SOLAR, 9 years of operations as external payload on the ISS: The technical challenges overcome

Abstract

After exactly 9 years of operations, the SOLAR mission was ended on 15 February 2017. This was an extraordinary achievement knowing the mission was originally set to only last for 1.5 years! SOLAR is a payload of the European Space Agency (ESA), mounted on one of the external platforms of the Columbus module of the International Space Station (ISS). The SOLAR platform hosts three instruments built to observe the spectral solar irradiance in the wavelength range 17 to 3080nm. The Belgian User Support Operations Centre (B.USOC) is the ESA Facility Responsible Centre that supported the operations for SOLAR throughout the whole mission, from the operations preparation to the first few commands sent, handling unexpected problems and operating the instrument with confidence to finally permanently switching it off after 9 years of successful operations. Nevertheless, the mission was of course not free of technical challenges, but these were always handled with the greatest attention to find a suitable solution that would have the least impact on the smooth continuation of the operations. This paper will present the operations constraints experienced by the payload operations team while operating SOLAR and how these challenges were overcome. Constraints covered by the paper include problems related to the ISS attitude and orbit, the mechanical limitations of the instrument, the availability of the various ISS resources, the external influences to the payload operations coming from the ISS itself and from other payloads, the instrument’s operational modes and the instrument’s degradation with time. This list is furthermore complemented by an overview of the operations products needed for dealing with these constraints during operations. Additionally, the so-called Sun Visibility Window bridging will also be mentioned, describing how a scientific requirement that could initially not be fulfilled by the platform as-built could still be occasionally reached by requesting attitude changes of the ISS at well-defined seasonal periods of the year. The paper will be concluded with some valuable lessons learned drawn from the solid experience gained after a 9-year long mission, which can bring useful advise to other teams planning operations of external payloads on the ISS or even to internal payloads, as some encountered constraints can apply to them too. This paper complements the abstract submitted by A. Michel et al. covering the evolution of the operational concept of the SOLAR mission.