image points. Trinocular vision measurement uses the distance difference square sum algorithm to reduce the rotation axis and rotation angle errors, to obtain high-precision pose information. Visible light camera-specific sensor characteristics are shown in Table 9. The PRISMA project is equipped with a VBS camera, with a resolution of 752\(\times\)582, from DTU [7]. The US Naval Research Institute uses a Bumble XB3 binocular camera, with a resolution of 1280\(\times\)960 and a frame rate of 16 FPS, in semi-physical experiments. After the two-dimensional image acquisition of the target is completed using the vision sensor, the stereo correspondence of the target satellite is established by using the image features. In addition, parts with regular geometric features can be projected, such as spacecraft solar panels and apogee thrusters, as a reference for the target satellite to complete feature recognition [44]. The visible light vision measurement technique is practiced in the CANYVAL -X project. VisNav (vision-based navigation) [40; 45] is a typical application of the visible light technique. It obtains the current measurement data through the position-sensitive diode and further calculates the estimated value of the sensor orientation. As shown in Figure 20, multiple visible light beacons of the target spacecraft are focused on the position-sensitive diode through the lens, and the relative position information of the target is obtained by using the photoelectric conversion characteristics of the sensor. This measurement technology has the disadvantages of limited applicab