On frequency plans for TianQin

Abstract In the space-based gravitational wave detection of the TianQin mission, gravitational perturbations from the Earth–Moon system induce relative motion between the satellites ( ± 4 m s −1 ), leading to a Doppler effect in the inter-satellite laser links. In heterodyne laser interferometry, the Doppler shift causes the beatnote frequencies to drift slowly over time, potentially exceeding the phase read-out bandwidth and resulting in loss of phase locking. Therefore, various frequency plans have been proposed to lock all lasers to a primary laser, which ensures that the beatnote frequencies remain within the phase read-out bandwidth throughout missions. Frequency plans for TianQin must take into account multiple clock sidebands, prevent beatnote signal crosstalk, and maintain frequency separations of 0.5 MHz. On top of the straightforward fixed-offset scheme, this paper proposes a Doppler-compensated scheme, where the frequency offsets are dynamically adjusted according to the Doppler shift, utilizing the precision orbit determination and prediction capability of TianQin. In contrast to the fixed-offset scheme, the Doppler-compensated scheme can halve the variation range of the beatnote frequencies and provides sufficient margin for the phase read-out bandwidth for all 36 laser locking configurations considered in the paper.