## 2 Materials and Methods
### Radar Sensors  
Frequency modulated continuous waves (FMCWs) are radar waveforms often used to measure distances in automotive applications [13; 28]. Anteral uRAD radar sensors for Raspberry Pi were tested here, shown in Figure 2. These are 24 GHz FMCW radar sensors which connect conveniently as extension boards to Raspberry Pi microprocessors [29]. Such microprocessors run a user-friendly operating system and can interface with purpose-built components. The automotive industry is increasingly using 77 GHz radar sensors, allowing for increased range resolution and accuracy [30]. These sensors can better separate closely spaced objects, and can be packaged in a smaller form factor. However, 24 GHz sensors are less expensive, and automotive-grade 77 GHz sensors which could be operated from Raspberry Pi microprocessors were not available when procuring the equipment. Thus, testing the proposed methodology using cheaper 24 GHz sensors was considered reasonable. While many 77 GHz FMCW radar sensors are more range-capable and have wider fields-of view, some 77 GHz radars, e.g., in [31], have shorter ranges than the uRAD sensors. Figure 2: Radar sensor outline [22]. The uRAD sensors have a theoretical distance range approaching 100 m, 30\({}^{\circ}\) fields-of-view both horizontally and vertically, and are able to detect up to five objects simultaneously. For each object, distance, radial velocity and reflected power (SNR, i.e., signal-to-noise ratio) are registered. In the context of truck platooning, both the sensor and the desired objects are in motion; so _relative_ velocities are detected. The velocity range (\(\pm\)0.2 to 75 m per second) is within the range of values encountered in road traffic. The use case and intended sensor placement were described to the supplier, which expected the application to be feasible, but did note that similar tests had never been carried out before. The sensor has been used for other purposes, the most relevant of which is as a stationary speed sensor [29; 32; 33]. However, transferability from the cited studies is limited, due to different use cases and configurations. Eight parameters were used to configure the sensors. These are detailed in Appendix A.1, alongside pre-trial testing of the mounting set-up and parameters. The most important considerations are detailed here. Firstly, the radar mode details the waveforms transmitted by the sensor. Triangular waves were chosen, which maximized the sensor range and the update rate for outputs (9-13 Hz) from the radar script. This mode also allowed for subsequent data filtering based on relative velocity. Maximizing range was important for capturing data even when trucks were located far apart, as the trucks were expected to drive with human-level gap sizes (2-3 s) at distances approaching the upper distance range. Adverse road geometry would presumably also lead to safer driving at larger gap sizes. Maximizing the update rate was seen as beneficial for obtaining as many measurements as possible. The update rate of the uRAD radar sensor is comparable to the 77 GHz sensors showcased in [14]. Secondly, number of targets detected (\(N_{tar}\)) and the detection distance (\(R_{max}\)) were maximized, to capture the most data, and to enable filtering of unwanted detections later. Thirdly, moving Target Indicator (_MTI_) was activated, for including data only from objects with motion relative to the sensor. The supplier stated that it would only eliminate objects which were absolutely static, such as detections of the windshield. The preceding truck would still be registered, even when moving at the same velocity as the sensor. Fourthly, for each truck, different values for ramp start frequency (\(f_{0}\)) and the duration of each wave ramp (\(N_{s}\)) were used for each sensor to avoid interference. Since each sensor had different \(N_{s}\) and \(f_{0}\) values, their theoretical maximum distance ranges also differed, at 75.0 and 73.1 m, for trucks 2 and 3, respectively, based on Equation A.1 in Appendix A.1.