## 6 Presentation and Analysis of Results
### Statistical Analysis  
While recognizing that the magnitudes for the 30 and 60 Hz spectral lines can help characterize vehicle mode or occupancy, additional features should be considered for reliable classification of gear engagements and number of occupants. As an example, for the GMC Acadia, significant vibration frequency features seem to fall mostly below 20 Hz, as noted in Figures 9-12. We present below some initial results of statistical analysis on the Kia Forte5 data to support these observations on feature extraction throughout the frequency spectrum. Figure 19: PSD comparisons for transient components between gear shifts for micro-Doppler radar and accelerometer data for OPRND case for the GMC Acadia from the front side. The accelerometer used to represent the front side is on the hood. Figure 20: PSD comparisons for transient components between gear shifts for micro-Doppler radar and accelerometer data for OPRND case for the GMC Acadia from the back side. The accelerometer used to represent the back side is on the license plate. Figure 21, displaying STFT data for the front side of a smaller vehicle (2015 Kia Forte5 hatchback), shows two distinct features for the gear shifting analysis. One feature is at 23 Hz and another is at 45 Hz. During the first 10 s of data, the vehicle is off, and both of these features are practically non-existent in this region. At 10 s, the vehicle is turned on, which creates a wideband pulse for a short duration, following which a 23 Hz signal appears. The second feature at 45 Hz only appears when the vehicle is in \"Reverse\" or \"Drive\" gears. The Figure 22 histogram confirms that when the vehicle is off, the received power seen at 23 Hz is distributed between 0 and \(-\)5 dB. Whereas when the vehicle is turned on, the received power at 23 Hz is reaching about 10 dB on average. Therefore, for this specific vehicle, 23 Hz is a good indicator for confirming if the car is on or off. Figure 23 illustrates the power distribution of the 23 Hz feature for all various gear modes. When the vehicle is off, most of the power stays below 0 dB. When the vehicle is turned on and shifted to any of the other gear modes, the 23 Hz feature stays above 0 dB. Notice that when the vehicle is in \"Drive\", the received power is at a maximum. Whereas when the vehicle is in \"Neutral\", the received power is at a minimum. Figure 24 illustrates that the 45 Hz feature reaches between 2 and 4 dB, while the other vehicle modes stay mostly below 0 dB. Therefore, for this specific vehicle, 45 Hz can act as a good indicator for detecting whether the vehicle is in reverse or drive (i.e., ready to move) or in park or neutral (i.e., stationary). A comparison of the histograms of the received powers in dB between the acoustic and the radar data provides some insights. Figures 25 and 26 show these comparisons for features at 23 Hz and 46 Hz, respectively. Figure 21: Spectrogram of a PRND recording on the front side of a small compact hatchback vehicle (2015 Kia Forte5). Figure 22: Received power histogram for the Kia Forte5 in the ON and OFF states at 23 Hz. Figure 25 illustrates the power distribution for each gear mode (OFF, PARK REVERSE, NEUTRAL, and DRIVE) on the front side of the Kia vehicle at the frequency of 23 Hz, since it has been inferred from the corresponding spectrogram as an important feature. The blue bins represent the radar Doppler samples and the red bins represent the acoustic samples. When the vehicle is OFF, the samples for the acoustic data hover around \(-4\) to 4 dB. Most of the samples in the radar data hover between \(-8\) to 0 dB. On the contrary, when the vehicle is ON and in DRIVE, the acoustic and radar samples are mostly above 15 dB. Acoustic samples mostly fall above 20 dB in DRIVE, which is most certainly because the accelerometers are physically attached to the vehicle, whereas the radar is stand-off at about 1.5 m, exhibiting radar path loss. However, the power distribution patterns remain the same.