## 3 The Zero-Baseline Experiment  
Our experiment was conducted on the roof of a high-rise building on the campus of Shandong University (Weihai), Weihai, Shandong Province, China. The surrounding environment of the experiment is open, which is convenient for conducting zero-baseline experiments. Our experiment was conducted on 7 March 2022. The day was sunny, and the weather conditions remained stable throughout the day, which was conducive to the operation of the zero-baseline experiment. The data receiving and acquisition system used in the zero-baseline experiment is mainly composed of a cooking-ring GNSS antenna and a four-channel IF signal recorder. Figure 5a,b record the field environment of the zero-baseline experiment.  
In Figure 5a, the geodetic GNSS antenna is erected and fixed by a tripod to keep it horizontal. The signal is led out from the antenna by one shielded cable. It can be noted that there is a fence on the top of the building, the horizontal plane of the erected antenna is higher than the height of the fence, and the other positions remain open. Therefore, it can be considered that the antenna signal is not affected by the surrounding obstacles. Figure 5b shows the wiring of the signal receiving and processing terminal. The shielded cable is led out from the antenna and connected to the four power dividers in Figure 5b. Before entering the recorder, the signal is divided into the same four branches by the four power dividers and connected to the four signal receiving ports of the recorder, respectively. It should be noted that the four data lines connecting the power divider and the recorder should be completely consistent to eliminate the difference in the line's thermal noise. The bandwidth of the recorder is 20.46 MHz, of which two signal channels will receive the B1 band with a frequency center of 1529 MHz, which can collect B1C signals with the center frequency of 1575.42 MHz. The other two channels will receive the B2 band with a frequency center of 1130 MHz, which can collect B2a signals with the center frequency of 1176.45 MHz. In Figure 5, the two input interfaces on the right correspond to the B1 band of the signal. It can be considered as the signal channel 1 and signal channel 2 of Figure 2. The two input interfaces on the left correspond to the B2 band of the signal. They can be considered as signal channel 3 and signal channel 4 of Figure 2. The recorder works at 62 MHz continuous sampling, and the collected IF data are quantized by 2 bits. After the signal is processed by the original IF signal receiver, the original IF signal data are stored in the data hard disk connected to the receiver at an interval of two minutes for subsequent data processing. In addition, in the later data processing, in order to minimize the impact of the multipath, the data with satellite elevation above 30 degrees were selected  
Figure 5: The field of the zero-baseline experiment: **(a)** Horizontally placed geodetic GNSS antenna and its tripod; **(b)** Quad-power divider and raw IF signal recorder.  
for processing. Thanks to the open surrounding environment during the experiment, the data can be arbitrarily selected from the satellite azimuth of 0 to 360 degrees.