First Analysis of Densities Inferred from Accelerometer Data on ESA’s Venus Express

Abstract:

After completing its primary science mission, ESA decided to do a risky aerobraking campaign with Venus Express (VEx) in order to gain operational experience as well as to measure high-resolution density profiles at high northern latitude. VEx was in a polar and highly eccentric orbit (e=0.84). In the aerobraking period from 24 June to 11 July 2014, the pericenter was at 75°N at an altitude of 130-134 km, and the local solar time drifted from 6.2 to 4.5 hr.

One density profile extending about 3° in latitude on both sides of the pericenter has been obtained for each of the 18 consecutive days at 1 Hz sampling, after ingestion of the 8 Hz accelerometer data in the GINS software. The uncertainty in the derived density is the sum of a systematic part due to the uncertainty in Cd, estimated to be 10%, and a noise and bias part due to the accelerometers. Using the accelerometer errors according to specifications, a signal-to-noise ratio of one is reached on average at 139 km altitude.

The validity range of the VEx densities can also be evaluated by means of comparison with a model. The VEx-to-VTS3 density ratios were computed for each profile, and these results are consistent with the specified instrument resolution. VEx densities are on average nearly a factor of 2 smaller than VTS3, which is in agreement with Precise Orbit Determination results obtained for higher altitudes (160-170 km). However, variability of up to tens of percent is visible in the form of wavelike activity as well as an altitude-dependent variation that is revealed by ratios that become smaller towards the start and end of the profile. The latter model error hints at an inaccurate the temperature profile, errors in constituent concentrations, or both. The waves can be interpreted as a superposition of two wave trains with wavelengths of around 100 km and 250 km. Average scale heights over the range of observations (130-140 km) are found to be 2.9+/-0.6 km.