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February
06, 2002 - update
THRUSTERS PRECISELY GUIDE EO-1 SATELLITE IN SPACE FIRST
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the Pulsed Plasma Thrusters providing pitch control, EO-1's
Advanced Land Imager (ALI) took this photo over Colorado,
demonstrating the ability of the thruster to maintain
precise control of the spacecraft needed while collecting
Earth imagery. |
A
new generation of thrusters has been used to precisely guide
and point a satellite in space, paving the way for use of
this technology on future spacecraft to save weight, fuel
and cut power consumption.
Engineers
used Pulsed Plasma Thrusters (PPT) onboard NASA's EO-1 satellite
as a precision attitude control actuator in space. A single
PPT unit with two opposing thrust nozzles controlled the 1166-pound
(529 kg) spacecraft's pitch (up and down) axis for 4 hours
as it made several orbits of the Earth.
Continued
testing of the EO-1 PPT is expected to demonstrate additional
attitude control capability and thruster performance characteristics.
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NASA's
Glenn Research Center, Cleveland, developed the PPT system,
and NASA's Goddard Space Flight Center, Greenbelt, Md., managed,
designed and implemented the PPT experiment into the EO-1
spacecraft.
"Our
initial operations with the PPT have been highly successful
and we have reached a significant milestone," said Chuck
Zakrzwski, Propulsion Systems Engineer at Goddard. "The
thruster and the entire spacecraft performed as expected during
the PPT calibration and closed loop attitude control tests."
"The
path is now open for PPT use on new missions due to the successes
in our flight validation and ongoing PPT development,"
said Scott Benson, Propulsion Programmatic Manager at Glenn.
The
PPT is a unique electromagnetic propulsion system that utilizes
solid bars of Teflon as fuel. Pulses of electricity, lasting
only one one-thousandth of a second, are fired across the
Teflon bar. Each pulse turns a minute amount of the Teflon
into an electrically charged gas that is accelerated out of
the thruster. Each pulse has approximately the same force
as dropping a paper clip into the palm of your hand from about
one-half inch away. Because the PPT also uses electromagnetic
forces, it is three times more fuel efficient than conventional
chemical thrusters.
NASA's
EO-1 spacecraft is the first to fly the smaller, lighter and
more efficient new generation PPT. Engineers tested the thruster's
precision attitude control ability by using the PPT in place
of one of the conventional reaction wheels. This is the first
time this has ever been done and demonstrates the technology
could be used as a precision attitude control actuator as
well as a precision orbit adjust thruster.
"Potentially,
a set of PPTs could be used to replace reaction wheels and
their associated electromagnetic torquers as well as the conventional
chemical propulsion for both orbit and attitude control,"
Zakrzwski said. "An analogy to this would be an automobile
that has its power steering and its forward motion supplied
by their common power source - the car's engine - rather than
separate, multiple power sources."
"Such
a configuration would offer better performance and likely
be more weight and cost effective than conventional systems,"
Benson said. "The PPT's extremely small impulse level,
high propellant efficiency, low power and compact size make
it well suited for a number of precision pointing and precision
positioning functions on future spacecraft."
The
Pulsed Plasma Thruster Technology Demonstration is the result
of a partnership between NASA's Goddard Space Flight and Glenn
Research Centers, General Dynamics Space Propulsion Systems
and Swales Aerospace. The EO-1 spacecraft is managed by Goddard
and was built by Swales Aerospace. The spacecraft is part
of the New Millennium Program managed by NASA's Jet Propulsion
Laboratory.
EO-1
SEES THE FUTURE WHILE LOOKING AT EARTH
In the future, exploration of the Earth will demand research
tools that can not only make some of their own decisions,
but deliver results faster, better and cheaper than their
forebears. NASA's launch of the experimental EO-1 satellite
is a significant milestone in that journey. Part of the space
agency's New Millennium Program, engineers designed EO-1 to
test advanced technologies for instruments pointed at our
own planet. The satellite will not only demonstrate significant
improvements in data collection capabilities, but will also
test methods for dramatically reducing costs and complexity
to achieve state of the art goals. As the first New Millennium
satellite designed to look at the Earth, NASA hopes EO-1 will
open a new era in exploration of our own planet.
