Evaluating Thermospheric Densities Derived from Orbital Drag Data

Thousands of satellites and debris reside in the thermosphere. The U.S. Space Command archives orbital tracking data for most known Earth-orbiting artificial satellites in the form of Two-Line Element (TLE) entries, which provide an expansive record of historical orbital element information and can be used to derive thermospheric densities. TLEs have been used to assess variability and trends in thermospheric temperature and density. Density can be extracted from TLEs through processing methods which propagate the orbital trajectory and assess orbital mean motion changes with time for a satellite. Comparisons between TLE-derived densities and high-accuracy accelerometer-derived densities for 8+ years of CHAMP and GRACE satellite data indicate that TLE processing choices, particularly those involving the time window of integration, significantly impact the agreement between the different density datasets. Integration time windows of >6 days improve the relative accuracy of TLE-derived densities, though analysis reveals a persistent bias in low-drag regimes where TLE-derived densities are underestimated compared to accelerometer-derived densities by up to 25%. These inaccuracies may be attributed to solar radiation pressure effects. The persistence of these errors in TLE-derived densities at GRACE altitudes (~500 km) has implications for the long-term density datasets and trends inferred from TLEs used to construct empirical models.