Before reading this article the user may wish to refer to article FAQ11365 which discusses the 1PPS-to-RF alignment that is completed on all simulators at the time of manufacture.
Acknowledging the 1PPS-to-RF alignment limits discussed in FAQ11365, this article will simply assume for the benefit of simplicity that 1PPS-to-RF is perfectly 0ns.
All Spirent GNSS simulators base the 1PPS output on GPS, even if GPS is not being simulated or even if the simulator does not have GPS capability! Of course in a 'perfect' simulation, where there are no system time offsets between the different constellations, then this reference to GPS actually does not matter, since all constellations will 'agree' on the alignment of 1PPS with their respective RF signals, i.e. 1PPS-to-RF is 0ns in all circumstances. However, the issue becomes important when you wish to simulate non-zero system time offsets between for the different constellations.
Yet the interface control documentation (ICD) for all constellations (at the time of writing, and expected to continue to be the approach used for any future system) have been developed with the intention of using GPS as the time reference from which any system time offset is based. For this reason Spirent choose to maintain GPS as the 'master' 1PPS time source, even if it is not being simulated. When a user chooses to apply a non-zero system time offset in a constellation file (this is available to SimGEN mode users of the PosApp software only) then PosApp will apply an appropriate offset of that constellation's RF signal to the 1PPS. For example, you add a 10ns GLONASS-to-GPS system time offset in the GLONASS constellation file; regardless of whether GPS is being simulated or not, the GLONASS RF signal will be offset from the 1PPS by 10ns (on the basis that the GPS RF signal is perfectly aligned with the 1PPS and hence has a 0ns 1PPS-to-RF offset).