# Design of Novel Laser Crosslink Systems Using Nanosatellites in Formation Flying: The VISION
## 2 Laser Crosslink Mission
### Pointing, Acquisition, and Tracking (PAT) Sequence  
To accomplish the laser crosslink, the LOS between each satellite is aligned precisely using body pointing and the FSM, which is termed the PAT sequence. The PAT sequence is classified into three stages: (1) bus initialization stage (BIS), (2) coarse PAT stage (CPS), and (3) fine PAT stage (FPS). Specifically, for the CPS and FPS, two types of beam divergence angles and sensors are used as beam detectors; one is the SWIR CAM (short-wave infrared camera) detector, termed the CAM, and the other is the Quadrant Cell (QC) detector. Figure 5 shows the PAT sequence in nanosatellite orientations with various beam divergence angles.  
Before the bus initialization operation, the nanosatellites locate each other by pointing to a target predicted by the mission-planning system of the ground segment. When they establish the RF crosslink, relative navigation is started to calculate the LOS vectors. The star tracker on each satellite is used to acquire its spatial orientation. The attitude maneuver using 3-axis RWs aligns the LOS with each other and compensates errors induced by orbital motions. When the estimated LOS error is smaller than the full field of view of the CAM or the field of uncertainty (FOU), they start to transmit a broad beam. The beam spot projected on the CAM is biased due to control errors and mechanical misalignment: the bias is defined as the AOA (angle of arrival). Applying the CAM feedback and attitude maneuver, each satellite sequentially corrects the AOA until the beam spot on both sides remains within a threshold of the FFOV of the QC. However, the jitter induced by the  
Figure 4: Operations mode flows diagram.  
platform remains. The FSM is activated to eliminate jitter. When entering the FPS, the beam divergence angle is narrow, and the QC is used for the FSM feedback with high-frequency measurements. Within the tip-tilt angle of the FSM, the LOS errors from the attitude maneuver and jitter residuals can be rejected, enhancing the optical link performance.  
Table 2 summarizes the details of the PAT sequence substages. The CPS should end within 5 min, and the FPS should be maintained for over 10 min. During the search operation, the CAM is used only to check whether the beam is projected within the active area of the detector. For the acquisition and detection operations, CAM provides the AOA to be corrected by attitude maneuvers. During detection, the FSM is used to reduce jitter. The pointing error over the CPS should be smaller than 1200 and 400 and for the bias and standard deviation errors, respectively. For FPS, the precision of the LOS jitter should be less than 1 \(\upmu\)rad. During tracking and communications, the CAM is operated as a backup detector to prevent the nanosatellites from missing each other. Furthermore, the RF crosslink is always enabled to execute relative navigations. Considering the bus and LCT operations, the optical link budget cases are classified into five cases for either PAT or communication.  
Figure 5: Pointing, acquisition, and tracking (PAT) sequence for laser crosslink: (**a**) nanosatellite orientations and operations, depicting the beam divergence angle; (**b**) beam spot on tracking sensor and pointing error profiles over substages. After the Acquisition substage is ended, the FSM is available to reject jitters.