# Functionality Investigation of the UAV Arranged FMCW Solid-State Marine Radar
## 3 Experimental Setup
The full view of the vehicle can be seen in Figures 4 and 5. Figure 3: Central part of the multi-rotor. On the (**left**)—3D model; on the (**right**)—semi assembled part. Figure 2: Deformation modeling of the multi-rotor frame. To control the UAV, open source hardware and software Pixhawk 2 (Cube) with Ardupilot onboard were used. The implementation of open source equipment allows for quite simple integration of the needed features into the UAV system (like the determination of a direction for the radar operations and etc.). To ensure the autopilot telemetry data link, a 868 MHz frequency system was used, which does not interfere with the radar's operations. In parallel to that, a 2.4 GHz manual multi-copter control system was implemented, which was mostly used for takeoff, landing or tuning of the vehicle's systems. One of the serious problems during initial testing was the amount of vibrations on the multi-rotor control system (Pixhawk autopilot). The vibrations caused some very serious control problems, especially in altitude control of the vehicle. Since the used Pixhawk autopilot is an opensource design (both hardware and software) no precise requirements for the vibration level are available. The acceptable or non-acceptable vibration levels are determined purely by the influence on autopilot control characteristics (which is an additional problem). To determine the vibrations practical tests were performed with vibration  
Figure 4: Drawing of the vehicle. Figure 5: Fully assembled vehicle. level measurements on the multi-rotor while in flight. As was predicted, the highest level of vibrations was determined on \(Z\) (vertical) axis having the peaks at 49 Hz of 22 m/s\({}^{2}\), and an RMS for the \(Z\) axis of 14 m/s\({}^{2}\). At such level of vibrations, the Pixhawk autopilot was not capable of providing a sufficient level of control of the vehicle (altitude control was affected especially). To deal with the problem a specially damped mounting plate for the autopilot was designed (Figure 6), consisting of specially designed plates and standard vibration damping mounts (used for camera suspension). The vibration mounts used are silicon-based \"ball dampers\", which are frequently used for camera stabilization. Unfortunately, precise damping parameters were not initially known, therefore the dampers were experimentally selected from the number of available ones by careful matching of the dampers and the additional damping weights. Implementing the mentioned equipment the vibration level was considerably decreased (Figure 7). Figure 6: Designed plates and standard vibration damping mounts. Figure 7: Autopilot vibration measurements. (**Left**)—on the frame without dampers; (**Right**)—with special dampers. As can be seen from Figure 7, the vibration level was decreased approximately 10 times, from an RMS of 14 m/s\({}^{2}\) for \(Z\) axis without dampening to 1.3 m/s\({}^{2}\) with damping. That decrease of the vibration level was quite sufficient to ensure stable and reliable both manual and fully automatic control of the vehicle shown in Figure 5. It should be noted again that no precise requirements concerning acceptable vibration level are available for Pixhawk equipment, only abstract recommendations and the need for experimentally based results. Since the UAV/radar conjunction was intended to be a proof of concept solution, the vehicle had a limited endurance of around 25 min and was mostly used in hovering flight mode (just to raise the radar to a certain altitude). In the mentioned hovering mode the multi-rotor was mainly controlled by the autopilot (in fully automated mode) with altitude and position data being transferred in real-time to the radar control hardware. The multi-rotor shown in Figure 5 was powered by a set of standard 6s (24 V) Li-Po batteries with several DC-DC voltage converters (step down conversion to 5 V) and Engine Speed Controllers (ESCs) for brushless (BLDC or AC synchronous) motors, of which the interference with radar operations are further described.