PRESS KIT -- Space Radar Laboratory 2
targeted launch date: September 30, 1994
Mission Objectives
SIR-C/X-SAR, part of NASA's Mission to Planet Earth, is
studying how our global environment is changing. From the unique
vantage point of space, the radar system will observe, monitor
and assess large-scale environmental processes with a focus on
climate change. The spaceborne data, complemented by aircraft
and ground studies, will give scientists highly detailed
information that will help them distinguish natural environmental
changes from those that are the result of human activity. NASA
will distribute the Mission to Planet Earth data to the
international scientific community so that this essential
research is available worldwide to people who are trying to make
informed decisions about protecting their environment.
Why Radar?
The unique feature of imaging radar is its ability to
collect data over virtually any region at any time, regardless of
weather or sunlight conditions. Some radar waves can penetrate
clouds, and under certain conditions, can also see through
vegetation, ice and extremely dry sand. In many cases, radar is
the only way scientists can explore inaccessible regions of
Earth's surface.
Radar is a lowercase acronym for radio detection and
ranging. A synthetic aperture radar transmits pulses of
microwave energy toward Earth and measures the strength and time
delay of the energy that is scattered back to the antenna. In
the case of SIR-C/X-SAR, the motion of the shuttle is used to
"synthesize" an antenna (the aperture) that is much longer in
length than the actual antenna hardware. A longer antenna
produces images of finer resolution.
Conditions on the Earth's surface influence how much radar
energy is reflected back to the antenna. An area with a variety
of surface types, such as hills, trees and large rocks, will
generally reflect more energy back to the radar than a less
complex area such as a desert. The resulting radar image of the
varied terrain will be brighter overall than the image of the
simpler area. The three frequencies of SIR-C/X-SAR will enable
scientists to view three different scales of features in the
images.
Results of STS-59
Launched April 9, 1994, SIR-C/X-SAR collected a total of 65
hours of data during the 10 day mission, roughly corresponding to
66 million square kilometers (26 million square miles). All
data were stored onboard the shuttle using a new generation of
high-density, digital, rotary-head tape recorders. The data
filled 166 digital tape cartridges (similar to VCR tape
cassettes). Twenty-five of those tapes were filled with X-SAR
data.
The mission returned 47 terabits of data (47 by 10^12 bits
of data) or the equivalent of 30,000 encyclopedia volumes. To
think of it another way, each of the radars generates 45 million
bits of data per second. When all the radars are operating they
produce 225 million bits of data per second, or the equivalent of
45 simultaneously operating television stations. The raw data
was processed into images using JPL's digital SAR processor and
by processors developed by Germany and Italy for the X-SAR data.
SIR-C data were downlinked, processed, and released to the
science team via the Internet within 24 hours after launch. X-
SAR data were processed in survey mode and displayed in real time
on NASA Select Television.
Science
The STS-59 mission achieved 100 percent of the SIR-C/X-SAR
science objectives during the April 1994 flight. In addition to
taking high resolution data at all of the planned sites, the
science team was able to adjust their timeline and observe events
as they were happening on the ground. SIR-C/X-SAR took data of
the severe flooding that inundated the mid-western United States
and Germany, as well as, three different views of tropical
Cyclone Odille as it formed in the Pacific Ocean. Scientists
also acquired a series of radar images over Canada documenting
the annual spring thaw of snow, ice and soil.
Scientists are using SIR-C/X-SAR data to study how our
global environment is changing. The SIR-C/X-SAR radar data
provides information about how many of Earth's complex "systems"
-- those processes that control the movement of land, water,
carbon and heat -- work together to make this a livable planet.
The science team is particularly interested in studying the
amount of vegetation coverage, the extent of snow packs, wetlands
areas, geologic features such as rock types and their
distribution, volcanic activity, ocean wave heights and wind
speed. STS-68 will fly over the same sites that were studied by
STS-59 so that scientists will be able to study any changes that
may have occurred in those areas between the missions.