A
View from Above
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The
hexagonal satellite called EO-1 carries a suite of instruments
designed to test new technologies while also conducting valuable
research about our home planet. Flying almost 438 miles above
the Earth, the satellite's scientific hardware can deliver
some of the data collection capabilities previously possible
only from satellites far larger and more complex.
Fully
outfitted, EO-1 weighs nearly 1166 pounds at launch. ItÕs
being sent into orbit on a Delta 7320-10 rocket launched from
Vandenberg AFB in California.
Clearing
the View
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The
Atmospheric Corrector Between the Earth and any satellite
on orbit lies an ocean through which all information must
pass. It's the atmosphere, and to the highly precise sensors
of delicate orbiting systems, it can be just like looking
through a cloudy or warped window. For researchers, this is
a problem that must always be taken into account when looking
at Earth from space. But the EO-1 project will test a new
device designed to compensate for atmospheric distortion.
It's called The Atmospheric Corrector (AC). If proven effective,
such a device will likely be applicable to other scientific
or commercial remote sensing missions where water vapor or
other particles in the atmosphere might cause measurements
of the surface to degrade. Until now, experts have generally
compensated for atmospheric distortion by using predicted
or modeled mathematical values for how much the atmospheric
layer between the Earth and their instrument causes changes
to images. But EO-1's Atmospheric Corrector changes that strategy.
By gathering actual, real time information about how the atmosphere
distorts images from the ground, scientists can calibrate
their sensors to create significantly clearer images of what
they're studying. The device should provide significant improvements
in generating accurate surface reflectance measurements for
land imaging missions. Further, the algorithms developed for
use with the Atmospheric Corrector will enable more accurate
measurements and classification of land resources and better
models for land management in the future.
Seeing
Earth's Quilt Anew --The Advanced Land Imager
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In
many ways, the Advanced Land Imager (ALI) embodies the engineering
ideal that less is more. A principle component to the EO-1
mission, ALI is an Earth observing instrument designed to
generate images of the planet based on various wavelengths
of light reflected from the surface.
Project
designers developed the instrument to be comparable with or
exceed the capabilities of Landsat's Enhanced Thematic Mapper
Plus. Further, the EO-1 project team designed ALI to deliver
these images at a significant reduction in weight, technical
complexity, and cost-- all vital features to facilitating
development of advanced Earth observing satellites.
http://eo1.gsfc.nasa.gov/Technology/ALIhome1.htm
Beyond
the Pale-Hyperion Imaging Spectrometer
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It's
not so much that the Hyperion instrument will be able to see
the Earth more "close up" or have a higher spatial resolution
than previous instruments. Yet Hyperion's goals are nothing
less than ambitious. The instrument is designed to gather
highly complex data from a given region on the Earth by viewing
the surface in terms of 220 distinct colors or "bands" of
light. Think of looking at a photograph in black in white
and then comparing the exact same frame in color.
Even
though there is no greater resolution to the image, no change
in perspective, lighting, or magnification, the amount of
data presented to the viewer has greatly increased. Project
managers designed Hyperion to fill in that kind of data in
observed regions on the ground. The uses for an instrument
than can make such fine spectral distinctions include studies
of land use, changes in land cover, mineral resource assessment,
research into coastal processes, changes in the atmosphere
and more.
http://eo1.gsfc.nasa.gov/Technology/Hyperion.html
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Advanced
Technology on EO -1:
Changing Directions - Pulsed Plasma Thruster
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As
a demonstration of experimental technology, much of the EO-1
project requires carefully considered scheduling. A good example
of this is the testing phase of the Pulsed Plasma Thruster
(PPT).
This
innovative design for a steering thruster uses solid Teflon
as a means for changing the satellite's spatial orientation.
