Precise Orbit Determination improvement evaluation using the Jason-2/T2L2 oscillator corrective model

In the context of the development of the next realisation of the International Terrestrial Reference Frame (ITRF2020), and in order to respond to the Global Geodetic Observing System (GGOS) requirement, to have a geodetic network accurate at 1 mm and stable at 1 mm/y, the four techniques of the space Geodesy (SLR, DORIS, VLBI and GNSS) need to provide stable and accurate measurements, by reducing systematic errors intrinsic to their technologies, and to the materials, and to the measurement strategies that they use. The improvements can take many forms, and need to be evaluated and validated before being integrated to the ITRF.

In the case of the DORIS (Doppler Orbitography Radiopositioning Integrated by Satellite) technique, radiation (especially protons trapped in the South Atlantic Anomaly (SAA) area) have shown to affect the residuals, the Precise Orbit Determination (POD), as well as the estimates of DORIS station coordinates and the Zenith Tropospheric Delay (ZTD), due to perturbations of the frequency delivered by the on-board Ultra Stable Oscillator (USO). This effect has been observed with varying magnitudes on different DORIS satellites: Jason-1, Jason-2, Jason-3, SPOT-5, Sentinal-3A, & Cryosat-2. The impacts of radiation can be mitigated by either down-weighting the data affected or by using a corrective model for the on-board oscillator. Models were developed to reduce the impact of radiation for Jason-1 and SPOT-5 satellites, allowing analysts to recover the affected data (JM Lemoine & Capdeville, 2006; Stepanek et al., 2013). More recently, we have developed a deterministic model of the behavior of the USO on Jason-2 using data obtained through the Time Transfer by Laser Link (T2L2 Experiment. The model is built on an analytical representation of physical and spatial effects that affect the USO (Belli & Exertier, 2018). This model, explains 99.99% of the frequency variation of the Jason-2 USO, and shows promising results to decrease the DORIS residuals, and to improve the Jason-2 POD, below the threshold of 0.3 mm/s.

In this paper, we investigate and evaluate the effects of this Jason-2 USO model, with respect to the Jason-2 DORIS residuals, the DORIS-derived ZTD, the DORIS station coordinates and the consequences on the Precise Orbit Determination of Jason-2, and other satellites of the DORIS constellation.