Satellite-/block-/plane- and constellation-specific GNSS LOD biases

Length of Day (LOD) describes variations in the duration of a single Earth rotation relative to the standard 24 hours. It is an important Earth Orientation Parameter (EOP) linking the International Celestial Reference Frame (ICRF) and the International Terrestrial Reference Frame (ITRF). For space geodetic satellite techniques (GNSS, SLR, and DORIS), the estimation of LOD is highly correlated with the precession of the satellite orbital ascending node, which is largely driven by the even low-degree spherical harmonic coefficients of the Earth gravity field (i.e. Earth flattening) and is also sensitive to orbit modeling deficiencies, such as out-of-plane empirical accelerations or solar radiation pressure (SRP).

In the case of SLR and DORIS, LOD estimation benefits from combining observations from multiple satellites with clear different orbital inclinations. Due to the different inclinations of the various satellites, SLR- and DORIS-derived LOD estimates are less correlated with other parameters which results in less biased LOD values. For the GNSS technique, GPS, Galileo and BeiDou constellations share the same orbital inclination of about 55 degrees, while GLONASS and QZSS employs an orbital inclination of 65 and 43 degrees, respectively. Given this small varying range of orbital inclination, modelling deficiencies lead to biased GNSS-based LOD estimates. Up to now, this was not handled, or a long-term constant (constellation-independent) bias was determined and applied at NEQ level.

In this presentation, we evaluate various LOD solutions computed from different satellites (including different satellite blocks, orbital planes, and constellations) and different SRP models.