The German Synthetic Aperture Radar (SAR) satellites TerraSAR-X and TanDEM-X have been in orbit since 2007 and 2010, respectively. Since 2010, both satellites have been operated in close formation with a typical separation of 250 m in a 505 km mean-altitude sun-synchronous orbit, forming a bistatic SAR mission. Although more than 15 years of formation-flying experience are now available, important operational lessons are still being learned.

The space weather conditions during the period from 2023 to 2025, characterised by high solar activity combined with moderate to severe geomagnetic storms, posed significant challenges for satellite operations in pretty low Earth orbit and proved extremely demanding for precise formation flying at these altitudes. Atmospheric drag modelling, and consequently the formation’s orbit and relative motion prediction, were often jeopardized by space weather prediction data of insufficient accuracy. The most severe impact on formation flying occurred during the strongest geomagnetic storm of the last 20 years, in the night from 10 to 11 May 2024.

This paper presents the flight experience gathered during such extreme events and summarises the associated lessons learned. A survey of space weather prediction data in combination with atmospheric density models is presented, together with a systematic investigation and numerical simulation of various data-model combinations applied to the TerraSAR-X / TanDEM-X mission. As a result, the complete operational processing chain - comprising of orbit determination and prediction as well as formation monitoring and control - was upgraded in March 2025 to a different atmospheric model and space weather prediction data. Finally, space weather events experienced thereafter, including the severe geomagnetic storm in November 2025, are presented, demonstrating the improved accuracy and robustness of close formation flying at low Earth orbit altitudes.