# A Novel Ship-Tracking Method for GF-4 Satellite Sequential Images
## 1 Introduction
However, limited to the LEO satellite's high pass speed and long revisit period, moving targets can only be observed in a short time interval and only a few plots or a short track can be obtained. In addition, most LEO remote sensing satellites have a narrow swath which is usually tens of kilometers. The limitations of LEO remote sensing satellites restrict their use for maritime surveillance to some extent. The GEO remote sensing satellite can provide medium spatial resolution, wide scan coverage and short revisit interval image series. Target-tracking methods for GEO remote sensing satellites have been discussed in some published literatures [3; 20; 21; 22]. However, compared to ground, aerial and satellite videos, the GEO remote sensing satellite sequential images have much lower spatial resolution. As shown in Figure 1, two ships are moving in five sequentially near infrared (NIR) band images from the GF-4 satellite (the size of chips is 128 pixels \(\times\) 128 pixels and acquisition Time (UTC) is given in Table 1), and some automatic identification system (AIS) information of two ships is shown in the right part of this figure. We can see that the ships have fewer pixels and lower discrimination, and ships can be regarded as weak point targets in the sea background. Meanwhile, the imaging interval of the GF-4 satellite (from tens of seconds to a few minutes per frame) is long, compared to traditional videos, and satellite images are also affected by light and other factors, resulting in higher false rate of detection. These new characteristics and problems of the GF-4 satellite cannot be directly solved with traditional video target-tracking methods. Image sequences of the GF-4 satellite are more suitable for direct detection processing for each frame, and then multi-frame multi-target data association processing. The accuracy positioning of ships using the GF-4 satellite is also important for ship tracking. Due to the long distance away from Earth, a small attitude control error of the GEO platform will cause a large position bias between the target corresponding to pixel point and image position. By directly using rational polynomial coefficients (RPCs) for geometric calibration, the absolute positioning accuracy of the GF-4 satellite's 1A level image is about 4 km. The large positioning error will affect ships association and tracking accuracy. At the same time, the motion of the space-borne camera will make the position between frames deviate. These problems can be resolved by means of ground control points (GCPs). However, the surveillance area is at sea and there are few or even no GCPs in sea area. AIS is an identification system that transmits a ship's identification, position and other critical information that can be used to assist in navigation, improve maritime safety and track ships for maritime domain awareness, search and rescue, environmental monitoring, and maritime intelligence application. Thanks to the wide use of AIS in marine surveillance, this paper uses AIS information as dynamic GCPs to register each frame to obtain accurate target position information without inter-frame registration. A method of ship detection and tracking for GEO remote sensing satellite sequential images is designed in this paper. The rest of this paper is organized as follows: the processing framework is described in Section 2. Experiments and results using GF-4 satellite sequential images are presented in Section 3. Finally, discussion and conclusion are presented in Section 4.