## 2 System Overview
### Sensing System: WSNs for Specific Data Acquisition  
The sensor network is responsible for acquiring on-ground relevant information and communicating the data to the base station, using WSN nodes (or motes). The motes are small devices equipped with sensors, processing systems and communications resources. The points to be taken into account are data acquisition operation, time and requirements for the communication systems, field dimensions in relation with nodes and sensors, crop requirements, _etc._ These subject matters are dealt in this section. Since the whole field-monitoring network is split into several clusters of different sizes, every part of the field is monitored by one or several clusters formed by one or more WSN nodes. The motes will acquire information about the soil, weather and/or crop status. These data will be relayed to the base station when an air vehicle flies over each cluster, sending the packets through a relaying node on the aerial drone, denoted as quadrotor. This physical topology has been defined in order to provide enough flexibility to the network [28, 29]. It also allows defining node layout according to terrain requirement and necessities, without considering communications constrains. In such manner, the number and performance of the nodes are increased, since node concentration varies according to the magnitude evolution in each terrain slide. In this sense, areas where the magnitudes-to-monitor do not highly change along the  
Figure 3: An illustrative example of the overall remote sensing mission. distance (e.g., temperature), it will be supervised with few nodes. On the contrary, zones where the variation of the magnitude is spatially unstable (e.g., gas concentration, vibrations) should be monitored with a tighter mesh. The best topology and layout for this network are determined experimentally: starting from a regular crystalline structure, magnitude evolution gradients are studied, in order to define variations focus. Having this data, it is possible to dynamically relocate the motes, obtaining finally a pattern adequate to the intrinsic terrain characteristics [30]. Nevertheless, communication links must be also taken into account. Node deployment shall optimize more performance, but will also try to perform a mesh topology, allowing complete interaction among neighbored nodes. It allows downloading all data by situating the relay node (_i.e._, the quadrotor) in the surroundings of the clusters, avoiding the necessity of locating it accurately. The capacity for obtaining load information from any nearby point also allows defining optimum trajectories to cover every field. It clearly enhances the global performance of the system, and derives the complexity to the communication protocol [31]. The systems work properly when no regular meshes are defined--both initially due to operation circumstances, but with decreases in their performance. The scattering of the nodes in relation to communications only becomes significant when node distribution is actually atomized. Even in this particular case, the system architecture is able to guarantee a normal operation by increasing flight time and requirements of the resources. Furthermore, not only mesh routing schemes should be considered. As the literature shows, most routing protocols are designed and defined for systems where nodes emit in discrete and punctual time intervals, being inactive and awaiting--with respect to communications--the rest of the time [32]. Nevertheless, this kind of architecture will saturate and overflow the network in our application, where big packet streams are simultaneously sent from every node, in order to minimize quadrotor harvesting/overfly time. Most of the Medium Access Control (MAC) layer protocols are based on probability (randomly waiting to resend packets which have been lost due to collisions occurring), so the synchronization and communication time could be huge [33, 34]. That is why a pipelined hierarchy organization was developed.