Localized In-Situ Density Measurement in Low Earth Orbit via Drag Torque Estimation

Abstract

Orbital forecasting is an essential part of precision satellite operations. The largest contributor to orbital propagation error in low orbits is atmospheric drag, which varies widely due to altitude, latitude, and solar activity. Accurate in-situ measurements would enable improved orbital forecasting, but conventional methods for density measurement require precision accelerometers, tracking systems, or processing on the ground. This work introduces the novel Satellite Producing Aerodynamic Torque to Understand LEO Atmosphere (SPATULA) concept, and provides supporting preliminary simulations of 1) the density recovery capability of a SPATULA satellite, and 2) the efficacy of estimating a global density map via a SPATULA constellation. Results suggest that a SPATULA CubeSat could provide measurement capability on par with current methods in both error and bandwidth using commercially available sensors. This measurement is enabled by considering drag torque instead of drag force; measuring in this domain eliminates many sources of perturbation, and leverages the large body of preexisting attitude sensors for small satellites to achieve a density measurement with root-mean-square error of 1 × 10⁻¹³ kg/m³ and bandwidth of 1 min⁻¹. The high accuracy and low expected cost of this method would enable a constellation to estimate a high-order spherical harmonic global density map in real time.