During communication between the high-altitude aircraft and the underwater platform, the high-altitude aircraft moves faster and passes through the area where the underwater platform is located in a shorter time. Additionally, limitations of the receiving field of view and laser emission window of the underwater platform restrict the effective communication time, impeding the establishment of a constant optical link. The utilization of a beacon light-based communication method extends the capture time and adds complexity to the link establishment in the underwater communication platform system. To overcome these challenges, a tracking and pointing system based on the orbit forecasting of the underwater platform is devised to establish an uplink between the underwater platform and high-altitude aircraft.

In contrast to the acquisition, tracking, and pointing (ATP) systems employed in space laser communications, the system we developed eliminates the need for an acquisition module. Instead, the underwater platform is required to obtain the real-time position of the aircraft during communication to achieve precise pointing of the aircraft. Several theoretical algorithms for orbit forecasting are assessed, and the Runge-Kutta method is selected for its computational efficiency. The laser pointing system structure is designed for an underwater platform, and the correlation between the motor rotation angles in two directions and the aircraft coordinates is derived. Subsequently, we develop computer software to simulate and analyze the orbit forecasting algorithm and pointing angles, which leads to the evaluation of the error results and running time. The findings support the feasibility of the tracking pointing method based on orbit forecasting. The system consists of several modules, including the laser receiver, transmitter, servo motor controller, attitude sensor, and field programmable gate array (FPGA) master control modules. To improve the reception sensitivity, the system utilizes a photomultiplier tube (PMT) with heightened sensitivity for reception. The laser receiver module is equipped with eight PMT, each with a maximum field of view (FOV) of 15°. These tubes are integrated to form a receiver array with a maximum FOV of 30°. The expansion of the receiving FOV enhances the communication coverage duration when the aircraft is in high-speed motion, thereby increasing the likelihood of receiving navigation parameter information. Additionally, the utilization of the diversity receiving technique enhances communication stability under low signal-to-noise ratios.

The simulation results show that the orbit forecasting error does not exceed 200 m on the X-axis, 160 m on the Y-axis, and 150 m on the Z-axis within 60 s [Fig. 5(a)]. The maximum error between the forecast position and the actual position is no more than 250 m [Fig. 5(b)]. In experiments, the errors between the forecasted and actual orbits, as well as the errors in the X, Y, and Z axes coordinates, are all less than 250 m within 60 s [Fig. 6(a)]. The optical pointing angle error due to the orbit forecasting error within 60 s does not exceed 0.51 mrad at most [Fig. 6(d)]. The actual pointing results are obtained by comparing the angle feedback from the servo motors to the theoretically calculated setup angle, resulting in a mean angular error of 0.20 mrad in the pitch direction [Fig. 7(a)] and a mean angular error of 0.16 mrad in the roll direction [Fig. 7(b)]. The system pointing error is a combination of the pointing angle error caused by the error between the track forecast and the actual track position, and the error between the theoretical pointing angle calculated by the track and the pointing angle feedback by the actual motor movement. The maximum pointing error is calculated to be 0.77 mrad.

We design a tracking and pointing system to forecast and track the orbit of a high-altitude aircraft from an underwater platform. By receiving coordinates and navigation parameters from the aircraft, the system can rapidly calculate and forecast orbit data within 60 s, with a calculation time of 0.6 s. Based on the predicted orbit data, real-time pointing action from the platform to the aircraft can be performed. The error between the simulated motion trajectory within 60 s and the aircraft's orbit trajectory calculated by the orbit forecasting algorithm does not exceed 250 m. In the experiment, the maximum error between the aircraft orbit obtained from the orbit forecasting and the actual running orbit is within 350 m, resulting in a pointing error not exceeding 0.54 mrad. The error between the calculated pointing result from the forecasted aircraft position and the actual feedback pointing result is 0.20 mrad in the pitch direction and 0.16 mrad in the roll direction. The maximum pointing error of the system is 0.77 mrad. Our study presents a system that aims to minimize the duration required for establishing links while ensuring precise communication through pointing. The tracking and pointing system, based on a short-time prediction of the high-altitude aircraft platform is usable.