## 2 Operational Environment and Sensor Network  
The environment where the whole system is deployed is the Gulf of Guinea. From our point of view, this environment is one of the most challenging in the world for the task we are targeting because of natural and man-made challenges.  
* Natural challenges:  
The Gulf of Guinea is located in the equatorial area where natural noise levels are greater than in any other part of the world [15; 16]. Also, very frequent gates occur on a nearly daily level in this region. Radar range is decreased due to the noise levels and the gates make detecting weak echoes difficult during gales, since surface wave propagation over a rough sea introduces a new attenuation factor into the radar equation.  
The aforementioned natural conditions make detection of vessels on the open sea a challenging task. The tracking algorithm at the radar level, described in [10] provides a solution for maintaining an unstable track, while multi-radar multi-target algorithm described in [11] forms unique radar tracks from all sources. More about natural challenges may be found in Appendix A of this paper.  
* Man-made challenges:  
Most of the challenges are a direct consequence of the lack of a strong regulatory body which defines the rules in the maritime arena. This leads to situations where AIS devices are very often turned off and their technical correctness may be questionable. Moreover, turning an AIS device off is often used as preventive measure which reduces chances of piracy attacks. Furthermore, transmitting another vessel's AIS data is a practice used during smuggling. At the end, SAIS data latency can be very high, practically sometimes it can be measured in hours.  
On the other hand, underdeveloped communication networks make data delivery quite hard. Since developing a full communication network for large areas only to provide consistent data for maritime surveillance is far from cost effective, data delivery is achieved through satellite communications. This introduces data delivery problems during bad weather conditions due to the increasing number of lost packages.  
All the aforementioned challenges make forming of unique operational picture at OTH distances in a Gulf of Guinea quite a difficult task. In order to provide the needed data, the following sensor network is formed:  
* Two HFSWRs with a nominal range of 80 nautical miles (approx. 150 km) for a Bonn express class of vessel [17] during night-time and sea states [18] up to 3. For larger vessels and during the day--time range can extend even beyond 125 nautical miles (approx. 230 km). Regardless of the time of day and vessel size, angle coverage is set to 120 degrees. More about used HFSWRs can be found in [19], while the HFSWR network coverage area over the Western part of Gulf of Guinea and it is shown in Figure 1,
* Six coastal sites equipped with AIS receivers and
* Data provided from SAIS provider, Orbcomm [20].  
In order to effectively combine data, all data is fed to the main data server located in a command and control center. The server performs three operations: tracking on single radar level, fusion of data from multiple HFSWRs and integration of SAIS and land AIS data with data provided from the fusion software layer. Although forming a single radar track in the main data server instead at the radar site might look less than optimal, it is the best solution in the given environment. Firstly, transferring plots from the radar site to the main server consumes less bandwidth than transferring the whole tracks. Secondly, the loss of a package of plots impacts the whole system far less since all tracking is done at the main data server.  
At the end of this paper, in Appendix B, sensor accuracy is briefly discussed and demonstrated on one example.