*2.3. Radar*

The radar used for this work is an AWR1843BOOST by Texas Instruments [12]. A radar detects the distance of the target by sending and receiving an electromagnetic signal through at least a couple of antennas. Using a multiple input multiple output array, it is also able to retrieve the direction of arrival. The sensor used is equipped with 3 TX and 4 RX antennas, which correspond to 12 virtual antennas, disposed as shown in Figure 3. In Figure 3, the *z* axis represents the altitude, while the *x* axis is left to right, and *λ* is the wavelength of the electromagnetic signal. This arrangement of virtual antennas achieves a good azimuth resolution and a poor elevation resolution, the elevation resolution being related to the inverse of the *z*-distance between the antennas.

For the current application, the elevation resolution is used only as an angular cut-off. In other words, the value of elevation measured by the radar is not used for mapping, and it is set equal to zero for each target, but it is used as a spatial filter for rejecting the target outside of a selected angular area. We set the angular field as ±45 deg in azimuth and [0,20] deg in elevation. Therefore, all targets outside this interval are not used for mapping.

Since the radar is not able to provide the elevation of the target, we decided to always consider zero as the elevation of the object. This is equivalent to assuming that each target is on a horizontal plane at the same height of the drone. This hypothesis is not as restrictive as it seems since usually a target at *z* = 0 m (at the same height of the UAS) is the most reflective. Under this hypothesis, the map generated using the radar is bi-dimensional. It becomes three-dimensional by changing the drone altitude [12].

**Figure 3.** Virtual antennas position of AWR1843BOOST [12].

In order to reduce the number of false alarms, the radar signal was filtered using two constant false alarm rate (CFAR) algorithms [12]. The first CFAR algorithm discriminates the physical targets from the thermal noise and from possible clutter using a moving threshold on range direction. The second CFAR algorithm was applied in the Doppler direction (which means on the speed) to further discriminate the possible targets from false alarm. The radar provides the coordinates of each target that exceed the CFAR thresholds.

In the current application, the radar range resolution was 0.5 m with a maximum range of 120 m and 10 Hz frame periodicity.
