Sources of Uncertainty in Atmospheric Drag: The Drag Coefficient

Atmospheric drag describes the main perturbing force of the atmosphere on the orbital trajectories of near-Earth orbiting satellites. The ability to accurately model atmospheric drag is critical for precise satellite orbit determination and collision avoidance. Assuming we know atmospheric winds and satellite velocity, area and mass, the primary sources of uncertainty in atmospheric drag include mass density of the space environment and the spacecraft drag coefficient, C_D . Historically, much of the focus has been on physically or empirically estimating mass density, while C_D is treated as a fitting parameter or fixed value. Presently, C_D can be physically modeled through energy and momentum exchange processes between the atmospheric gas particles and the satellite surface. However, physical C_D models rely on assumptions regarding the scattering and adsorption of atmospheric particles, and these responses are driven by atmospheric composition and temperature. Modifications to these assumptions can cause C_D to change by up to ~40%. The nature and magnitude of these changes also depend on the shape of the spacecraft. We can check the consistency of the C_D model assumptions by comparing densities derived from satellite drag measurements and computed C_D values for satellites of different shapes orbiting in the same space environment . Since all of the satellites should see the same density, offsets in the derived densities should be attributable to inconsistencies in the C_D model. Adjusting the C_D model scattering assumptions can improve derived density consistency among the different satellites and inform the physics behind C_D modeling. In turn, these efforts will help to reduce uncertainty in C_D , leading to improved atmospheric drag estimates.