# A Survey of Precision Formation Relative State Measurement Technology for Distributed Spacecraft
## 3 Analysis of the Characteristics of Relative State Measurement Technology
### Laser Measurement Technology  
The laser devices used for the relative state measurement of spacecraft include laser rangefinders and LiDARs [31]. The principle of laser ranging is divided into pulse measurement, phase measurement, triangulation, and interferometry. The phase ranging method obtains the distance information via the phase change of the modulated continuous light wave during round-trip propagation; the pulse ranging method obtains the distance information via the propagation time of the pulsed laser. Compared with the phase measurement, the receiver requirements of the pulse measurement are lower, the transmitter is relatively simple, and the accuracy is at dm level. The accuracy of phase measurement can reach 1 mm, and the effective distance is 100 km. The angle information is realized via four-quadrant detection, and the accuracy can attain the second level [32]. The advantages of laser measurement are its strong anti-interference ability and high precision. The disadvantage of the proposed method is that the phase measurement is a good choice for the phase measurement. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Technical** & **Positioning** & **Scope of Action** \\  & **Difficulty** & **Accuracy (m)** & **(km)** \\ \hline  & Position Differential & Very easy & 10 & 100 \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Pseud orange \\ differential \\ \end{tabular} } & General pseudopitch differential & Easy & 5 & 100 \\  & differential smooth pseudo distance of phase & General & 1 & 100 \\  & Wide area differential & Very difficult & 1 & 1000 \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Phase differential \\ \end{tabular} } & Quasi-carrier phase differential & Difficulties & 0.5 & 50 \\  & Carrier phase differential & Very difficult & 0.05 & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Performance comparison of GPS measurement methods. Figure 16: The AFF subsystem schematic diagram. vantage is that the beam is narrow and cannot be used for multi-target measurement. It is more suitable for auxiliary guidance, in combination with other measurement systems. LiDAR acquires point-cloud data on the target surface by autonomously emitting and receiving laser beams. It reduces the complex image processing steps but also has the disadvantages of large size, high resource consumption, and high energy consumption. The current LiDARs used in autonomous relative navigation are divided into scanning and array types. The scanning type scans the field of view through the movement of the detection component, while the array type uses a fixed detection array to obtain point clouds at one moment in time. Flash LiDARs measure using the round-trip beam time, while TOF cameras measure using the phase difference. LiDAR is mainly composed of a laser-transmitting module, laser-receiving module, system control module, etc. As shown in Figure 17, the system consists of an image processing unit, a servo actuator, and a CCD to form a target capture unit. The QR four-quadrant detector is used to aim and capture the target, the optical antenna or CCD assists the radio frequency system for rough positioning, and the light-spot unit is used to measure the space angle of the target spacecraft. In recent years, laser interferometric ranging technology has become increasingly advanced. According to whether the frequency of the interfering laser is the same, it can be divided into two specific forms of laser interference: heterodyne and homodyne, corresponding to a dual-frequency interferometer and single-frequency interferometer, respectively. Single-frequency laser interferometers are suitable for distance measurement between targets that remain relatively stationary or that move relatively slowly. The dual-frequency laser interferometer is suitable for measuring the distance between targets with a relatively high moving speed.