The noisefloor power can be expressed as = 10log[kTB] (units of dBW)
where
k = Boltzmann's constant (1.38x10^{-23})
T = operating temperature
B = signal bandwidth

It is common to quote thermal noise as power 'per Hz', i.e. in a bandwidth of 1Hz. This simplifies the above expression to = 10log[kT] (units of dBW/Hz)

All Spirent GNSS simulators have a noisefloor that is dominated by thermal noise and for that it is common to use a value of T = 290K.

Thus, Spirent simulators have a noisefloor level, dominated by thermal noise of:-
10log[290*(1.38x10^{-23})] = -174 dBm/Hz (-204 dBW/Hz)

By comparison a GPS antenna pointing at the sky at high elevation would have a noise temperature of T=50K. This will give you a thermal noise = -182 dBm/Hz (-212dBW/Hz). Therefore the simulator noise floor is approximately 8dB higher than the live sky.

It is not possible to change the noisefloor level via some command sent to the software or hardware.