When small amounts of the propellant are turned to plasma
by an electrical discharge, it's accelerated out of the steering
nozzle by an electromagnetic field. But that's where the careful
scheduling of the tests comes into play. Because of the plasma
discharge, project managers will test the thruster after most
of the other systems on the satellite have put through their
paces. This will help insure that there's no contamination
or corruption of any instrumentation on board from the thruster
plume. As designed, the Pulsed Plasma Thruster is not expected
to cause any problems for other systems, but the flight of
EO-1 will put that assumption to the test. The new thruster
will be used to maintain fine pitch attitude control for the
spacecraft--essentially small navigational adjustments. If
the technology is proven to work as designed, it could have
a wide range of uses in future spacecraft systems. One of
the principle advantages of the Pulsed Plasma Thruster is
its lack of liquid propellant, removing the potential for
sloshing fuel inside spacecraft tanks to affect delicate instrument
measurements.
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Shedding
Light on new Solar Technologies
Lightweight
Flexible Solar Array Spacecraft must either produce their
own electricity from chemical processes or solar power. Most
unmanned satellites rely on the sun. Above the atmosphere,
light from our nearest star is far brighter than after it
passes through miles of air and water vapor to us on the ground,
and it's never diminished by clouds. But there's still the
issue of its collection and conversion into useful electricity.
It's hard enough to design, build, launch, and maintain a
satellite in orbit. But without working instruments on board,
there's little point. On the ground we're used to simply plugging
virtually infinite electrical devices into waiting outlets.
But in space, power is at a premium, and even with abundant
solar energy, the hardware necessary to harness the sun still
exacts significant engineering demands. The EO-1 satellite
will have two solar panels. The main panel --the wide, wing
looking extension attached to the spacecraft--is basically
a variant of proven technology, flown on many previous missions.
The array is composed of cells made from either silicon or
gallium arsenide on Germanium, essentially crystals grown
in a lab and carefully placed on a fragile supporting wafer.
But on one side of the satellite's underbelly, engineers have
attached a new, experimental solar panel in an effort to generate
significantly more power at a fraction of the weight, size,
and complexity. It's called the Lightweight Flexible Solar
Array (LFSA). Unlike its larger sibling, it employs a surface
of copper indium diselinide, originally deposited on a lightweight,
flexible backing in the form of a vapor. Not only is it significantly
lighter than solar cells designed as crystals, but it can
also operate on a flexible, less rigid surface, with significantly
higher returns on its electrical output. That is, where the
main solar array can generate roughly 40 watts per kilogram,
the new solar panel is expected to be able to generate roughly
100 watts per kilogram. Additionally, with the inclusion of
new hinges on the release mechanism of the panel based on
advanced shape memory alloys, further weight and risk will
be reduced, compared to traditional technologies that use
pyrotechnics for deployment. Flight validation of a technology
like this is imperative, because certain real-world conditions
cannot be simulated adequately on the ground. One of the more
interesting hurdles for the system to confront concerns how
it stands up to the rigors of a near-Earth environment. Even
in low Earth orbit, there are traces of gas and particulates
which can have real effects on spacecraft performance. In
the case of the Lightweight Flexible Solar Array, engineers
want to test how well it can stand up to atomic oxygen. This
form of oxygen can dramatically degrade delicate, thin chemical
films. To prove its usefulness in future missions, the new
solar panel will be placed on the side of the satellite that
most directly bears the brunt of any atmospheric pressure,
called the ram side, so as to maximize the effects of atomic
oxygen on the solar panel. Successful development of Lightweight
Flexible Solar Array technologies will dramatically enhance
future satellite engineers ability to maximize the value of
what they can put into orbit by delivering more reliable sources
of ever precious power in greater quantities.
http://eo1.gsfc.nasa.gov/Technology/lfsa.html
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Pushing
the Edges of Technology
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Touring
some of EO-1's Systems EO-1 is primarily a test bed for new
technologies and techniques. In this series, we point out
several of the devices that comprise the satellite. Advanced
Land Imager Atmospheric Corrector Hyperion (Hyperspectral
Imager) X-Band Phased Array Antenna Carbon-Carbon Radiator
Lightweight Flexible Solar Array Autonomous Star Tracker
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The
Landsat Duet: Enhanced Formation Flying, Part I (Two Way EFF
animation
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By
design, EO-1 is primarily tasked to study the surface of the
Earth. But the satellite's reason for being is the essentially
the next logical step in fulfilling the mandate put forth
by the 1992 Land Remote Sensing Policy Act (Section 105 of
Public Law 102-555). That act calls for the development of
a sound data policy for information collected by Landsat 7.
