# Data Collection for Target Localization in Ocean Monitoring Radar-Communication Networks
## 1 Introduction  
With the development of marine activity and scientific expeditions, precise and reliable data link communication plays a pivotal role in ocean monitoring [1; 2; 3]. As an important supplement to the satellite geodesy, unmanned aerial vehicles (UAVs) assisted satellite geodetic techniques to achieve both data acquisition endpoints for receiving sensor-generated data and cooperating with satellites to transmit more refined information [4; 5]. There have been a few works researching using UAVs in marine activities as aerial data collectors [6], aerial relays [7], and node complements [8] in integrated air-space networks. Considering the improvement of communication performance, UAV-assisted communication has been regarded as the core technology in the next generation of maritimecommunication due to its flexibility and mobility, which resolves the long transmission and wide coverage problem in the maritime communication system. In order to sufficiently exploit the characteristics of the UAV, researchers have conducted extensive investigations on resource management [9; 10], channel modeling [11], safety control [12], and performance analysis [13]. The application scenarios of UAV-assisted communication can be summarized into three types, i.e., the UAV serves as a data collector, the cooperative relay and the aerial base station, respectively [14]. Due to maneuverability, UAVs can improve transmission quality by creating line-of-sight (LoS) channels in communication-unfriendly environments. Since most IoT or marine acquisition devices usually have limited transmit power and communication coverage, the flexibility of UAVs can effectively reduce the communication distance and select the optimal location to complete communication tasks. For instance, how to effectively transmit data collected from the sea surface sensor nodes in the ocean monitoring network to the ground base station through UAV cooperative communication is introduced in [15]. In this case, deploying the UAV as a data collection device was devoted to being a profitable and capable scheme. Furthermore, combining the non-orthogonal multiple access (NOMA) and UAV into the wireless network has been regarded as a promising way to promote data collection and spectral efficiency simultaneously. The authors of [16] proposed a new cooperative NOMA scheme for which a UAV transmitted the signals to several base stations (BS) and all of the base stations decoded the message cooperatively. However, a cooperated BS is hard to deploy in an ocean scenario, which makes it not manageable. A UAV-assisted NOMA network where the UAV cooperatively served the ground user to communicate with the base station is investigated in [17]. The NOMA precoding and UAV's trajectory are jointly optimized to maximize the sum rate. However, such a scheme is not appropriately used in maritime tasks where the base station's location is usually far from the equipment. In [18], the study of a UAV equipped with a phased-array antenna to receive the signal from the satellite and then transmit the information to the land-based user was investigated, where the trajectory of the UAV and NOMA power was jointly allocated and optimized to achieve a higher energy-efficiency purpose. Unfortunately, considering the complexity of satellite communication systems, using a UAV as a relay to transmit data to ground users will challenge energy efficiency regardless. Furthermore, all of the above research only focuses on UAV-assisted downlink transmission, which means that UAVs are employed to serve ground users or base stations instead of data collection. Applying the radar sensing function to the sixth-generation communication system opens up possibilities for broadening oceanographic missions [19; 20]. Employing the radar function in the UAV-assisted communication system can help the UAV to detect the environment which can avoid potential threats and find the optimal location of the UAV to construct a more stable communication link.