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Design Concept



Door Assemblies - Earth View Door


Each MODIS instrument has three “doors” that protect the internal components from contamination, damage, and in some cases help the instrument’s self-calibration processes.

The Earth-view door (Nadir Aperture Door, or NAD) covers the opening in MODIS that faces Earth’s surfaces. This door usually remains open, and in this position helps keep MODIS data free of contamination from optical scatter. When the door is closed (usually when the instrument or satellite is in safe-mode), the door prevents data or anything else from entering the instrument. The inside of the door is painted black to minimize optical scatter, while the outside is painted white to help control the instrument’s temperature. The door opens and closes via a motor/gear-head system, with a spring system as a backup.

Technical Description

Each of the door assemblies were latched closed for launch, then opened in orbit by energizing the High Output Paraffin (HOP) actuators. Energizing the HOP actuators caused the release of captured torsion springs, which in turn rotated the latch arms to the open position. Each of the latches were used only once while in orbit, but have a re-settable feature that allowed them to be tested before launch. Stepper motor/gearhead sets with redundant windings were used to drive each door assembly to the open position and can be used again to close the doors when MODIS is directed by the spacecraft to go into the “safe mode.” All doors have failsafe mechanisms to provide fault-tolerant door operation.

The NAD weighs 3kg. The door is constructed of Kevlar, the outer surface of which is coated with S-13GP/LO white thermal control paint, and the inner surface with z-306 black paint. The NAD has two latches to anchor the door closed, and in case of a problem with the primary motor/gearhead, a mid-span failsafe linkage with stored spring energy.

The SVD weighs 7kg, and is coated with the same paint as the NAD.

The SDD (including the panels, actuator/latch assemblies, and thin aluminum screen) weighs 1.9kg.




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Curator: Brandon Maccherone
NASA Official: Shannell Frazier

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