The major driver for the Polar Platform/Envisat satellite configuration has been the need to maximise the payload instrument mounting area and to meet the viewing requirements within the constraints of the Ariane 5 fairing and interfaces. Additionally, the configuration also has been driven by the reuse of the SPOT Mk II service module concept and the ERS payload accommodation experience (concept of instrument electronics in an internal enclosure with externally mounted antennas).
In flight, the major spacecraft longitudinal axis (the Xs-axis) is normal to the orbit plane, the Ys-axis is closely aligned to the velocity vector and the Zs-axis is Earth pointing. This configuration concept provides a large, modular, Earth-facing mounting surface for payload instruments and an anti-sun face for radiative coolers, free of occultation by satellite subsystem equipment.
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