The Importance of Atmospheric Modeling for Next-Generation Venus Mapping Missions

Upcoming Venus missions will feature a range of radar and multispectral instruments to image and map the surface, which is inaccessible to optical sensors due to the thick Venus atmosphere. Of these missions, the X-band VISAR synthetic aperture radar (SAR) instrument on the VERITAS spacecraft will be used to create a global, high-resolution Digital Elevation Map (DEM) with a posting of less than 250 m and a vertical accuracy of better than 10 m. How can the VERITAS mission achieve such performance if the Venus atmosphere, with its high pressure and temperature, introduces consequent apparent range delays to the radar measurements of several hundreds of meters, and only limited in-situ measurements of the deep atmosphere from earlier missions are available?
Our answer to this problem is a Venus atmospheric model that combines available data on the atmospheric constituents with wave propagation physics of X-band signals under such conditions. Based on previous work by Duan et al. (2010), we have built a Python-based toolkit to compute the signal delay and attenuation for given atmospheric composition, refraction and absorption models, and instrument viewing geometries. We investigate using simulations how the mismodeling of the atmospheric parameters can lead to an inaccurate georegistration of the SAR imagery (based on work by Gisinger et al., 2015, 2017), which in turn would significantly degrade the quality of the derived DEMs.
We perform our investigations on a global scale and over the entire VERITAS science phase of approx. 3 years, and compare our results with analytical expressions. We also show that the expected variability of temperature, pressure, and ionospheric density with latitude and solar time only plays a negligible role in the performance degradation. Finally, we aim to share our atmospheric modeling code with the community, such that we may incorporate improvements to the model, and to help sensitize future users of radar (or multispectral) data from the VERITAS, EnVision, or other missions, to the importance of atmospheric corrections.