Operational Experience of Close Formation Flight at 500 km Altitude during Severe Geomagnetic Storms

Abstract

The TerraSAR-X (TSX) and TanDEM-X (TDX) satellites have operated as a bistatic SAR formation since 2010, providing more than 15 years of close formation-flying experience. High solar activity and strong to extreme geomagnetic storms between 2023 and 2025, especially the 10-11 May 2024 event, significantly challenged atmospheric drag modelling and orbit prediction in low Earth orbit. This paper summarizes the operational experience and lessons learned from these events, including a systematic assessment of space weather prediction data and atmospheric density model combinations for the TSX / TDX mission. The resulting upgrade of the operational orbit determination, prediction, monitoring and control chain in March 2025 reduced along-track prediction errors and restored robust compliance with the formation-flying requirements, as demonstrated during subsequent space weather events including the severe geomagnetic storm in November 2025.