PATHFINDER Density Model and Future Update

Abstract

Accurate orbit and realistic covariance propagations are required for reliable conjunction assessment. However, dynamics in low earth orbit is strongly affected by atmospheric drag. Therefore, it is essential to accurately model the drag and properly estimate its uncertainty. Japan Aerospace Exploration Agency has developed a prototype of the “PATHFINDER” density model. This paper introduces PATHFINDER and simulates conjunction events using the model to explain the importance of considering atmospheric drag uncertainty. Then, this paper proposes methodologies to enhance the density modeling capability. The proposed method applies spherical harmonics expansions not only to direct temperature corrections but also to temperature corrections due to geomagnetic activity and inconsistency of geodetic height in Roberts' temperature profile. This paper estimates the spherical harmonics coefficients by a batch least squares estimator using observed densities derived from accelerometers and/or precise orbit determinations. Numerical simulations discovered that the proposed method could significantly improve density modeling with globally observed densities. Furthermore, the proposed method could reduce an absolute mean relative density error against actual Swarm-A's accelerometer-derived density from 10.0 % to 0.1 % on a high solar activity day in 2014 compared with the Jacchia Roberts density model while a prior study could only reduce the error from 10.0 % to 5.3 %.

Key Words: Atmospheric Density Model, Dynamic Calibration of Atmosphere, Orbit Prediction, Space Situational Awareness