## 2 Data Fusion for Object Localization and Identification  
Let us start with the real-world requirements of airport security:  
Location of an object with radar accuracy (resolution);  
Identification of an object is mandatory;  
Possible identification of many objects: at least 50 units/second;  
The personal unit must be portable, with acceptable size and weight;  
The battery life of the unit must be at least equal to shift time (10 h);  
The system must be automatic, i.e., the operator must be relieved of the constant need to classify the objects appearing and their status. The main idea of the presented wireless personal monitoring system arises from the need to locate and identify persons and vehicles in a wide-open outdoor area, e.g., airport apron, military traverse, game field, dock, or pasture. Among different systems, a fusion of two of them seems to fulfill the provided requirements, i.e., a combination of radar and GPS positioning together with an independent device--an automatic object identifier, which will be called the mobile unit (MU) in short later [16; 17]. This system combines the advantages of the location accuracy of a radar system (which, compared to GPS, is significantly better and constant, both temporally and spatially) and possible identification based on the MU equipped with a GPS module (Figure 1). The radar sends the position of an object every one second and must be consistent with a 50 MUs response time (airport requirement). This means that the round-trip time for the position query from the server (PC, e.g., a system operator personal computer) via the base station (BS) to a MU (acquiring the GPS position) and then the round-trip time for the response MU \(\rightarrow\) BS \(\rightarrow\) PC is limited to 20 ms. Such strict timing demands require a special radio front-end and customized transmission protocol design [18]. Figure 1: General idea of system for object localization and identification. Thus, the stand-alone system consists of base stations (BSs) and mobile units (MUs) [19; 20]. Each MU contains an integrated GPS receiver, an RF transceiver, a power management unit, a microcontroller, and can optionally be equipped with a keyboard, an LCD display, or a biometric sensor (e.g., fingerprint reader) to confirm personal authorization (on demand). The structure of a BS is similar, with the addition of a communication module (e.g., RS-422/485 or Ethernet) to establish a link with the server (PC). A BS has a constant and known location, so it may also contain a GPS receiver to calculate the difference between the real and the reported location. It can be used to correct the location of the MUs (e.g., using simple DGPS techniques). An MU is attached to an object (person or vehicle); therefore, its unique identification number can be assigned to the object. If an MU has a biometric module, the link between the person and the MU can be made automatically (Figure 2). The idea of automatically assigning object status is derived from a comparison of the locations indicated via radar and the MU (GPS). The proposed algorithm takes into account the movement history and the current difference between the two locations. The most important element is the introduction of two temporary states:  
* _Friend with unknown ID_--for objects (two or more) that are authorized to be in the area. Such a status does not generate an alarm in the system, but all objects will change their status if and only if an ID is assigned. Figure 3 shows such a situation for two objects--if the system's resolution cannot distinguish between ID1 and ID2, both are tracked without any action. The ID is reassigned if the distance is greater than the system resolution (\(d_{min}\));
* _Foe Suspected_--for objects (two or more) that are currently close enough to foe object and none can be uniquely identified. The distance between _friend_ and _foe_ depends on the accuracy of the system, e.g., radar, GPS, etc. Therefore, it must be customized for a particular system.