Chapter 8

UAV Remote Sensing in Precision Agriculture

UAV Platforms

UAV or drone-based platforms are a technology that can be used to obtain quantitative plant information for tens or even hundreds of lines in a crop field using noninvasive imaging techniques and protocols. Fully integrated remote sensing platforms consist of unmanned aircraft fitted with multiple sensors and use communication and global navigation satellite system (GNSS) tools to acquire crop canopy images from the field. UAV classifications are globally based on their wing design, which impacts their autonomy, size, and weight. There are three major types of UAVs—fixed-wing, rotary-wing, and hybrids, also known as vertical takeoff and landing (VTOL) drones. UAV characteristics (Table 8.1) within these three categories vary in their aerodynamic features and have a significant impact on flight time, altitude, speed, cost, resolution, and so forth. The choice of drone platform depends on the survey area and project requirements. UAV platforms are chosen based on project area size and location, flight range, optimal flight time, and payload (sensors to be deployed).

Fixed-Wing UAVs

Fixed-wing drones are designed like more traditional types of aircraft—which look like airplanes (Figure 8.1). They are made of a central body that has two wings and a single propeller. Once in the air, the two wings generate lift that compensates for its weight—allowing the aircraft to remain in flight. Fixed-wing UAVs can be launched by manually throwing them into the wind or by accelerating down a runway into the wind. While in flight, fixed-wing UAVs are controlled using aileron and other controls. Piloting a fixed-wing UAV requires much practice—especially when following flight lines at specific heights above ground level.

Advantages

Fixed-wing aircraft can fly for a longer period of time and distance from higher altitudes on a single battery cycle than a rotary wing. This makes them ideal for mapping very large or linear areas because they do not have to fly home for a battery replacement as often during a single mission. However, as technology improves, the gap closes.

Disadvantages

Piloting a fixed-wing UAV requires much practice—especially when following flight lines at specific heights above ground level. Nevertheless, in this respect, pre-designed flight plans facilitate piloting and data acquisition. Fixed-wing aircraft require a larger takeoff and landing zone for flight, which can make them ill-suited for some use cases.

Rotary-Wing UAVs

Rotary UAVs can be differentiated based on weight and the number of rotors (Figure 8.2). Generally, they are more affordable and easier to handle, suitable for daily operations on small to medium scales. They are further classified into helicopters and rotary UAVs. Helicopter UAVs feature a single set of blades connected to a central shaft, which rotates at a specific speed to generate lift for takeoff. Additionally, a counter-rotor is located at the tail to control yaw.

Advantages

Unlike fixed wings, rotary-wing aircraft can perform vertical takeoffs and landings. This means that they require less space to take flight, can hover mid-flight, and can maneuver above and around objects for easy inspection, mapping, and modeling. This also makes them ideal for area mapping due to the number of flight legs often required to get sufficient overlap to make a quality map. For inexperienced operators, they are the easiest way to get up and running quickly.

Disadvantages

Each rotor on a multi-rotor UAV has an electric motor; together, they significantly shorten battery life compared to fixed-wing UAVs with one propeller and motor. Most multi-rotor drones can fly for about 30 to 60 minutes in ideal conditions before returning home for battery replacement. One can offset this downside by purchasing additional batteries.

Vertical Take-Off and Landing UAVs

Vertical takeoff and landing (VTOL) drones, commonly known as hybrids, are becoming more common because autopilot, gyroscope, and accelerometer technology can be integrated into the UAV to facilitate remote piloting of these complex systems (Figure 8.3). VTOLs combine many of the best attributes of fixed-wing and multi-rotor drones. VTOLs have the capability to take off, hover, and land vertically without the need for a conventional runway. VTOL aircraft combine the advantages of fixed-wing airplanes (speed, range, efficiency) with the vertical lift and landing capabilities of helicopters.

Advantages

VTOL drones don't need to use the runway to take off, saving space and not being limited by terrain. There is nearly no operational limitation of VTOL UAVs and they can be widely used in mapping, surveillance, surveying, and other fields. The ability to vary the relative speed of each rotor creates changes in thrust and torque, so VTOL drones have more efficient maneuverability. VTOL drones have high agility and high cruising speed compared with traditional air vehicle.

Disadvantages

The VTOL system greatly increases the weight of the aircraft and limits the payload that can be carried. VTOL systems are more complex than traditional UAV systems. It requires advanced technologies such as multiple engines and lift systems, which makes them more difficult to operate.

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