globsky/SignalSim ? reverse-engineered prompt

Build me a GNSS signal and data simulator like the one described here, with a simple way to load a scenario from JSON and generate the different outputs the project supports.

I want it to be able to take receiver trajectory, navigation data, and timing settings, then produce observation data, baseband correlation output, and digital IF samples. It should support GPS, BeiDou, Galileo, and GLONASS, and handle common bands like L1, L2, L5, B1, B2, B3, E1, E5, E6, G1, and G2. Please also include the core pieces for ephemeris parsing, trajectory calculation, signal modeling, and RINEX navigation and observation file handling.

If there are parts that depend on exact GNSS message details or format rules, use the docs in the repo and look up current reference info online if needed. Make it runnable from the command line with clear input and output files, and keep the code organized so it can be extended later.