Upgraded modelling for the determination of centre of mass corrections of geodetic SLR satellites: impact on key parameters of the terrestrial reference frame

Abstract

We present improved modelling to compute centre of mass (CoM) offsets for spherical geodetic satellites and derive new values for the spacecraft LAGEOS, LAGEOS-2, Etalon-1/2, Starlette, Stella, LARES, and Ajisai for all system configurations of the laser ranging network, past and present. The updated CoM values, i.e. the average distances between the centres of mass and the optical reflection points, reveal that a major contributor to the error budget in satellite laser ranging (SLR) for reference frame realisation has been a limited accuracy of the CoM corrections hitherto employed to model some of the systems of the SLR network. Our modelling strategy takes into account target characteristics, detection hardware, and mode of operation. For the optical modelling, we have employed high precision data from the single-photon, high-repetition rate SLR system at Herstmonceux (UK), performing the relevant computations for all laser frequencies ever used for tracking operations in the SLR network. For stations that do not operate in single-photon mode, we performed Monte Carlo simulations to estimate the CoM offsets, in this way taking into consideration previously neglected aspects of the detection process. The newly derived values reduce by up to 50% the biases estimated for the two biggest targets, Etalon and Ajisai, and eliminate the generally positive trend in range biases previously observed across the network. These CoM values, mostly lower than the previous ones, imply higher estimated station heights, with a consequent increase in the scale of the terrestrial reference frame that is realised by SLR on the basis of LAGEOS and Etalon observations. This increase in scale, of about 0.65 ppb, is confirmed by geodetic solutions reported here using nineteen years of observations from the ILRS network of tracking stations. We also examine the consequences for the determination of the geocentric gravitational constant, GM, that result from using the new CoM corrections.