## 1 Introduction  
Laser communication is a technology that uses a laser beam as a carrier to transmit information in space [1,2]. The laser beam used in space laser communication is emitted at an angle that is near the diffraction limit. The divergence angle of very narrow communication beams requires both communicating sides to maintain extremely high dynamic alignment accuracy. With the ultimate aim of realizing a global-scale laser communication for practical use, many significant achievements have been made in the past decades, such as the demonstration of links for building-to-building communication [3,4], airborne applications [5, 6, 7], inter-satellite applications [8,9], satellite-to-ground [10, 11, 12, 13, 14, 15, 16], and ship-to-ship communication [17, 18, 19]. However, currently, most of the experiments only involve space and ground communication links or short-distance communication between ships. Since water bodies occupy more than 70% of the earth's surface and given the rising demand for maritime communications, the global-scale communication network is bound to extend from the ground to the sea in the future. Huge volumes of data need to be transmitted by satellite. Thus, there is an urgent need to conduct experimental verifications of laser communication between satellites and ships at sea. Satellite-to-sea laser communication mainly refers to data transmission between satellites and shipboards. Acquisition, tracking, and pointing (ATP) is the key technology used in laser communication, which requires the rapid acquisition and alignment of line-of-sight between two platforms and the establishment of communication links with high precision, high probability, and high dynamic tracking. The ATP system's rapid and high probability acquisition of a satellite terminal's line-of-sight under shipborne conditions has a direct impact on the communication time. Currently, optical links between ships have been verified. The link is usually initiated actively by an ATP at one end to cover an uncertain area with beacon light, then scanned or stared at by an ATP at another end to establish the link. This method of establishing an optical link is the most common method in laser communication, which requires high pointing accuracy. For short-distance communication links of tens of kilometers between ships or in air [20; 21], the divergence angle and optical power of the beacon light can be improved, and the requirement of the ATP with respect to the pointing accuracy can be reduced to reduce the difficulty of acquisition. Another method for realizing short-range ship-to-ship laser communication is using modulating retro-reflectors (MRRs). An MRR link uses an active ATP at one end and a small semi-passive MRR at the other [22; 23; 24; 25]. For one end of the link, MRRs no longer have tight pointing requirements. However, satellite-to-sea laser communication is very different from ground ATP systems. For ship-to-ship laser communications of tens of kilometers, the divergence angle of the uplink beacon is usually only a few milliradians due to resource constraints. The MMR method is less likely to achieve communication over hundreds or thousands of kilometers of links. Moreover, the complex nature of the ocean environment significantly increases the difficulty of acquisition and tracking by the ATP system. It is difficult for shipborne ATP systems to achieve high pointing accuracy to complete initial acquisition under conditions such as random and sharp waggling along with the sea, which poses a higher challenge to its technology. As a first step toward satellite-to-sea laser communication, it is essential to develop shipborne ATP and test its performance. In this paper, we report the acquisition and tracking of low-Earth-orbit satellites using shipborne ATP and verify the establishment of optical links between laser communication satellites and a ship at sea.