# Functionality Investigation of the UAV Arranged FMCW Solid-State Marine Radar
## 6 Discussion  
Even though we reproduced both kinds of interferences in the laboratory, the origin and penetration of the circle-like interferences during the flight test is not fully clear. It is obvious that it penetrates not through the antenna. We know that the isolation of the power supply chains is sufficient. There was no response from the switching power supply. The amplitude of the supply voltage pulsations was 0.1 V, the frequency--50 kHz. The frequency of 50 kHz corresponds to the 15 km distance, though no circles of this range were obtained. The possibility of the induction signal with the frequency of 7.5 kHz being caused by UAV's PWM circuit is not plausible because the frequency (radius) of the circles stays constant during various stages of flight. Because the intensity depends on altitude, there is no correlation to the load on the motors.  
A possible origin of the circle-like responses could be the interfering radio signal penetrating the IF circuits or low-frequency induction affecting the wiring of the UAV. Low-frequency currents in the wiring may occur due to non-linear effects (such as inter-modulation or frequency conversion) in semiconductors when two or more high power radio signals affect the circuits.  
Figure 21: Observation of the vessels by pulse compression radar and RTL-SDR receiver on a drone on low (7 m) altitude.  
No correlation between the apparent rings and the frequency of the rotors or their harmonics was observed, thus the possibility of the rotors causing additional reflections and/or Doppler shifts was ignored. No changes of the first ring's radius during various stages of flight were observed, only the appearance of the additional rings due to the higher intensity of harmonics at higher altitudes.  
The UAV's radar has a rotating antenna inside its radome. It allows for rapid scanning at the cost of additional weight and volume of the rotator and circular radome. In this paper, the performance of the radar such as angular resolution and the range was prioritized over the speed, flight duration and range of the UAV. Using a relatively small UAV with a mass of several tens of kilograms the antenna diameter was limited to 0.5 m with an angular resolution of 5\({}^{\circ}\) due to weight. Initial experiments were performed using a 2 m long antenna on a rotorcraft UAV where scanning was done by rotating the aircraft. This concept can be implemented in a fixed-wing UAV, for which the antenna is integrated into the wing as presented in the patent application WO/2019/003194. [25].  
Additional experimentation and measurements were performed at A (Figure 22), a new site, which was approximately 6 km from the previous measurement site B (Figure 22). Measurements acquired at the new site show that the interferences are directed at a different angle towards the Curonian Spit. There were no interferences to the city of Klaipeda's direction. The circular interferences were less significant and differed in frequency, which was 8.05 kHz (in the inner circle).  
We can determine the source of interference, which was near the first measurement site, but travelling further the interferences were diminished. In this case, if the multi-rotor was to fly further away from the shore towards the sea, the circular interferences should decrease significantly.