Each MODIS instrument has three “doors” that
protect the internal components from contamination, damage,
and in some cases help the instrument’s self-calibration
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.
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 SDD (including the panels, actuator/latch assemblies,
and thin aluminum screen) weighs 1.9kg.