EnVision gravity experiment: Joint inversion of Doppler tracking data and tie-points monitoring from SAR images.

Abstract

Abstract:The gravity field of planets is determined using the Doppler radio-tracking data of orbiting spacecraft. However, this method is intrinsically limited by the orbit of the spacecraft, the noise on the Doppler data, the temporal and spatial coverage of the tracking dataset, as well as the inaccuracies of the non-gravitational perturbations of the spacecraft motion and of the rotation and orientation of the planet.The monitoring of tie-points identified on planetary surface images is helpful to improve the determination of the pole precession and rotation rate of the planet performed with Doppler data alone, and in turn the determination its gravity field.This study investigates the joint inversion of tie-points identified on SAR images data and Doppler tracking data scheduled for the science phase of the EnVision mission to Venus due to be launched in 2031.1- EnVision gravity experimentThe gravity experiment onboard Envision will use the telecommunication system onboard the spacecraft to establish a coherent Doppler link (X-band uplink and dual X/Ka band downlink) between this spacecraft and on ground deep space tracking stations. This link will be established at each telemetry slot providing at least 3,5 hours of effective tracking per day throughout the science phase of the mission, i.e. 6 Venusian days (4 Earth-years). The orbit of the spacecraft is foreseen to be near-polar (inclination of 88°) and slightly elliptical (altitude between 220 km to 525 km).Numerical simulations of this Doppler tracking dataset have been performed to assess the expected precision of the determination of the gravity field (accuracy and spatial resolution) as well as of some geophysical parameters related to the internal structure of the planet, i.e. k2 tidal potential Love number, tidal phase lag and moment of inertia (MoI) of the planet (Rosenblatt et al., 2021).These simulations show that the spatial resolution is better than 200 km over almost all the surface of the planet (Rosenblatt et al., 2024). The k2 tidal Love number is determined with a precision of about 1%, the pole precession with a precision yielding to an uncertainty of 2% on the MoI value, and the tidal phase lag with a precision of about 60% (see Table 1).The uncertainty on k2, tidal phase lag, and MoI values will allow to further constrain models of the internal structure of the planet, in particular the viscosity profile of the mantle (single or multi layers models) (Dumoulin et al., 2017; Musseau et al., 2024).The uncertainty on the rotation rate or length-of-day (LOD) of the planet is 0.03 minutes which is 0.4% of the 7 minutes range of values obtained at different periods and using different methods (see Lévesque et al., 2024).Table 1: Expected uncertainty (3 times the formal error) on geophysical parameters determined with the Doppler tracking data alone.Parameter 3-sigma uncertainty Tidal Love number k2 (real part) 3.6x10-3 (1.2%) Moment of inertia (MoI) 6.4x10-3 (1.9%) Tidal phase lag (in degree) 0.3° (62.2%) Load Love number k’2 0.18 (54%) Length Of Day (LOD) 2x10-5 days (0.03 minute)  2- Simulations of tie-points In order to improve the error on the planet orientation parameters related to the interior structure (i.e. pole precession), we have simulated the monitoring of tie-points that could be built with the radar images that will be taken by the VenSAR instrument throughout the mission science phase.The radar images will be collected over Regions of Interest (RoI) corresponding to about 20% of the planetary surface, offering the opportunity to monitor, over the 6 Venusian days duration of the mission, the position variations, in a celestial reference frame, of several tie-points associated with surface craters, coronae and so on. The spatial resolution of the radar images is foreseen to be 10 meters and 3 meters for some high-resolution images at dedicated local areas. As the geographical coordinates of these tie-points are known in a frame tied to the planet, it is possible to monitor the orientation parameters as well as to determine its rotation rate.We have performed numerical simulations of the retrieval of the orientation parameters using the spatial and temporal coverage of the future VenSAR images. We have identified 104 tie-points and estimated the orientation parameters.Then, we have merged both Doppler and tie-points simulated data to improve the resolution of these parameters and in turn of the gravity field and k2 Love numbers in order to further constrain models of interior. The uncertainty on the geophysical parameters (table 1) is expected to be improved since the tie-points provide additional information on the relative position between the spacecraft and the surface of the planet (Cascioli et al., 2023).