# A Millimeter-Wave Radar Tunnel Obstacle Detection Method Based on Invalid Target Filtering
## 2 Materials and Methods
### Filtering Methods for Invalid Millimeter-Wave Radar Targets  
In practical conditions, due to noise and vibration in the working environment of underground electric locomotives and the unstable operation of the millimeter-wave radar itself, most of the targets detected by the radar are invalid targets, for only a few are target obstacles. Invalid targets are divided into empty targets, false targets and non-threatening targets, which are filtered out in different ways. Empty targets are easy to filter out and can be judged and filtered out directly based on the distance value of the target measured by the millimeter-wave radar; based on experience, a horizontal and vertical distance threshold is set and the horizontal and vertical coordinates of the non-threat target are compared to the threshold and filtered out if their absolute value is greater than the threshold. A thread is created to combine the results of successive radar frames and set a threshold to filter out false targets using the valid target lifecycle method. The valid target lifecycle method is divided into three phases: generation, persistence and extinction (as shown in Table 4). The status parameter FindCount describes the number of times the target has been detected, and the status parameter LostCount describes the number of times the target has been continuously lost. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Signal** & **Starting Position** & **Length** & **Minimum Value** & **Maximum Value** & **Resolution** \\ \hline Object\_ID & 0 & 8 & 0 & 255 & 1 \\ Object\_DistLong & 19 & 13 & \(-\)500 m & +1138.2 m & 0.2 m \\ Object\_DistLat & 24 & 11 & \(-\)204.6 m & +204.8 m & 0.2 m \\ Object\_VrelLong & 46 & 10 & \(-\)128.00 m/s & 127.75 m/s & 0.25 m/s \\ Object\_VrelLat & 53 & 9 & \(-\)64.00 m/s & 63.75 m/s & 0.25 m/s \\ Object\_RCS & 56 & 8 & \(-\)64.0 dBm\({}^{2}\) & 63.5 dBm\({}^{2}\) & 0.5 dBm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Contents in the Object_1_General message segment. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Signal** & **Starting Position** & **Length** & **Minimum Value** & **Maximum Value** & **Resolution** \\ \hline Object\_ID & 0 & 8 & 0 & 255 & 1 \\ Object\_ArelLong & 21 & 11 & \(-\)10.00 m/s\({}^{2}\) & 10.47 m/s\({}^{2}\) & 0.01 m/s\({}^{2}\) \\ Object\_ArelLat & 28 & 9 & \(-\)2.50 m/s\({}^{2}\) & 2.61 m/s\({}^{2}\) & 0.01 m/s\({}^{2}\) \\ Object\_Orientation Angel & 46 & 10 & \(-\)180.00\({}^{\circ}\) & 180.00\({}^{\circ}\) & 0.4\({}^{\circ}\) \\ Object\_Length & 48 & 8 & 0.0 m & 51.0 m & 0.2 m \\ Object\_Width & 56 & 8 & 0.0 m & 51.0 m & 0.2 m \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Message contents for Object_3_Extended. The reasons for the loss of millimeter-wave radar targets are: 1 random bumps and oscillations during the exercise of the mine tramway locomotive, the millimeter-wave radar measurement signal may be lost for a short period of time; 2 the presence of sloping lanes in the tunnel and uneven terrain may lead to abnormal millimeter-wave radar detection and may even temporarily lose the original valid target; 3 the original valid target is temporarily blocked by the presence of a temporary target; and 4 the original valid target has left the set target screening area. The cumulative count threshold \(C_{D}\) and the lost count threshold \(C_{L}\) can be obtained experimentally. In this paper, the thresholds \(C_{D}\) and \(C_{L}\) are set to three and five, respectively. By setting the valid lifecycle of a target detected by the radar, false filtering of valid targets can be prevented. For non-threat targets, a suitable target area is selected according to the actual application scenario, and judgment is made based on whether the target is in the target area or not. In this study, the motor truck is used as the research object and the operating area where the underground electric locomotive is located, and the adjacent areas that can pose a danger are selected as the target area.