## 2 Materials and Methods
The experimental multi-rotor used in the investigation has a significant drawback--limited flight time (up to 15 min), which often is not sufficient for conducting measurements. For this reason, during the development, a larger multi-rotor aircraft (with 12 kg take-off mass) was designed and verified to ensure a longer flight time (up to 35 min); the vehicle is shown in Figure 5.  
The implemented SignalHound BB60C spectral analyzer had no specific modifications. The measurements were performed at two frequencies, to allow for measurements of two radio transmitters at a time. The spectral analyzer was controlled through an onboard NUC Mini PC. The drone had minor modifications for radio frequency measurements--a specially designed and manufactured 1 mm aviation-grade aluminum box was installed with all of the radio sensitive equipment in it. The aluminum box was implemented to decrease the influence of external radio frequencies on the spectral analyzer performances (though no such effect was noted during real-life testing). The receiver antenna (spectral analyzer antenna) was mounted on a special glass-fiber stand to move it as far away as possible from the drone sources of radio interference (telemetry transmitter, wide-band data transmitter, etc.) and the possibly reflective parts of the drone (carbon fiber main board, batteries, etc.). Figure 4: (**a**) Experimental unmanned aircraft with measurement equipment. (**b**) Hexacopter aircraft during flight. The implemented RTK GNSS receiver (single-frequency U-Blocks MSP-2) has manufacturer-stated positioning accuracy of (with RTK) 0.025 m, ensuring sufficient accuracy for our implementation. The use of a hexacopter unmanned aircraft ensured easy and fast take-off and landing, as well as sufficiently accurate flight route tracking. However, hexacopter aircraft have several key drawbacks:  
* An average flight time of 15-30 min; nonetheless, this is much better than the first multi-rotor version. * Low flight speed (up to 10-12 m/s). Such flight speed is both a drawback and an advantage. The drawback is that it is difficult to ensure measurements over long distances and large areas. The advantage is more accurate measurements. * Limited flight distance (due to short flight time). To improve the measurement capabilities, the measuring equipment was also installed on a fixed-wing (airplane-type) unmanned aircraft. As mentioned earlier, the use of a fixed-wing unmanned aircraft ensured the following:  
* Longer flight time (up to 4 h, depending on the power source);
* Higher flight speed (16-30 m/s);
* Ability to conduct measurements over longer distances (due to higher speed and longer flight time). At the same time, the use of a fixed-wing aircraft for measurements has certain drawbacks:
* Maintained flight trajectory is not as precise, which can affect measurements;
* During flight, the aircraft's flight azimuth can change, which can affect measurements (due to the measurement antenna diagram). For the above-mentioned reasons, fixed-wing aircraft are used for collecting a larger quantity of data in larger areas, as well as for creating broader 3D antenna diagrams (which was demonstrated during the research). The unmanned aircraft 'Spartan' (see Figure 6), which is in production by the \"Zvelk Auksciau\" company, was selected for the installation of the measurement equipment, with the following main characteristics:  
* Take-off weight--up to 14 kg;
* Payload weight--up to 2 kg;
* Flight time--up to 4 h (depending on conditions);
* Powerplant--two electric motors;
* Launch--hand-launched;
* Landing--belly landing. Figure 5: (**a**) Hexacopter aircraft view from the side. (**b**) Hexacopter aircraft view from the front. In all cases, two computers were used for flight and measurement equipment control and management: one for controlling and managing the aircraft's flight and the other for controlling and managing the measurement equipment, as shown in Figure 7: