Couldn't this project from the link below be used for testing?
GPS-SDR-SIM generates GPS baseband signal data streams, which can be converted to RF using software-defined radio (SDR) platforms, such as ADALM-Pluto, bladeRF, HackRF, and USRP.
https://github.com/osqzss/gps-sdr-sim
Perhaps the GPS signal needs some variation to pass through the system stages, a fixed carrier signal could be like a muted radio signal:

https://electronics.stackexchange.com/questions/353664/gps-noise-figure-analysis

How to Simulate a GPS Signal
A GPS signal simulator mimics the radio frequency (RF) operation of a GPS satellite. Digital modulation techniques are used to multiply, mix, and combine the output of a central 10.23 MHz master clock in stages to simulate a space vehicle (SV) broadcast.
A GPS satellite constellation simulator configures key variables into pre-programmed or customized test scenarios without ever leaving the test bench. This includes the quantity and position of satellites, power levels, transmission band(s), constellation health, and almanac data.
A GPS walking simulator is established by adapting signal transit times along with refection and interference settings to simulate a receiver in motion. The trajectory of incoming signals can also be adjusted to approximate SV orbits.
GPS record and replay capabilities capture output from live satellite antennas at a high rate of speed. These functions combine the authenticity of actual satellite signals with the flexibility of on-demand testing.
https://www.viavisolutions.com/en-uk/products/gps-signal-simulators