# An Indoor DFEC Ranging Method for Homologous Base Station Based on GPS L1 and BeiDou B1 Signals
## 1 Introduction
The principle of pseudolite positioning is the same as that of GNSS, but due to the complexity of the indoor environment, there is also a problem that the range value jumps and the range measurement is not accurate. In the research process of Wuhan University, by constructing an indoor RTK method, using the method of UKF and known initial points to estimate the ambiguity of whole cycles and improve the ranging accuracy to achieve positioning, but in actual tests, this method is only suitable for simple indoors Environment, the difference correction value in complex environment is generally difficult to assist the ambiguity analysis [16; 17; 18; 19], the other is our unit, the pseudolite system designed by our unit is currently the leading domestic. Our unit is the first country to introduce BeiDou pseudo-satellite technology indoors, and has achieved good results. By constructing an array-type pseudo-satellite network indoors, utilizing the characteristics of the pseudo-satellites and the network-derived network, and controlling the consistency of multiple signals through PLL control. However, the system still has the problem of inaccurate ranging indoors. For this reason, we have designed two positioning algorithms, an innovative fingerprint location algorithm [20] and the doppler positioning algorithms with known initial points [21]. These methods can currently achieve indoor positioning services at the centimeter level by avoiding real-time raw ranging value. However, from an application-orientedperspective, we do not expect the existence of initial values. When we enter a place to navigate, letting customers find the initial point before providing services is not conducive to technology promotion to a certain extent. The most desired state should be the state of entering or navigating. Therefore, the accuracy of ranging is again concerned. Here, we introduce the principle of ranging. We can understand it as a problem similar to the pursuit of multiple wavelengths. When the first frequency signal propagates a wavelength length, the second frequency signal propagates the sum of the wavelength length plus the length of the wavelength difference between two signals. When the first frequency signal propagates N wavelength lengths, the second frequency signal propagates N wavelength lengths plus N wavelength difference between the two signals, so we can infer how many full wavelength lengths the two signals currently propagate based on the phase distance difference within a cycle between the two signals. The sum is the propagation distance of the current position signal at the current moment. The ranging diagram is shown in Figure 1.  
The remainder of this paper is organized as follows: Section 2 introduces the working principle of the dual-frequency multi-channel signal base station and describes the ranging process in detail. At the same time, the phase accumulation cross-cycle problem and the frequency selection principle used in the ranging process are analyzed. Section 3 carried out a series of tests on the feasibility of the DFEC ranging method from the transmitting base station side, receiver side, and influence of noise and multipath in the wireless environment, and obtained the relevant test results. Section 4 Combined with the existing test results in Section 3 above, we carried out related positioning experiments at the pseudo-satellite artificial intelligence test site, and gave the positioning error analysis under this ranging method. Finally, on the basis of the existing results, we summarize the technical advantages of the current method and give the follow-up research directions.