3.1. Unmanned Aerial Systems
An Unmanned Aerial System (UAS) is an aircraft with capability of flying autonomously or controlled by a remote pilot. All UASs include a command, control, and communications system, and personnel necessary to control the unmanned aircraft . UASs used to be primarily employed in military operations; however, their use for other purposes such as agricultural remote sensing has rapidly increased in recent years.
Payload and flight endurance were two main challenges of using UAS for agricultural purposes. During the 1990s, UAS capable of carrying powerful and accurate sensors tend to be large and very expensive, while small platforms lacked adequate payload capacity to carry heavy sensors and deliver precise data. As a result, one of the major conclusions of the Environmental Research Aircraft and Sensor Technology (ERAST) program and subsequent UAS science community workshops in the late 1990s was the need for sensor miniaturization to enable the use of small and affordable UAS platforms for survey-grade data delivery .
Using small Unmanned Aerial Systems (sUAS) or drones, which includes drones weighing less than 25 kg (55 pounds), in agricultural tasks started in the early 2000s  mostly for management purposes and then expanded swiftly to the point that it was listed as one of the ten breakthrough technologies by MIT Technology Review in 2014  and became increasingly popular and widespread since then. According to Federal Aviation Administration (FAA) report in March 2018 , the number of commercial drones will be quadrupled in the next five years, from 110,604 in 2017 to 451,800 by 2022, where drone technologies are estimated to provide service worth $127 billion .
Affordability, reliability, user-friendliness, improved mission safety, and flight repeatability due to improving autopilots, heavier payloads, and being standalone (no need to infrastructure) are among the essential factors facilitated the expansion of sUAS, especially in agricultural activities. Nowadays, a sUAS equipped with cameras and software, cost less than $1000, can easily survey over 100 acres in a short flight . Additionally, growers can fly over and monitor fields on a monthly, weekly, daily and even hourly basis  depending on the desired temporal resolution. It is promising that drone-based remote sensing can provide inter- and intra- field variability of crops information and help optimize irrigation schedule and increase water use efficiency . Newer, more powerful, and efficient drones are emerging every day in different shapes. Multirotor, single-rotor, and fixed-wing drones are commonly used in agriculture . Fixed-wing drones, which resemble conventional airplanes, is the fastest in mapping a larger area compared to other types. Besides, they are very energy efficient and can fly for a much longer time. Nevertheless, the complexity of launching and remotely controlling a fixed-wing drone are two main drawbacks that are not present in rotary drones. Vertical take-off and landing (VTOL) and hovering capability of rotary drones make them very popular in agriculture, yet flight endurance and energy efficiency are serious cons. Because of this, some companies tried to combine fixed-wing and rotary drones to have both advantages of VTOL and flight endurance. In table 2, different types of drone were compared based on their advantage/disadvantage, application, and cost .
Table 3- Comparison common drones used in agriculture.
Usually, cameras and sensors account for a significant share of the cost in a drone-based field mapping, especially for state-of-the-art sensors with high precision and reliability. Although the higher price of these sensors and cameras limits their practical implementation by farmers and technicians, they are critical for unraveling complex problems and paving the road for designing simpler sensors. Apart from the high price restrictions, limited payload, and short flight endurance, some other challenges might also be considered when choosing a drone for an application. Technological advances and increasing operational capabilities of drones have generated new challenges to flight operators, end-users, and aviation authorities. Therefore, some legal provisions have been legislated that either allow, prohibit, or restrict flight operations. Such regulations significantly impact how, where, and when data can be captured and also restrain the drones' characteristics . For example, in the U.S. flying beyond visual line-of-sight (BVLOS) needs special approval, and maximum flying altitude is limited to 122 meters above the ground level. Additionally, specific locations in the proximity of controlled airspace and airports are limited.
Despite all the aforementioned drawbacks and restrictions, a high-resolution camera when flying in low altitude provides very high spatial resolution imagery in 1 cm level , or even less, which justifies their widespread use. Moreover, a great deal of effort has been put into improving the UASs' drawbacks. To ameliorate limited flight times, for instance, the implementation of new battery technologies (fast charge with higher energy density) and deploying solar panels on UASs can extend flight time from 40–50 min up to 5 h . Additionally, an efficient mission planning can improve the flight coverage by eliminating unnecessary energy consumptions. All in all, UASs, specifically multirotor drones, have attracted a rapidly growing attention in agricultural practices.
According to table 2, more than 66% (6 out of 8) of the sUAS used in nut studies are multi-rotor drones capable of collecting RGB and MS images. This percentage goes down to around 22% (2 out of 9) for fixed-wing drones, and it is even less (around 11 %, 1 out of 9) for single rotor drones. Another interesting point for comparing different drone types is that most studies with multirotor drones are conducted recently (2014, 2017, 2018, 3*2019), while studies based on the single rotor (2012) and fixed-wing (2014) are older. This clearly shows the growing popularity of multi-rotor drones in the remote sensing application of nut orchards. Moreover, as discussed earlier, fixed-wing drones have higher endurance and can cover larger aeras from higher altitudes that are quantified in table 2 and are in accordance with table 3. On average, studies that used fixed-wing drones cover more than 150 ha, while this number for multi-rotor drones is less than 20 ha.
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