So, if the law speaks about Landsat, how does EO-1 fit in?
If the technologies prove their promise, the new experimental
satellite begins to light a way for future, continued development
of the Landsat data legacy. EO-1's Advanced Land Imager, it's
Hyperion hyperspectral imager, and the new Atmospheric Corrector
all have direct application to the issue of providing next
generation Landsat type data; the two satellites share a common
ancestry. To that end, a novel experiment will be conducted
with EO-1 and Landsat 7 working in concert. In the first satellite
maneuver of its kind, EO-1 and Landsat 7 will assume an orbital
"formation", flying approximately one minute apart on the
same ground track. In terms of distance, this will place the
two spacecraft approximately 270 miles (450 kilometers) apart,
plus or minus 30 miles (50 kilometers) or so. This affords
scientists and engineers the opportunity to do some valuable
tests. By flying the same route so close together, nearly
identical images taken by each satellite can be compared on
the ground. As potentially powerful improvements to existing
technologies, use of the Advanced Land Imager and the Hyperion
instruments on EO-1 in concert with Landsat overflights refine
the calibration. Of more immediate interest is the opportunity
to try EO-1's Atmospheric Corrector as a tool for refining
data collected by its fellow satellite Landsat, flying one
minute ahead. There's a lot of information expected from EO-1.
For each scene the spacecraft generates, over 20 gigabits
of scene data from the Advanced Land Imager, Hyperion, and
Atmospheric Corrector will be collected and stored on the
on-board solid state data recorder at high rates. When the
EO-1 spacecraft is in range of a ground station, the spacecraft
will automatically transmit its recorded image to the ground
station for temporary storage.
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A
String of Pearls: Enhanced Formation Flying, Part II (Four
Way EFF animation)
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The
New Millennium Program is not simply about designing individual,
experimental satellites. NMP efforts also strive for new ways
to design and implement software and computer programming
techniques, as well as ambitious project goals that often
include multiple systems working in harmony. With EO-1, NASA
plans to fly a spacecraft in formation with three other research
satellites for the first time. Soon after achieving orbit,
the satellite will become part of a carefully choreographed
constellation, joining Landsat 7, the Earth observing flagship
Terra, and SAC-C, an instrument designed and managed by the
space agency of Argentina. The operation offers unique research
possibilities, including highly precise cross calibration
of instruments, and atmospheric correction of data acquired
by Landsat 7 and the MODIS instrument onboard the Terra spacecraft.
Terra will fly in the fourth position of the EO-1 constellation;
SAC-C third; EO-1 assumes the second slot, and Landsat 7 takes
the front of the line. Enhanced Formation Flying (EFF) tests
highly sophisticated software systems, including so-called
"fuzzy logic" algorithms to resolve navigational and operational
conflicts that inevitably occur in flight. Some of the benefits
of flying satellites in formation come in the area of risk
management. By using small fleets of less expensive, less
complex satellites in place of singularly large, highly sophisticated
platforms, a catastrophic failure does not necessarily cause
irreparable harm to an overall mission. Further, by flying
a suite of sensors in formation, researchers can essentially
create one enormous "virtual" satellite by integrating the
data collected individually by each smaller instrument.
http://eo1.gsfc.nasa.gov/Technology/FormFly.html
Large
Ambitions in a Small Package
SAC-C SAC-C is an international cooperative mission between
NASA and the Argentine Commission on Space Activities (CONAE).
The
Argentine space agency in providing the spacecraft and some
of its instrumentation, while NASA will provided the launch
vehicle and other components. The SAC-C program also encompasses
technical contributions from Brazil, Denmark, France, and
Italy. SAC-C will study terrestrial and marine ecosystems,
monitor atmospheric temperature and water vapor, examine variabilities
in the ionosphere, study the interaction between the Earth's
magnetic field and the Sun, provide multi-spectral images
of the Earth in order to monitor the condition and dynamics
of the terrestrial and marine biosphere and environment, and
more. The satellite even includes an experiment to track the
migratory route of the Franca whale. By working in concert
with NASA, SAC-C will be able to significantly add to the
growing pool of orbiting observatories that are seeking to
better understand how our planet works.
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