Calibration Techniques for Studying Venus and Mars Atmospheres

The European Space Agency Venus Express mission (VEX) was sent to Venus in 2005 to unveil the unsolved mysteries regarding its atmosphere, the plasma environment and its temperatures. Radio occultation experiments performed by VeRa radio science instrument probed the planet’s atmosphere by studying the frequency shift on the radio signal sent by the spacecraft to Earth-based ground stations. This study carries out the calibration of the radio frequencies within a radio occultation experiment in order to correct the main sources of error as: thermal noise, spacecraft clock, spacecraft trajectory, and plasma noise. Any uncalibrated effects will bias the retrieval of atmospheric properties. A comparison of the occultation experiments between Venus and Mars is presented, both from the engineering and scientific point of view, through the analysis of Venus Express and Mars Global Surveyor (MGS) occultations data, highlighting stronger calibrations required for VEX, the extreme, hostile, thick Venus’ atmosphere, and a friendly, thin Mars’ atmosphere. This investigation analyzes Venus Express data recorded by the NASA Deep Space Network in 2014, and the results are compatible to previous studies of Venus atmosphere with VEX between 2006 and 2009.