The Harmony mission is ESA’s 10th Earth Explorer, designed to advance our understanding of Earth system dynamics through innovative observations. It consists of two companion satellites flying in formation with each other and with Sentinel 1C/D. They are equipped with a passive Synthetic Aperture Radar (SAR), to receive Sentinel 1C/D echoes, as well as a Thermal Infrared (TIR) instrument, composed of 5 cameras. This configuration enables the detection of small scale motion and deformation in the Earth’s surface, ocean-atmosphere interactions, monitoring the dynamics of the cryosphere, and clouds. The mission is scheduled for launch by the end of 2029, on a VEGA-C rocket from Kourou.

The Harmony mission is organized into distinct operational configurations, each defined by specific formation flight characteristics with Sentinel-1C/D to maximise scientific return over its five-year lifetime. The Along-Track Interferometric phase, also known as the “Stereo” formation, places the two Harmony spacecraft keeping an along-track separation of 350 km with respect to Sentinel-1C/D, with one Harmony spacecraft leading and the other trailing the formation. In contrast, the Cross-Track Interferometric (XTI) phase, or “Helix” formation, positions both Harmony spacecraft at nearly the same orbital phase—either both trailing or both leading Sentinel-1C/D—while maintaining the same 350 km along-track separation from Sentinel-1C/D as in the Stereo phase. The Helix configuration is achieved by applying a separation in the relative eccentricity and inclination vectors, following D’Amico and Montenbruck (2006) [1], which ensures radial and cross-track separations of approximately 400 m and 1800 m, respectively (parameters a?Δe and a?ΔΩ). The mission profile foresees a minimum of three main phases: starting in the Helix formation for one year, transitioning to the Stereo, and concluding again the final fifth year in the Helix formation. Additionally, during the Stereo phase, occasional and opportunistic orbital transfers between Stereo and Helix are planned to complement the mission return.

This paper presents the results of a study on the Harmony and Sentinel-1C/D convoy safety. The analysis addresses the safety of formation flight in both, the Stereo and Helix configurations. The effect of differential drag is analysed under varying ballistic coefficients, including assumptions for nominal flight, safe-mode conditions, beginning and end-of-life scenarios, and including design margins, ensuring compliance with established safety thresholds. For the Helix configuration, the study evaluates the time window during which passive safety can be guaranteed without active control. Finally, the work investigates strategies for robust and safe transitions between the Stereo and Helix formations, aiming to achieve these transfers in a delta V efficient manner.

References:

[1] D’Amico S. and Montenbruck O.; Proximity Operations of Formation-Flying Spacecraft Using an Eccentricity/Inclination Vector Separation. Journal of Guidance, Control, and Dynamics. Vol. 29, No. 3, May-June 2006. https://arc.aiaa.org/doi/10.2514/1.15114