## 3 Detection Method Design and Implementation  
Before designing the two-sensors data fusion vehicle detection method, we first analyze the data characteristics of geomagnetism and received electromagnetic wave's signal strength. We built a test-bed to collect and analyze the geomagnetism and received signal strength data. Generally, the speed of vehicles is 5-80 km/h on urban roads, the length of vehicles is 3-5 m and the distance between the front and rear vehicles on the same lane is 2-30 m. From the above  
Figure 3: Simplified model of radio signal caused by vehicle influence. traffic parameters, we can see that the vehicle's retention time above the sensor is generally more than 200 ms and the interval between the front and rear vehicles is generally more than 1 s. According to the Nyquist Theorem, as long as the sampling rate of the sensor is greater than 10 Hz, the vehicle running on the urban roads can be detected accurately. In order to obtain as many characteristics of sensor signals as possible, we use a geomagnetic sensor to collect the \(Z\)-axis of the geomagnetic signals and the sampling rate is 100 Hz. The received signal strength data collected by a FM radio module and the sample rate is also 100 Hz. The verification circuit board of the test-bed is placed on the middle of a road. A USB (Universal Serial Bus) to UART (Universal Asynchronous Receiver/Transmitter) serial cable is used to connect the microcontroller UART interface to a USB port of the laptop placed on the roadside and the baud rate of the UART interface is 115,200 bps. Microcontroller will start the geomagnetic sensor to sample the geomagnetic signal and the FM module to estimate the received signal strength at the frequency of 100 Hz which is generated by a timer of the microcontroller. After that, the microcontroller will enter into sleep mode. Once the microcontroller is woken up by the periodic timeout event, it will read the magnetic data from the geomagnetic sensor and the received signal strength data from the FM radio module. Then, the collected geomagnetic data and the received signal strength data will be sent by the microcontroller to laptop for further analysis. In order to collect and analyze the characteristics of geomagnetic signal and received signal strength disturbed by the high-chassis vehicle, we use a bus with the length of 11 m to pass over the sensors at different speeds, the speeds are 10 km/h and 60 km/h. The collected data shown in Figure 4.  
In Figure 4, the geomagnetic data is raw data read from the corresponding register of geomagnetic sensor, so there is no unit of the measurement. The received signal strength is the value from the corresponding register of the FM radio module and calculated according to the module's datasheet. Therefore, the unit of the received signal strength is dBm. We also analyze the absolute changes in geomagnetic signal and received signal strength as shown in Figure 4c,d. From Figure 4a,c we can see that the area between point 7500 and 12,500 of \(x\)-axis is the data of geomagnetism and received signal strength when the front wheel of the bus was passing. The area between point 12,500 and 20,000 of \(x\)-axis is the data of the vehicle body between the front and rear wheel. The area between data point 20,000 and 27,500 of \(x\)-axis is the sensors' data when the rear wheel of the bus was passing. Figure 4: The geomagnetism and RSS data when a bus runs at different speeds. From Figure 4a,c we can also see that there is an obvious geomagnetic signal blind area between data point 12,500 and 20,000 of \(x\)-axis. Fortunately, we find that the received signal strength data has obvious signal characteristics in the geomagnetic signal blind area. This phenomenon provides the possibility to realize two-sensors data fusion detection method in a vehicle detector, which can improve the detection accuracy of large vehicles running at low speed. We also analyzed the characteristic of the geomagnetic signal and received signal strength data when the bus running at high speed.