## 2 System Overview
### Communications Workflow: Relaying Data to the Base Station  
As previously mentioned, the air robot is responsible for shipping the bridges between the parcels and the base station. This mobile node is attached to one of the four frames of the quadrotor, taking into account drone mass distribution and inertia. Nevertheless, the motor throttles should be balanced in order to compensate mass deviation. Additionally, it was also necessary to consider electromagnetic interferences (EMI) when placing and configuring the communication systems, in order not to affect quadrotor avionics, or more capabilities (see Figure 7). Figure 8 shows the onboard mass distribution in the quadrotor employed, where the more can be observed--including the communications and control devices--using less than 50% of the overall payload capacity. In order to cope with the requirements of the mission such as dealing with the problems of long distances between the vineyard parcels and the base station, a packet-oriented mobile data connection service based in General Packet Radio Service (GPRS) has been adopted in order to communicate the drone with the base station. The GPRS connection allows relaying the sensor data from the motes within the vineyards until the base-station through wide distances. This solution is only limited by the  
Figure 8: Quadrotor on-board mass distribution. Figure 7: Quadrotor flying over the vineyards shipping a mote. The attached mote is depicted in the bottom right inset. quality of service (QoS) of the network provider, so it can be considered as robust and reliable. Unlike fixed GPRS modems, the modem shipped on the drone allows transmitting data overcoming GPRS shadow areas--commonly found outside urban areas--in the parcels. The data is stored and then later transmitted as soon as the drone reaches areas with communication coverage,  
An additional feature for the system can be using Short Message Service (SMS), which allows sending messages to farm managers periodically, and thereby increasing their situation of awareness. The system previously mentioned is based on Arduino Uno board and GPRS Quad band module, which include a HiLo SAGEM communication module (see Figure 9). The overall Ground-Air communication system is shown in Figure 10, and works as follows: the main component of the air sub-system (shown on the top of Figure 10) is the Arduino board that reads the data from the quadrotor, likewise from the TelosB mote. The data is read from the quadrotor at a frequency of 2 Hz, and from the mote at a frequency of 4 Hz. The Arduino is always on stand-by. The reading mode is activated every time the onboard node senses a reliable sign from a ground WSN distributed over a parcel (_i.e._, the signal quality should be greater than a defined threshold). This data is then dispatched to the base station that is shown in the bottom layer, with respect to the ground sub-system, and consequently sent by SMS to the farm manager if the temperatures drop the critical values. It allows a previous reaction (earlier to the damaging effects, but reactive to some past circumstances). In the base-station, the biophysical parameters (e.g., temperature, humidity, _etc._) are stored with respect to the waypoint that maps the parcel. It should be taken into account that each waypoint corresponds to a parcel, which is indeed covered by a single WSN. The waypoint coordinates are computed with an iterative genetic algorithm (GA) that computes an optimal hovering position within each parcel. We understand optimal hovering position as the position where there is more ground nodes covered by the aerial node. This stored information could be analyzed, managed and visualized using a specific tool specially developed for this application. This instrument is prompted as a Graphical User Interface (GUI), and provides crop managers with a simple way to define the mission targets and likewise to store-visualize and manage the data acquired over the parcels (see Figure 11).