## 3 System Design  
### Prescription Map Generation and Interpretation  
In order to get the prescription map, the ArcMap software (Environment System Research Institute, ESRI) was used to generate a prescription map with different prescription values, so that the variable spray system can be guided by the prescription map. At the same time, in order to verify the effectiveness of the system, a 40 m \(\times\) 60 m field experimental field was selected in the Zengcheng Experimental Teaching Base of South China Agricultural University in Guangzhou, China (113\({}^{\circ}\)38\({}^{\prime}\)15\({}^{\prime}\) E, 23\({}^{\circ}\)14\({}^{\prime}\)37\({}^{\prime\prime}\)N). The target area was divided into small slices of 10 m \(\times\) 10 m. The UAV flew over the center of each operation unit. The flight path and unit division are shown in Figure 2.  
Figure 1: Schematic diagram of the structure of a prescription graphical interpretation of a variable spray system. PWM: pulse width modulation. **Note**: 1. Medicine box; 2. micro-diaphrag pump; 3. digital pressure gauge; 4. pressure nozzle; 5. hall flow sensor; 6. liquid crystal display (LCD); 7. buck module; 8. 12 V direct-current (DC) power; 9. drive amplification module; 10. prescription figure; 11. GPS. Figure 2: Sketch map of target plot. Each 10 m \(\times\) 10 m area is an operational unit, and a spraying amount is presented in each operational unit. According to the performance of each component of the variable spray system, five different levels of dosage (7.5 L/hm2, 15 L/hm2, 22.5 L/hm2, 30 L/hm2, and 37.5 L/hm2) were set up. The prescription value of each operation unit was selected randomly from the above 5 gradient prescription values, which are shown in Figure 3.  
Before the experiment, the working area was located by LocaSpace Viewer software (Beijing 3D Vision Technology Co., Ltd.). Then, the geographic position information of the working field was obtained, and the longitude and latitude were input into Microsoft Excel (Microsoft Corporation, USA) file. As a result, the ArcMap software was imported to generate the line map layer grid [12; 13; 14]. Eventually, the prescription map was created. By adding element classes to generate fishing nets, the grid can be edited and read. Finally, the longitude and latitude information of each unit and the dosage information were inserted by the linear difference method to generate the prescription map of the work area. The prescription diagram mainly contains three layers of information, as shown in Figure 4. The first layer is raster information, which is a rectangular raster of equal size according to the effective spray amplitude and flight speed of UAV. The second layer is prescription value information layer, which is based on the grid dosage obtained by the expert system of pesticide application, and different colors represent different prescription values. The third level is the geographic information layer, which is mainly the latitude and longitude information of raster rows and columns. Figure 3: Prescription value setting in each operation unit. The prescription map generated by the ArcMap software simulation mainly includes information such as the latitude and longitude and the amount of spraying of each unit, and programs the information of each unit to be stored in the STM32 controller. During the spraying operation, the on-board high-precision GPS transmits the position information of the plant protection drone to the STM32 controller in real time, and matches the position information contained in the work prescription map to determine the unit area where the UAV is currently located. When the position information of UAV is successfully matched, the prescription value of the current location is obtained. The obtained spray quantity information is transmitted to the variable spray controller to perform spray operation. The schematic diagram of hardware representing the prescription graphic translation system is shown in Figure 5.  
Figure 4: Structure chart of prescription map.