VNIR and SWIR FPA designs for 3MI instrument: thermal, mechanical and dimensional stability challenges

Within the framework of the 3MI instrument for METOP-SG satellite, Sodern develops the Focal Plane Assemblies (FPAs) for the SWIR and VNIR detectors. The main functions of these FPAs are the thermal cooling, the dimensional stability of the detectors and the mechanical withstanding with respects to the thermal and mechanical environments. From a mechanical point of view, the mechanical requirements are identical for the two FPAs with notably stiffness above 500Hz. On the other hand, from a thermal point of view, the temperature environment and thermal control principle between SWIR and VNIR FPAs are different by the use of a heater for SWIR and a TEC for VNIR. At last, the SWIR FPA turns out to be the more challenging case with a high insulation, up to 200K/W, between cold parts (-90°C) and hot interface (25°C) and also a high coupling of 2.7K/W with the cold sink. This paper shows how a single architecture composed of a hexapod structure made of 6 titanium shafts and a thermal strap made of low diameter copper strands permits to achieve the best compromise between thermal and mechanical requirements for both FPAs. In addition to technical aspects, this common architecture also answers to the aim of cost cutting for the different phases of the project: design sharing, grouping of supplies, mutual qualification tests, shared integration tools and processes. Several models have been integrated and tested to verify the performances and the withstanding to thermal and mechanical environments.