# Remote Image Capture System to Improve Aerial Supervision for Precision Irrigation in Agriculture
## 1 Introduction
### Wireless Sensor Networks in Agriculture and Food Industry  
The implementation of wireless sensor networks (WSNs) in precision agriculture enables increasing efficiency, profitability and productivity, and at the same time reducing possible impacts on the environment and wildlife. WSNs powered by batteries include different types of components: sensors, processors and radio frequency (RF) modules. The motes or sensor nodes have wireless connectivity to transmit the obtained data to a coordinator node or base station either directly or through a gateway. The real-time information obtained in the field provides a useful base for the managers to define the adequate strategies. This way, the decisions are not taken based on supposed typical conditions, which could not be realistic, but on real and updated data.  
The use of WSN technologies depends on the propagation of radio signals in practical environments, being difficult because of shadowing of the signal, multipath propagation and attenuation. In agriculture, radio frequency (RF) must face problems due to the placement of nodes for a large coverage and adequate link quality over the crop canopies. WSN must operate in different conditions like vineyards and orchards, in flat land or abrupt areas, and in all weather conditions, all of which affect radio performance [27]. In these cases, the link power budget depends on the state of the crop and the land, as well as other frequent factors such as node spacing and the height of the antenna [28]. Normally, a received level of the signal between 10 dB and 20 dB over the sensitive limit of the receiver is a convenient rate for the link budget [28].  
The proportion of messages lost or packet reception rate is also vital in WSN development and must be measured for any application. In accordance with the operating environment, an important signal loss may occur at some frequencies, especially when radio nodes need line-of-sight for optimum performance, being 2.4 GHz less robust than 900 MHz. Looking for better results, an alternative could be putting intermediates nodes, thus allowing peer-to-peer connection to the base node. Another option can be testing other frequencies, such as 916 MHz or 868 MHz, which are used in some nodes commercially available, or developing nodes with higher RF power which are able to reach longer distances. Additionally, connectivity ca be improved using directional antennas and optimizing the orientation of the antenna, its shape and configuration [29]. Other authors have focused in the development of autonomous image acquisition modules, such as the interesting work presented in [30], where a thermal-RGB color space capture module is described with WiFi and Ethernet connectivity.  
The weather is especially important in agricultural applications, since it affects the signal loss due to atmospheric conditions [28]. Outdoor use should consider the consequences of moisture due to humidity and precipitation. Some authors have analyzed the propagation of radio signals in potato fields using 433 MHz nodes, finding that the propagation of better in wet weather [31]. Moreover, rain and high humidity produced bigger signal strength in the receiver. Nevertheless, experiments using 868 MHz and 916 MHz nodes indicated the reverse results; the gateway only received 60% of sent messages when the relative humidity along the day was high. This ratio can grow to more than 70% in the driest day [32].  
Ambient temperature also affects the performance of the motes. Low temperature has an undesirable effect in the life of the node battery; tests in cooling chambers at diverse conditions proved that battery has a shorter life at low temperature. For instance, for 2.4 GHz ZigBee nodes, the battery life at 0 \({}^{\circ}\)C is reduced by half in relation to its life at 20 \({}^{\circ}\)C [33]. Finally, crop canopy has also been proved to influence the signal. It has been observed that signal strength and attenuation depend on the line-of-sight losses and heights less than the Fresnel zone radius [34].