# Remote Sensing for Irrigation of Horticultural Crops
## 3 Sensors and Platforms
### Remote Sensing Platforms
#### 3.2.2 Airborne Platforms  
Airborne platforms (from 50 m to 50 km) carry out downward or sideward sensors to obtain earth's surface images. Airborne platforms give images with a spatial resolution of 20 cm or less, much higher than satellite maps, but they have low coverage area and high cost per unit area. For these reasons, it is not profitable to monitor and map large areas. Airborne missions are one-time operations,whereas satellites provide continuous target monitoring [72]. The quality of images acquired may be degraded by factors that introduce blurring, improper focusing, atmospheric scattering and target motion [34; 40]. The fields of application of airborne remote sensing are numerous, such as biodiversity, biogeochemical cycles, climate change, eco-hydroology, infectious disease, land use, invasive species.  
For natural and agricultural environments, it is worth mentioning airborne remote sensing studies on vegetation: (i) cover amount; (ii) dominant type; (iii) canopy structure (height and Leaf Area Index); (iv) crop status; (v) canopy chemistry (nitrogen index); (vi) crop greenness; and (vii) crop health in relation to topography (elevation, slope and aspect) [73; 74; 75; 76; 77].  
Most data seen in Google Earth, and the imagery of towns and cities, are supplied by Bluesky and DigitalGlobe, and are not taken from a satellite. On a plane or helicopter it is simple to remove/replace the old sensor with a new one that is more precise and accurate. Furthermore, it is possible to take several flights until the proper and correct image is acquired. Moreover, a plane or helicopter, different from satellites, can fly with unfavorable weather conditions (high-level clouds). Even imperfect images can undergo a post-processing stage for correcting atmospheric problems. Helicopters are usually used for low altitude applications where the ability to hover is required. Airborne hyperspectral imagery has been evaluated for estimating crop yields [78; 79], green biomass [80], and crop disease [81].  
In one of the first examples of application of airborne imaging to crop farming, [PERSON] and [PERSON] [82] set up an experimental field capable of creating a continuous gradient of water table level during the rainy season. Four inclined water tables were obtained (Figure 1). Figure 1b was taken in summer 1986 by an aircraft with conventional imaging equipment. Figure 1c is the conversion of the color photo to grayscale image, and then conversion to false colors. The upper part of the image (darker soil color) corresponds to the most drained part of the field during the winter season, which was then excluded from measurements. Results of the experiment are published by [PERSON] et al. [83], reporting the effect of shallow water tables on peach trees grafted on three peach seedlings.  
There was was a pioneering attempt to achieve airborne RS, unfortunately not supported by knowledge appropriate to the task, and this is the main reason that the RS image was not orthogonal. The goal was to integrate ground photos of each plant with airborne images to calculate the leaf area of each tree. For this goal, several trees were fully defoliated to have an accurate measure of Leaf Area Index (LAI).