Orbit Determination of the EnVision D-InSAR mission to Venus

EnVision is proposed to the European Space Agency for launch in 2020-22, to detect geological change on Venus and to address key questions from Magellan and Venus Express, particularly: how active is the surface of Venus now? The global distribution of impact craters has been shown (Romeo and Turcotte, 2010) as consistent with a range of resurfacing models, with rates of volcanic and tectonic activity comparable to that of plate interiors on Earth, i.e., perhaps up to 10 mm a-1 in localised tectonic movement and less than 1km3 a-1 (Stofan et al., 2005) in global extrusive activity. Capturing, during the lifetime of a single spacecraft, evidence of an individual volcanic eruption or tectonic movement under the clouds of Venus is only feasible using Differential Interferometric SAR (D-InSAR). D-InSAR is a radar interferometry technique using repeat-pass images of a surface area to measure terrain deformation using radar phase differences at the beginning and end of the observation period. To separate the effects of topography, at least three images are required: a pair closely spaced in time, in which any phase differences are solely attributable to topography, and a third acquired later but from one of the orbital positions used previously. Such technology is now routinely used in earth orbit for terrestrial applications. EnVision is a mission that will use these techniques at Venus to determine the rate and location of geological activity (e.g., tectonics, earthquakes, or volcanic eruptions) on Venus. The EnVision radar will be designed to detect rates of movement as close to the theoretical limit of 1 mm a-1as possible, from a quasi-circular, quasi-polar orbit of 300 km altitude, over a period of up to 10 years. Hence, EnVision must repeat its initial orbit throughout its 10 year mission and in synchronism with surface rotation to within 100 m in all three axes. This paper describes how we propose to determine EnVision's orbit to the necessary degree of accuracy by cost-effective use of the techniques listed in the table.