Multi-constellation GNSS signal simulator and its software implementation

A multi-constellation GNSS signal simulator has been developed, which is currently capable of simultaneously generating five carrier signals of three GNSS constellations, including GPS-L1, Galileo-E1. The architecture of this GNSS signal simulator consists of the following three main parts: simulator software running on PC server; IF GNSS signals generator based on the platform of DSP and FPGA; RF up-conversion and combining module. And the first part will be focused on in this paper. The simulator software consists of the following functional modules: simulator Graphical User Interface(GUI); dynamic receiver trajectories generation; GNSS satellite constellations simulation; signal propagation channels and error sources simulation; the navigation messages generation; GNSS signal parameters computation for IF/RF signal generation; data frame making and transfer to the signal generator hardware via PCI-E interface. The receiver trajectories generation allows the user to define a wide range of receiver motion scenarios. The corresponding software module in this signal simulator has been implemented by a parameterized trajectory representation, which supports real-time three-dimensional receiver trajectories generation. The long-term satellite constellation simulation and the corresponding navigation messages generation are critical to some user simulation missions. It has been realized in this signal simulator by the circulatory use of ephemeris data files and the ephemeris parameters extrapolation. Due to the uncertainties of the GNSS systems in the development or improvement phase, the software based solutions are proposed and realized in this signal simulator, for changes in future have minimal impact on the simulator architecture. Under the requirement of multi-frequency and multi-system, the multi-thread scheme and the communication interrupt scheme have been designed in the signal simulator for efficient and real-time processing purpose. The testing and validation of this GNSS signal simulator at the data level has been performed. First, the ground traces for the satellite constellations determined by the orbital parameters are analyzed. Second, the simulated error sources are configured, and the resulting pseudorange errors are evaluated. As for multi-path, by using a COTS (Commercial Off-The-Shelf) GNSS receiver the multi-path error envelopes are measured. Third, by connecting this simulator to the COTS GNSS receiver, a static scenario is specified, and the positioning performance is analyzed. And fourth, a set of motion scenarios are specified, and the receiver trajectories are measured and compared. The generated GNSS signals are fed into the corresponding COTS GNSS receiver where each composite IF signal is digitally sampled, split into 12 channels, and processed to extract the navigation messages and measurements. With no pseudo-range being considered and 8 satellites in view, the positioning performance for a single static receiver is shown in Fig.7. In the figure, E, N, U represents the east, north and up coordinate axis directions respectively. The conditions are the same as for Fig.7, but the user is dynamic, traveling in a line, the positioning result is shown in Fig.8. While Fig.9 and Fig. 10 shows the positioning results for dynamic users traveling in a circle, and in a combination of motions, respectively. (Figure Presented).