# Maritime Over the Horizon Sensor Integration:
## 3 Description of Data Fusion Algorithm  
Each radar in the radar network performs its own tracking procedure, as described in [13]. The tracking procedure relies on Joint Probabilistic Data Association-Unscented Kalman Filtering (JPDA-UKF) algorithms. The HFSWR data-fusion algorithm is an iterative process triggered when a set of single radar tracks from any radar in the network is received.  
Each track formed by any radar in the network has an identification (Id) assigned by the radar which formed the track. When the track appears for the first time, it is placed in the so-called track-waiting list. All tracks in the track-waiting list are not forwarded for further processing, until they appear N times in M consecutive periods. After a radar track passes this threshold, it is moved from the track-waiting list to the tracking list.  
Figure 1: HFSWR network coverage area (taken from [21]).  
The data fusion algorithm creates so-called fused tracks and operates with them. A fused track is formed from at least one single radar track. Upon the formation of a fused track the algorithm assigns a new Id to the fused track. Besides this new Id, every fused track maintains the list of Ids that represent the identifications of radar tracks which are used to form it. It is important to note that one fused track may not contain more than one track originating from the single radar.  
The main steps of the data fusion algorithm are shown in Figure 2.  
During the first step, the algorithm is trying to update already formed fused tracks using newly arrived radar tracks. For each fused track algorithm, it is checking if the originating radar tracks are present in the newly arrived data. If the Id is matched, the algorithm checks if the radar track still falls within the fused track gate radius. This gate radius is calculated based on the vessel's speed, so only the range radius is taken into account. From our point of view, since all single radar tracks are already preprocessed by JPDA-UKF algorithms, there is no need to include the state vector again. When both conditions are satisfied, the fused track is updated. If at least one of the criteria is not satisfied, the radar track is forwarded to the following step of the algorithm.  
During the second step of the algorithm, all previously unassigned radar tracks that are considered potentially suitable for an association with existing fused tracks are processed. This means that for all of the unassigned radar tracks, the gating with existing fused tracks is checked. Please note that one fused track may not have more than one radar track coming from each radar. For simplicity's sake, this procedure is shown in Figure 3.  
Figure 2: Main steps of the data fusion algorithm (taken from [18]).  
If a fused tracks loses all of its source radar tracks, it is moved to the so-called predicted track list. The fused track stays in this list for K consecutive update periods, and it is referred to as the predicted track. If no source (either new or old) is found within this time frame, the track is declared non-existent and the algorithm stops further processing.  
The third step of the algorithm operates on predicted tracks in a manner similar to the previous step of the algorithm: all the tracks which are currently in the predicted track list are compared with the remaining radar tracks. If a matching track is found, the track is returned to the fused track list.  
During the last step of the algorithm, new fused tracks are formed from the remaining unassigned radar tracks. It is important to remember that the fused track may not have more than one track originating from each of the radars.  
At the end, the predictions and corresponding gate radiuses for all of the tracks present in the fused track and predicted track lists are calculated. These predictions as well as the gate radiuses will be used when the next data set from radars arrives, in order to create, maintain or delete fused tracks.