# A Survey of Precision Formation Relative State Measurement Technology for Distributed Spacecraft
## 3 Analysis of the Characteristics of Relative State Measurement Technology
### Composite Relative Measurement Technology of Spacecraft Formation
#### 3.6.4 Visual Composite Close-Range Measurement Technology  
Tyvak's CPOD (CubeSat Proximity Operations Demonstration) mission [66] was performed using two identical 3U (U, volume 10 \(\times\) 10 \(\times\) 10 cm) CubeSats for formation, close-range operations, rendezvous, and docking. The measurement between satellites is completed using the PROD module [67], including a narrow field of view camera (NFOV), a docking camera, and an infrared camera (IR1/IR2), as shown in Figure 25. The image processing system reads the image from the imaging sensor and compares the a priori 3D model of the target with the image. It obtains position and pose information, using edge detection and feature extraction. Each camera can obtain a measure of the object's position and orientation. At great distances, measurements are mainly obtained using GPS and ISL. When the mission target is several kilometers away, the measurement value of NFOV occupies a larger proportion of the result. The docking camera and IR1 and IR2 account for the largest proportion of results in the short-distance measurement. At longer ranges, the target may contain only a few pixels, and the system maintains visual tracking. When the target is close to occupying more frames, pose estimation and ranging are performed. The a priori 3D models are managed by means of an on-board database, including various operations such as modification, deletion, and addition. This solves the known errors, including solar scintillation and Earth albedo effects.  
Vision measurement algorithms that are based on optical image processing have high accuracy. Their disadvantages are a large number of calculations, a long processing time, and a short effective distance. In order to better utilize the advantages of vision measurement technology, combined navigation methods, such as vision-inertial navigation and vision-laser ranging, have begun to receive attention. Inertial navigation uses angular velocity information, combined with kinematic equation integration, to obtain the target altitude angle information. It can jointly filter and correct the results of visual measurement technology to gain higher precision and a more stable measurement output, as shown in Figure 26[41]. Additionally, the pose data output of the vision measurement can correct the drift error of the inertial element and improve the accuracy by closing the loop. The laser-ranging instrument can accurately and efficiently obtain the distance information of the target spacecraft with the guidance of visual measurement technology [46; 68]. It is also used as the baseline distance input of the vision measurement system for precise  
Figure 24: Schematic diagram of GPS + visual combined relative state measurement.  
Figure 25: RPOD module diagram.  
measurement. This method can greatly simplify the complexity of the vision algorithm, reduce the operation time, and improve the real time and robustness of the composite measurement system.