The Building of an On-Orbit Sensor
by Carol Bruegge, May 1996
A space satellite is useful only because of the various sensors, or
instruments, that it carries. This brief article tells what happens during the
building of a typical sensor instrument. It is based on the experience we have
had with the Multi-angle Imaging SpectroRadiometer (MISR), and mentions specific
aspects of the MISR activities. The overall story is similar to that of building
any on-orbit sensor, -- some are much more complex and more expensive than MISR,
while others are much simpler and correspondingly less expensive, depending on
the requirements.
Building the instrument
Select the image for a full-sized picture of the main MISR mechanisms
after assembly, -- 587 x 458 pixels, 98 kilobytes.
Each of the EOS spacecraft sensors, or instruments, has been designed and
built to meet unique specifications. In this sense they are each a custom
design, tailored to a specific scientific application. For MISR, over 100
individuals were assigned to the task. At the lead is a chief scientist, known
as the "Principal Investigator." He works with team of scientists, the "Science
Team", to define the specific data products desired from the instrument's
on-orbit data. The desired scientific applications, and the way that the data
received from the insturment will be structured and processed, become the basis
for an overall design concept, and later are used to develop performance
requirements that guide the details of the instrument design.
Design specialists using computer-aided drafting software then draw the
instrument concept and the parts required, defining the parts to less than a
thousandth of an inch accuracy. Engineers guide the formation of the drawings
and specify materials and thicknesses to achieve the balance between stiffness
and weight. Machinists use the drawings to cut metal and plastic parts to highly
precise tolerances, and technicians assemble these parts to make component
sections that form the instrument. These sections are called subsystems of the
instrument, and consist of assemblies such as lens barrels, detector packages,
electronics, and computers.
Another technician team assembles the various subsystems into a working
instrument. The instrument is then tested thoroughly to ensure that it will
function in space as required. Scientists and data analysts develop computer
tools that will be used to view the images made during the testing of the
instrument. These computer tools will then be used to interpret the images
received from the instrument when it is orbiting around Earth.
Where the instrument is built
The main center of operations where MISR is being built is a facility called
the High Bay. This is a huge room dedicated to the assembly and test of the
instrument. The MISR High Bay is 50 x 100 feet, and 30 feet in height. The
biggest challenge to this facility is that of maintaining a clean environment.
An air circulating system moves filtered air from the ceiling to the floor.
Small particles are removed, so that less than 10,000 particles per cubic foot
of air remain.
The biggest source of contamination to the room occurs when humans enter. The
human body sheds up to one million hair and skin cells per minute!! Because of
this the engineers and technicians who work in this room wear special garments.
Booties cover the shoes, a jump suit covers the clothes, arms, and legs. A face
mask and rubber gloves are worn, as well as a hat to cover the hair. These
garments are specially designed and laundered. Carbon fibers in the garments
prevent the humans from damaging the instrument with static electricity. The
material is lint-free. Access to this room is limited. The people who work in
this area not only hove to dress in the garments described here, but as they
enter, they also must take an "air shower" in a small chamber which sends jets
of air streaming out onto them at many heights.
Why great care is required
Select the image for a full-sized picture of the MISR outside body
structure, -- 651 x 482 pixels. 85 kilobytes.
Why is all of this care taken? It must be remembered that the space
environment is very different from that here on Earth. The biggest concern is
that the stronger UV light in space will harm the instrument. You might have
learned that the ozone layer of Earth absorbs UV light that would otherwise be
damaging to the eyes and skin of humans. Well, in space this UV absorption has
not occurred. The energy within this light can be damaging to space instruments
as well. A mere fingerprint on a camera lens turns black when exposed to the UV
light in space, thus preventing the camera from taking the images they were
built to take. Another concern is that the MISR instrument is made of many
microcircuits. A small particle of dust could make the electronics malfunction.
The individual detectors are less than 1/1000th of an inch in size!!! This can
be compared to the size of a human hair. A small dust particle lying on a
detector could make the camera inoperative.
Testing the sensor
Instrument testing must verify that the cameras will work even after
undergoing the stresses of the launch, the vacuum conditions of space and
extreme temperature changes that occur when the instrument orbits in and out of
the sun (when it goes behind Earth.) Thus, the test facilities include test
chambers where air can be removed to form a vacuum and the temperature can be
lowered or raised. The instrument is also fastened to shake tables that can
simulate the shocks and vibrations that are encountered during launch.
The extent of the testing is driven by the knowledge that there is no way to
bring an instrument such as MISR back to Earth for repair. The Hubble Space
Telescope repair was made possible because the spacecraft was in a relatively
low orbit, accessible the shuttle astronauts. However, as well as flying higher,
the MISR instrument will be in a polar orbit, i.e., passing over the North and
South Poles rather than around the Equator after the fashion of the Space
Shuttle. It will thus never be possible to retrieve or repair MISR after it is
launched.
Launching the sensor, the start of the mission
The launch of the instrument like MISR thus follows five years of design, two
years of building and testing, and millions of dollars funding. When the first
images from these on-orbit sensors are received at stations on earth, it is a
day to celebrate. The collected images represent an end to a decade of
preparation. Ten years of planning, designing, building, and testing have been
performed to ensure the instrument can withstand a rough launch harsh space
environment and still send back the information desired. The first images
received are the beginning of the era of scientific investigation and learning
for which the instrument was planned and built.
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