## 3 Developed Drone System for TEM
### Aeromobile Part
#### 3.1.2 UAV  
To perform geophysical surveys at low altitudes and in conditions of difficult terrain, original multi-rotor systems with electric batteries were used. They are quite simple to manufacture in the conditions of university workshops, are cheap to operate, are characterized by a significantly lower noise level compared to systems with internal combustion engines, and a high ratio the mass of the payload to the mass of the carrier. In this case, a typical version of the UAV from the SibGIS UAS complex was used. It was developed specifically for geophysical surveys in difficult conditions of geological expeditions [2], which is a hexacopter with a frame diagonal of 1 m (Figure 3). For several years of operation of these UAVs, their design has been brought to the maximum degree of strength, ease of maintenance, low operating costs, and minimization of electromagnetic interference of the carrier. For example, UAVs of this design can withstand a fall with the engines turned off from a height of 40 m onto a surface with at least some vegetation (not bare stones) without damaging the main structural elements, while only propellers and specially weakened elements manufactured by 3D printing, and can be replaced in the field in 5 min. A high-precision magnetometer ceases to feel any influence of carrier interference at a distance of about 3.5 m below the UAV frame. It should be noted that UAVs were originally designed for a payload mass of up to 4.5 kg (a spectrometer with a CsI 8 \(\times\) 10 cm detection unit or the overhauser magnetometer and a CsI 3 \(\times\) 15 cm spectrometer at the same time) [2; 10; 11], but they are also capable of carrying a larger payload with a reduction in flight time. In the case described in the article and at this stage of the development of the UAV-TEM method, there was no need of a flight time of more than 20 min, since during this time, at a UAV cruising speed of 7-8 m per second (\(\sim\) 25 km/h), it was possible to get the operating radius 2.5 km, taking into account the time spent on takeoff, landing, flying in and out of the survey profile. This was significantly higher than the previously justified value of 1.7 km and allowed us to survey areas of at least 5 square kilometers using one take-off point, which was not only sufficient for experimental work, but also, in our opinion, is quite economically profitable. The UAV was equipped with RTK GNSS system and a laser altimeter that recorded a series of beam reflections, which measured the distance to the ground, rather than to vegetation. These solutions ensured high accuracy of marching mission and data georeferencing.  
The main problem of the used design of the UAV-system in this case was the weight of the UAV and the towed system are almost the same. In that case the center of gravity of the whole system is floating. UAV is able to move such a load for the required time, but it turned out that it is very difficult for the carrier to stabilize its flight after turns, as well as to start and stop. The high-quality data could be received after the UAV reaches cruising speed and stabilizes the measuring system. In this regard, in the central part of the geophysical sections, arrived in Section 4 of this article, where the UAV begins and ends its flight along the survey lines, the replacements are discarded shooting areas. This could be avoided by dividing the flight missions in such a way that they went 150-200 m beyond the surveying area, this distance would be enough for acceleration. However, a better solution was to reduce the payload mass, in the next field season it will be reduced to about 5 kg, as described in the previous section.