# Functionality Investigation of the UAV Arranged FMCW Solid-State Marine Radar
## 4 Results of the Primary Radar  
The flying site was chosen on the seashore in Klaipeda (Lithuania) near the seaport. This site provides acceptable safety and a good field of view into sea's direction. In the vicinity of the seaport there are industrial and urban areas from where potential interference  
Figure 8: Communication system structure between the multi-rotor, radar and ground station. may originate. Additional marine traffic data was monitored by the Marine Traffic website as presented in Figure 9.  
The first radar image and the AIS vessel positions (Figure 10) were taken when the UAV was on the ground (on the roof of the car). The dark blue insert in the centre is generated using open CPN software. Red objects are the primary radar's responses, green triangles are vessel positions, received from the Secondary radar (AIS). Positions of vessels correspond to positions presented in Figure 10. Unfortunately, the only AIS data we have received were from the vessels in the port. As we find later, the short-range of the AIS receiving was caused by a frequency drift of the heterodyne of the cheap RTL SDR dongle receiver. During takeoff, additional objects appeared on the radar's response chart as presented in Figure 11. These objects are not related to real targets but are radio interferences. Ray-like objects are typical interferences caused by signals on the radar's frequency. Similar results are presented in [23], where interferences to a meteoradar from Wi-Fi devices were obtained. The origin of the rays can be explained by the properties of the radar's antenna. The highly directional rotating antenna can receive signals from directions very close to the antenna's pointing direction. The frequency of the signals should be close to the radar's operating frequency. The width of the rays depends on the antenna's directivity. Due to  
Figure 10: Radar and AIS data when the UAV is on the ground. Figure 9: Positions of vessels by Marine Traffic website. rays being pointed in the direction of land, we can assume that this kind of interference is caused by land infrastructure of communication systems. Another kind of interferences occurs as rings. The first ring appears at an altitude of approximately 15 m. The diameter of the ring is equivalent to 2280 m. At higher altitudes, rings with diameters two, three, and four times wider occur. The higher the altitude, the higher the intensity of the rings (Figure 12). The distance to the target in the FMCW radar corresponds to a frequency, thus we can assume that the equal steps of the diameter of the rings in the image correspond to the harmonics of the interfering signal. It is obvious that the interference does not come through the antenna because the intensity of the interference's image does not depend on the antenna's pointing direction. This is a reason why we cannot estimate the direction to the source of the interfering signal. We can estimate the frequency of this signal \(R\) when knowing the distance \(R\) and parameters of the transmitted signal. \[R=\frac{c_{0}|\Delta t|}{2}=\frac{c_{0}|\Delta F|}{2\Big{(}\frac{\Delta f}{ \Delta t}\Big{)}} \tag{1}\]  
Figure 11: Radar and AIS data when the UAV is at the altitude of 20 m.  
Figure 12: Radar and AIS data when the UAV is at the height of 120 m.  
Here \(c_{0}\) is the speed of light, \(\Delta t\) is the delay time, \(\Delta f\) is the measured frequency difference of FMCW chirped pulse, \(F\) is the detector frequency, \(R\) is the distance between the antenna and the reflecting object, \(\Delta f/\Delta t\) is the frequency shift per time unit. When the frequency chirp is linear, \(\Delta f\) is equal to the frequency span, \(\Delta t\) is equal to the pulse length. When the Simrad 4G(tm) radar is working in maximum range mode, the following values were measured: \(\Delta f\) = 703 kHz, \(\Delta t\) = 1.42 ms. The radius of the first ring (Figure 12) is 2280 m.  
We can calculate the frequency in the detector using the range Equation (1). When the range \(R\) is 2280 m, then the frequency \(\Delta f\) is 7525 Hz.