## 3 RS Applications in Ocean
### Ship Detection (SD)
#### 3.6.3 HF Radar  
HF radars based on electromagnetic scattering and surface wave propagation can detect and track targets far beyond the conventional microwave radar coverage [339; 340]. HF radars exploit the HF band (3-30 MHz) to offer a board coverage extended to more than 200 km in the range [340]. However, the radio frequency interferences and external noise restrict the detection capability of the HF radars [184; 185]. Regarding SD using HF radar data, the sea clutter is considered to be self-generated interference and creates the first- and second-order resonant scattering effects in the Doppler spectrum. The first-order scattering is produced by ocean waves with half of the radar wavelength, while the second-order scattering is caused by the interaction of crossing sea waves [340]. SD using HF radar is mainly based on the CFAR algorithm following the beamforming data process. The CFAR threshold is usually calculated by the Neyman-Pearson approach and assuming a fixed PFA [341]. With this approach, a detection decision is made for each Range-Doppler (RD) cell.  
Many studies have so far utilized HF radar data for SD. For example, in [PERSON] and [PERSON] [341], the adaptive threshold is obtained by applying the conventional curvilinear regression analysis [342] along with the Doppler and range cells to detect ships. Additionally, the gating approach was used to associate each target plot with a track in polar coordinates in order to track targets such as ships. Moreover, in [PERSON] et al. [343], target detection was accomplished by applying the space-time adaptive processing technique to each range cell. This algorithm was operated on a so-called data cube including complex samples, obtained from many coherent pulses for various range cells to acquire test statistics.  
Although the aforementioned approaches are applicable for SD using HF radar, several issues may emerge when an HF radar observes the ocean surface and monitors moving ships. The first issue is that ships' maneuvering during a long coherent processing interval creates Doppler blur in the Range-Doppler map. Therefore, SD may fail when the CFAR detector is applied. The second issue is that the Bragg scattering may mask a ship signal or be a considerable source of false detection since Range-Doppler cells associated with the Bragg scattering have a significant magnitude. In this regard, [PERSON] et al. [344] attempted to decrease the effect of ship maneuvers by applying a median filter before CFAR detection. Additionally, the decision fusion technique can be used for improving the detection performance to solve the second issue [70]. [PERSON] et al. [345] proposed a method to simultaneously solve the two abovementioned issues by adding a signal-enhancement procedure before CFAR detection. Finally, [PERSON] et al. [346] proposed a DL method for SD using a combination of SAR and optical images. They developed a novel, robust framework based on a real-time object detection model, called YOLO, to improve the SD results compared to previous methods in the context of multisource, multiscale, and multisensor datasets. It was reported that their model could detect ships at any time with any available RS imagery with a high accuracy.