Improved Orbit Propagation Using Drag Terms Updated by the Analytical Gas-Surface Interaction Model and Neutral Atmosphere Models

Regular orbit propagation methods are commonly used to determine predictive satellite ephemerides. In particular, the Simplified General Perturbation (SGP4) propagator is routinely used with the NORAD Two Line Elements (TLE) to calculate orbital state vectors of satellites and space debris. However, SGP4 uses a constant drag-term (B*) obtained from the TLEs, which causes significant error for longer propagations. During geomagnetic storms, increases in atmospheric drag are observed due to increased neutral densities and wind velocities, which are not modeled in the current SGP4 routines. Our work aims to provide an accurate estimation of drag by modelling the B* term in the TLEs. We use the analytical Gas-Surface Interaction model to estimate the drag co-efficient (c_D) and the effective cross-section area (A_ref) of a satellite to calculate the drag term as a function of time. An updated drag-term for every orbit is used in the SGP4 propagation, which shows improved orbital decay estimation compared to regular SGP4. Present work focuses on implementing this method to estimate CNOFS and GRACE orbital decay during selected storm periods. The neutral densities and wind velocities at satellite co-ordinates are obtained from Global Ionosphere Thermosphere Model (GITM). The results are compared with propagation obtained from the normal propagation using the B* term from the TLEs and quiet-time neutral densities obtained from the JB 2008 atmospheric model using quiet-time solar indices.