# Monitoring Vehicles on Highway by Dual-Channel \(\varphi\)-OTDR
## 5 Vehicle Locating on a Highway  
After validating the performance of the DC-\(\varphi\)-OTDR over the campus road, we proceeded to assess its feasibility for monitoring running vehicles on a highway. In this case, two of the optical fibers integrated into an armored four-core cable are used as the double sensing channels. In this test, the driving speed of the vehicle on the highway is in the range of 80-100 km/h. The cable is \(\sim\) 22 km away from the monitoring center and buried along the highway with a depth range from 0.2 m-0.5 m and a closest horizontal distance from the highway of \(\sim\) 1m. Originally, the cable was paved along the highway not for sensing purposes, but for transmitting the traffic violation videos taken by cameras to a surveillance room. Therefore, some sections of the cable are suspended in the drainage ditch and do not make contact with the soil, which hinders the vibrations caused by the running vehicles to be transmitted to the cable through the soil. For this, we have chosen the cable located between z = \(\sim\)7 km and z = \(\sim\)9 km being the sensing unit for the vehicle measurement, over which the cable is buried in soil as shown the inset of Figure 1c. Another reason for selecting this section of the cable is that there is a video camera mounted at z = \(\sim\)8.5 km, by which we could confirm our vehicle positioning obtained by the DC-\(\varphi\)-OTDR. In the monitoring room (z = 0), the probe pulses with a repetition rate of 4 kHz are injected into both _fiber_ 1# and _fiber_ 2# (as shown in Figure 1), and the backscattered Rayleigh optical signals from each sensing fiber are converted into electric signals and sampled by data acquisition, then processed by the CDM and CDM&V, respectively, to obtain the accumulated vibration signals. In the experiment, 100 consecutive Rayleigh traces are measured over 25 ms (N = 100) for each sensing fiber. Again, the upper limit of the separation between the two differencing Rayleigh traces is restrained to be N/2. Figure 5 shows the measurement results at z = 8.3 km for different differential steps: Figure 5a\({}_{1}\)-a\({}_{3}\) are the accumulated vibration signals from _fiber_ 1#; Figure 5b\({}_{1}\)-b\({}_{3}\) are the accumulated signals from _fiber_ 2#; Figure 5c\({}_{1}\)-c\({}_{3}\) are the synthesized signals by the fusion of both sensing channels, in which all curves are normalized to their respective maxima. It should be noted that the value of k (differential step) is arbitrarily selected from 1 to N/2; other values in this range have no significant impacts on the results. We both employ the CDM (based on Equation (1) and Equation (2)) and the CDM&V (based on Equation (3) and Equation (4)) to detect the running vehicle. Figure 4: Synthesized detection signals obtained by the CDM and CDM&V when the vehicle is driving over the campus road. (**a**) z = 200 m, (**b**) z = 218 m, (**c**) z = 242 m, and (**d**) z = 252 m.  
Figure 5a\({}_{1}\)-c\({}_{1}\) compare the results obtained by the CDM for k = 5 and 30. It can be seen that when the differential step is chosen to be 5, _fiber_ 1# fails to detect the vehicle and _fiber_ 2# detect the signal with a small SNR of 0.5 dB. To efficiently locate the vibration, the differential step of the CDM must be greater than 5. With k being increased to 30, both _fiber_ 1# and _fiber_ 2# show an ability to locate the vibration signal with a respective SNR of 1.5 dB and 0.8 dB. We also notice that there are some spikes around the vibration peak, which may be attributed to the spreading effect of the vibration caused by the moving vehicle. From the two synthesized signals shown in Figure 5c\({}_{1}\), it is observed a considerable enhancement in SNR for the larger differential step (3.9 dB for k = 30 vs. 0.7 dB for k = 5). Again, the results indicate the strong sensitivity of the CDM algorithm on the differential step. Meanwhile, the synthesized signals show greater SNRs compared to the accumulated ones either from _fiber_ 1# or _fiber_ 2#.