*2.2. Multispectral Sensors and UAV Platforms*

Two types of multispectral sensors were compared in this experiment. The Sequoia camera [43] has a total of five imaging sensors, including four multispectral sensors and one RGB sensor (Figure 2). The spectral response function of Sequoia is shown as the solid line in Figure 3, which was provided by the manufacturer. The focal length of the Sequoia camera is 3.98 mm, the image size is 1280 × 960 pixels, and the sensor size is 4.8 mm × 3.6 mm. It is equipped with a sunshine sensor that can record the illumination information of each image, facilitating the calibration of multispectral images. The self-provided calibration panel can be used for radiometric calibration, and the reflectance data can be obtained directly.

**Figure 2.** UAV platforms and multispectral sensors (Sequoia and P4M). The Sequoia sensor is carried on the EM6-800 hexarotor UAV (left), and the P4M uses its own aircraft (right).

**Figure 3.** Spectral response functions of Sequoia (solid lines) and P4M (dashed lines).

The other multispectral sensor considered in the study is the P4M (Figure 2). The P4M camera [44] has a total of six imaging sensors, including five multispectral sensors and one RGB sensor. The spectral response function of P4M is shown as the dashed line in Figure 3, which was provided by the manufacturer. The focal length of the P4M camera is 5.74 mm, the image size is 1600 × 1300 pixels, and the sensor size is 4.87 mm × 3.96 mm. The P4M camera is also equipped with a sunshine sensor, but the reflectance data cannot be obtained directly. Table 1 shows the band information for Sequoia and P4M.

**Table 1.** Spectral band information for the Parrot Sequoia (Sequoia) and DJI Phantom 4 Multispectral (P4M). Their approximately equivalent bands (green, red, red edge, and near-infrared) were compared in this study.


During the data collection, the sensors were carried on different UAV platforms. The Sequoia was mounted on a hexarotor UAV called EM6-800 which has the advantages of low cost and high stability. Its payload is 800 g, and the maximum flight time is 40 min; under the maximum load of 1.2 kg, its maximum flight time is 25 min [45]. The UAV is equipped with an onboard flight controller, which includes a compass; an inertial unit; and gyroscopic, barometric, and global positioning system sensors. The onboard flight controller can be used to control the flight missions through flight route and measurement point settings. It can also record and access the data obtained by the mounted sensors for postprocessing. Unlike the Sequoia, the P4M has its own UAV platform, so it can complete the data collection task independently without the help of other aircraft. It has a takeoff weight of 1487 g, and the average flight time is 27 min.
