# Pseudorange Measurement Method Based on AIS Signals
## 6 Field Test Verification  
Based on the above results, we present a pseudorange measurement scheme based on the existing AIS communication system, and the specific block diagram is shown in Figure 14:  
Figure 13: Algorithm performance comparisons.  
The RF circuit processes the received wireless signal by the frequency shift to obtain the intermediate frequency (IF) signal with a central frequency of 455 KHz. When the IF signal is processed by the automatic gain control module, the received signals of different intensities are normalized. The digital IF signal (obtained by sampling the analog IF signal through an analog-to-digital converter (ADC)) is sent to the signal processing module, which is implemented by FPGA for data decoding and TOA measurements.  
In the signal processing module, the Gaussian baseband waveform (the research object in this paper) is recovered by the GMSK demodulation module from the digital IF signal. We used the coherent demodulation method based on phase locked loop [30], which can theoretically improve the SNR by 3 dB. The subsequent signal processing flow is the engineering realization of the pseudorange measurement scheme described above. The bit numbers corresponding to the timestamps were provided by the message sequence recovered by the bit decision module. When a frame of message demodulation was completed, the signal processing module could output a TOA measurement value. The engineering prototype is shown in Figure 15a.  
In order to remove the error caused by the clock source interference at both ends of the transmitter and receiver, we used a rubidium atomic clock which had been synchronized by GNSS as the clock source. Additionally, we simultaneously used a Yagi-Uda antenna (Figure 15b) as the transmitting and receiving antenna to reduce the influence of multipath interference on the measurement results [31; 32].  
The field test site was located in Xinghai Bay, Dalian City, Liaoning Province. To ensure that there was no obstruction between the two points, we set up one fixed transmitting station and three receiving stations (Figure 16). The distance shown in the figure is the actual distance calculated from  
Figure 14: AIS transmission delay measurement scheme block diagram.  
Figure 15: Science of field test. (**a**) Engineering prototype; and (**b**) Yagi-Uda antenna.  
the longitude and latitude coordinates. The accuracy of the pseudorange measurement was statistically analyzed by the method of fixed-point multiple tests.  
The test was divided into short- and long-message ranging. In the short-message ranging experiment, the transmitter station only sent Message 1 and the length of the link layer packet was approximately 224 bits. In the long message ranging experiment, the transmitting station only sent Message 8, and the length of the link layer packet bit was approximately 1064 bits. In both test conditions, the contents of the transmitted message were not fixed, but encapsulated according to real-time information. In order to protect the transmitter's power amplifier, the transmission interval was set to five seconds. For different test content, each receiving point continuously collected about three hours of test data. The pseudorange measurement results are shown in Figure 17.  
Figure 17 shows that the pseudorange measurement scheme proposed in this paper is valid under different distance conditions. For the test prototype, in the case of a one-slot short message,  
Figure 16: Schematic diagram of field test site layout.  
Figure 17: Field measurement data analysis results.  
the pseudorange measurement accuracy was better than 60 m (\(\delta\)); and in the case of a five-slot long message, the pseudorange measurement accuracy was better than 28 m (\(\delta\)).