## 2 Related Works  
The AWARE project [1] aims at integrating a sensor network of resource constrained ground nodes with mobile sensors, carried on the ground by UGVs and in the air by UAVs. In a large sense this work is closely related to ours and an idea that is shared in both is the combined use of ground sensors and UAVs taking part in the same sensor network and cooperating to achieve surveillance mission goals. In [16] a rendezvous-based data dissemination protocol to deliver critical alarm messages issued by the static sensor nodes to the mobile ones in the context of AWARE project is presented. This approach divides the network of static sensors in a set of hexagonal cells, like a honeycomb, which is a solution used in other applications such as cellular phones. Both alarm messages from the static sensor nodes and queries from the mobile ones are forwarded to central parts of the network following the hexagonal organization of the nodes, and then forwarded to their destinations. Our approach is more flexible because the nodes do not need to keep information about their neighbors, as it is needed in clustered organizations such as the proposed hexagonal one in AWARE. Moreover, in our approach there is no preferable region of the network to where the messages should be forwarded, such as the centre of the hexagons in their work. In [17], an approach exploring the concept of scent marking used by primates for territorial demarcation is proposed to provide cooperation among mobile sensors, by using static sensors and Radio Frequency Identification (RFID) tags. On one hand, this work is closely related to ours because its concept of scent marking is very similar to the stigmergy approach behind the pheromones used in our work. In their proposal, mobile sensors are robots that move around an area, and when they find events of interest in a given region they leave RFID tags or sensors covering this region. If they need help to complete a given task the information stored in the tags will drive other robots towards the region where their help is required. The main difference from our work is that they deploy new nodes in the network, in an _ad hoc_ fashion, as an attempt to disseminate the information that must be received by the other mobile nodes, in this case a help request. On the other hand, our approach assumes an already deployed network of static nodes, which may perform a number of sensing missions, with which the mobile ones cooperate. In this sense, we try to use the resources available in the network in a rational way so as to not waste them, while the proposal in [17] relies on additional resources being added to the network according to the needs. In [18], a biologically inspired approach for division of labour in Sensor/Actuator Networks (SANETs) is proposed. They also explore the ant pheromone mechanism, in a scenario closer to what is discussed in [17], but focusing on a statistical method to perform route decisions. In order to perform these decisions, the proposed networking mechanism is based on information obtained during a route discovery phase, which is an important difference in relation to our work. In our proposal the route is discovered by the message that has to be delivered to the destination node, avoiding the overhead of a precedent route discovery phase, as proposed in [18]. A proposal to solve the problem of message dissemination in a multi-level WSN composed by static and mobile sensors on the ground and UAVs moving in the air is presented in [19]. Their approach is based on the epidemic routing concept, but uses a probabilistic decision to forward or not incoming messages, so that a more efficient usage of the communication medium is achieved. As the nodes on the ground are also able to move, and considering the urban scenario addressed in their work, nodes may have different numbers of neighbors during system runtime.