Day 4

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Paper title Improved radiation pressure modelling for the Swarm satellites
Authors
  1. Natalia Anna Hładczuk Faculty of Aerospace Engineering, TU Delft Speaker
  2. Christian Siemes TU Delft
  3. Jose van den IJssel Delft University of Technology
  4. Pieter Visser TU Delft/Faculty of Aerospace Engineering
Form of presentation Poster
Topics
  • B2. Earth Explorer missions
    • B2.05 Swarm - ESA's Extremely Versatile Magnetic Field and Geospace Explorer
Abstract text The Swarm mission provides thermosphere density observations derived from the GPS receiver data for all three satellites and, as a separate data product, from the accelerometer data for the Swarm A and C satellites. Deriving thermosphere density observations requires the isolation of the aerodynamic acceleration by reducing the radiation pressure acceleration from the non-gravitational acceleration. Uncertainties in the radiation pressure modelling represent a significant error source at altitudes above 450 km, in particular when solar activity is low. Since the Swarm satellites spent several years at such high altitudes during periods of very low solar activity, improvements in radiation pressure modelling are expected to yield a substantially higher accuracy of the thermosphere density observations, in particular for the higher-flying Swarm B satellite.

In order to improve the radiation pressure modelling, it is crucial to account for the detailed geometry and the thermal radiation of the satellites. The former is achieved by augmenting the high-fidelity geometry model of the Swarm satellites with the thermo-optical properties of the surface materials. The augmented geometry models are then analysed using ray-tracing techniques to account for shadowing and multiple reflections (diffuse and specular), which is not the case for commonly used methods based on panel models. Another important factor which we want to address in this study is the sensitivity of the thermosphere density observations to errors in thermo-optical surface properties, i.e. errors in the coefficients for specular and diffuse reflection, and absorption, which are not accurately known and might change over time due to aging effects of the surface materials.

The thermal radiation can be calculated directly using the in-situ measurements from thermistors that monitor the temperature in a number of locations on the outer surfaces of the satellites. Whilst this is expected to give the most accurate results, it also offers the opportunity to optimize a recently developed thermal model of the satellite. The model consists of a set of panels that heat up by absorbing incoming radiation and cool down by emitting radiation. It can be optimized by adjusting its control parameters, which are the heat capacitance of the panels, the thermal conductance towards the inner satellite, and the internal heat generation from the electronics, batteries, etc. Such an optimised thermal model is expected to provide valuable insights for other missions, such as the CHAMP, GRACE, and GRACE-FO missions, for which thermistor measurements are not publicly available. While the positive effect on density observations is most pronounced at higher altitudes, we anticipate that at lower altitudes crosswind observations will benefit.

In our presentation, we will show how to improve the radiation pressure modelling by (1) using the detailed geometry model of the Swarm satellites and (2) accounting for the thermal radiation. Further, we will determine the impact of radiation pressure mismodelling on the thermosphere density observations. This analysis could help resolve critical issues such as errors in Swarm B data (manifested by negative density observations), which are currently addressed by providing extra information about the orbit-mean density. Additionally, other missions such as CHAMP, GRACE, and GRACE-FO could benefit from a knowledge transfer, which will make a significant portion of the thermosphere observations more reliable.