## 3 Application Scenario and System Model  
### Application Scenario  
The scenario studied considers surveillance systems using two different types of sensors: static on the ground and mobile in the air (UAV-carried or simply UAV). There is a number of static ground sensor nodes spread in the area of interest, called mission area, according to a given distribution, which can be random or following a predefined pattern. The ground sensor nodes perform simple measurements, such as differences in magnetic field and CO\({}_{2}\) concentrations, among others, which can indicate the presence of objects or occurrence of events of interest, such as vehicles or people, or the occurrence of a fire. Some UAVs fly over the mission area, following a random or predefined movement pattern. UAVs are equipped with more sophisticated sensors, such as visible light cameras, infrared cameras, and synthetic or inverse synthetic aperture radars. The UAV sensors typically provide more detailed information compared to the static sensors on the ground. The number of UAVs is however much lower than the ground sensor nodes, and the idea is to make them work cooperatively, so that they complement each other. The distribution of static ground sensor nodes is assumed to ensure that they cover the whole area and, when they identify a possible event of interest, they trigger an alarm that is sent to the UAVs, equipped with sophisticated sensors, to perform more accurate observations.  
The sensor nodes communicate with each other via wireless links within a tunable, but limited, communication range. Due to the broadcast nature of wireless media, all nodes in the range of a sending node receive the sent messages.  
The system behavior is defined as follows. Ground sensor nodes are configured to detect phenomena indicating possible threats, which are defined by a set of threshold levels of their measurements. When the acquired measurements reach a configured threshold level, a \"match\" with the detecting criterion is achieved. In the occurrence of a match, the sensor node issues an _alarm_, which is received by all nodes that are within its communication range.  
Alarm messages are sent as single communication packets containing a timestamp, the position of the issuer node, and the type of the possible threat. The first two components of the alarm message enable unique identification, avoiding alarm duplication, assuming the atomicity of the events reported by the alarms, which means that each alarm that is sent indicates a different threat. Consequently, if the indicated threat is a group of persons or vehicles, they are handled as a single threat. This assumes that neighboring nodes on the ground cooperate to aggregate information needed to characterize threats before issuing an alarm.  
The main elements of the described scenario are presented in Figure 1. In the figure it is possible to observe the detection of a possible threat by a ground sensor node. This node issues an alarm that is received by all its neighbor nodes. One of these neighbors relays the alarm, which is then received by the neighbor static sensor nodes and by a nearby UAV.  
At the occurrence of an alarm, one UAV, equipped with more sophisticated sensors, takes the responsibility for the alarm and flies towards the area where this alarm was issued, in order to gather further information about the possible threat and confirm it as a real threat, e.g., an intruder or a fire spot.  
Figure 1: Overview of the application scenario illustrating threat detection, alarm issuing and alarm reception.