CW hardware injections have been started back up after a long hiatus and their amplitudes have been adjusted for O4 running. All other parameters of the 18 signals (ranging from 12 Hz to 2991 Hz) remain as they were in O3. As described in LIGO-T2100373, six of the amplitudes are meant to allow daily monitoring, with the other 12 allowing long-term monitoring over the full band covered.

The same amplitudes are used for corresponding H1 and L1 signals, Because of the dramatic improvement in the sensitivities of both detectors since O3 at high frequencies, the amplitude reductions for signals above 140 Hz have been reduced by factors close to two, with some variations. For the time being, signal amplitudes at lower frequencies remain the same as in O3. A table of the injection parameters can be found here.

Attached are ldvw spectra of H1:CAL-INJ_X_OUT_DQ for May 16, 2019 (O3 amplitudes) and May 16, 2023 (nominal O4 amplitudes) at the same time of day (to give approximately equal antenna pattern factors).

Although the non-amplitude intrinsic signal parameters remain the same as in O3, the actuation function used has been updated with what Evan G posted here  last week, for which there is an overall 2% change in magnitude and a negligible change in phase.