## 2 Materials and Methods
### Principles in Zero-Baseline Experiments  
Zero-baseline means that the antenna phase centers corresponding to multiple signal channels are consistent, i.e., the same antenna is used to receive signals. In the study, the GNSS signal recorder used in the zero-baseline experiment has four signal input channels corresponding to different frequencies and signal branches. As shown in Figure 2, after the signal is led out from the antenna, the power distributor is used to connect the signal to the four signal ports through the same transmission conditions, and the zero-baseline condition is formed between the four signal receiving terminals. For the actual recorder, the signal channel 1 and signal channel 2 will operate on the same frequency point, and signal channel 3 and signal channel 4 will operate on another frequency point. Since the four signal channels were connected with the same antenna, the error influence is consistent among the four signal channels in the process of the GNSS signal transmission through satellite, ionosphere, and atmosphere propagation and the signal line before obtaining access to the recorder. Therefore, theoretically, the data collected by signals of the same signal frequency between different signal channels should be consistent. However, in practice, owing to the influence of the structure of the ranging code itself, the performance of the ranging code cannot be fully idealized. It leads to certain differences in the ranging code data of different signal channels at the same epoch and frequency, which can be used to reflect the ranging performance of the signal frequency ranging code directly. The code signal path differences can quantitatively reflect the difference in the ranging code data. In order to obtain the values of code signal path differences, the code-level waveform diagram of the two signal channels were obtained by phase correlation between local codes and two-channel signals. Figure 3 shows the code-level waveform diagram obtained after two signal channels pass through the same epoch. Generally, the waveform figure at a single epoch has a power maximum near the central position, where the path delay is 0 and the power on both sides decreases with the increase in the absolute value of the path. Delay difference between the extreme positions of the waveform between two channels code signal path differences of two signal channels. In the observation period, the code signal path differences of each epoch can be obtained through the same data process under different epochs that meet the observation requirements, and the changing image of code signal path differences with time can be obtained. Figure 2: Diagram of the zero-baseline experiment in this study. Ideally, the code signal path difference of the waveform signals in the same epoch should be strictly 0 due to the zero-baseline environment. Therefore, the code signal path differences can be used to evaluate the performance and ranging accuracy of the ranging code directly. The smaller the code signal path differences, the better the performance and ranging accuracy of its corresponding ranging code. In order to obtain the code-level signal path differences for each signal channel, we use a 4-channel IF signal recorder corresponding to Figure 2 that can receive B2 and B1 band signals simultaneously to collect the raw IF signal. There is two signal receiving channels under each signal frequency point. The collected raw IF signal is processed by a self-developed SDR based on MATLAB to obtain the code signal path differences of two signal input channels at the same frequency with a zero-baseline. The raw IF data processing of GNSS signal recorder under a single channel can be shown in Figure 4.  
After the signal is transmitted to an input port through a four-power divider, it is first divided into two sub-channels inside the receiver channel for further processing. SDR will process the data epoch by epoch.