There are more than 400 sites on Earth where data will be
taken during the mission. Nineteen of those have been designated
as "supersites," making them the highest priority targets and the
focal point for many of the scientific investigators. There are
an additional 15 backup supersites. If problems should occur
during the flight that would drastically reduce the team's
ability to collect data, the supersite data will take precedence
over other data acquisition.
During STS-59 the scientists who were working in the Payload
Operations Control Center in Houston were in daily communication
with the researchers who were part of the "ground truth" teams.
The ground teams at several of the supersites made simultaneous
measurements of vegetation, soil moisture, sea state, snow and
weather conditions during the mission. Data was also collected
from aircraft and ships to ensure an accurate interpretation of
the radar data taken from space. In addition, the astronauts
recorded their personal observations of weather and environmental
conditions in coordination with SIR-C/X-SAR operations.
Supersites
The supersites were chosen to represent different
environments within each scientific discipline, and they are
areas where intensive field work will occur before, during and
after the flight.
Ecology
Manaus, Brazil
Raco, Michigan
Duke Forest, North Carolina
Ecologists study life on Earth and how different species of
animals and plants interact with one another and their local
environment. SIR-C/X-SAR ecology investigations are focused on
mapping wetlands, deforestation and flooding under forest
canopies over the tropical forests of the Amazon basin in South
America and over the temperate forests of North America and
Central Europe. Scientists are also studying wetlands and are
using the data to validate computer models to determine
vegetation type, seasonal freeze/thaw transitions, and biomass.
The radar images will be used to study land use, the volume,
types and extent of vegetation and the effects of fires, floods
and clear-cutting.
Using early-release data, science team members have already
generated both tree classification and vegetation biomass maps of
the Raco, Michigan site and a
freeze/thaw map over the Prince Albert, Saskatchewan backup
supersite. A map of flooding near Manaus, Brazil has also been
produced, as the first step toward improving models of both
flooding and wetlands under dense forest canopies.
SIR-C/X-SAR's three radar frequencies interact with the
vegetation on different scales, providing three independent views
of the forest. The radar's multi-polarization ability allows
scientists to look beneath the thick vegetation canopy of the
forest in these often cloud-covered regions of the world to study
the trunks of the trees, which reflect the vertical waves as well
as the tree branches, which reflect the horizontal waves. These
data give scientists a more complete picture of the conditions on
the ground.
Seasonal changes in the forest will be studied by comparing
data from the two SIR-C/X-SAR flights in April and October. For
example, data from two previous imaging radar missions showed a
decrease in the amount of forests along the Mississippi River
between 1978 and 1981. Deforestation threatens both temperate
and tropical forests around the world. SIR-C/X-SAR data will be
used along with ground data to understand the impact of the loss
of forests on local populations of plants and animals. By
studying the short-term and long-term changes in forests,
scientists can determine what effects changing environmental
conditions and land use have on the forests and, in turn, on
global climate change.
Hydrology
Chickasha, Oklahoma
Otztal, Austria
Bebedouro, Brazil
Montespertoli, Italy
Hydrologist study how water flows on land. In addition to
swamps, lakes, rivers, snow and ice, an important part of the
global water cycle is the large amounts of water stored as soil
moisture and in vegetation. This "hidden" water plays a major
role in determining whether a region is wet or dry, and it also
influences the way energy is distributed around the globe.
SIR-C/X-SAR hydrology investigations are focused on Brazil,
Italy and Oklahoma, where the radar data will be used to
determine soil moisture patterns. These studies will help
scientists develop ways to estimate soil moisture and evaporation
rates over large areas, which will ultimately be incorporated
into computer models to help predict a region's water cycle.
Eventually, continuous radar monitoring of an area's water
resources would help farmers determine the best type of crops to
plant, where to plant them and when.
Another significant part of hydrology is snow cover. Using
data from STS-59, investigators generated a snow and ice
classification map over the Oetztal, Austria supersite and a snow
wetness map of the Mammoth Mountain, Calif. backup supersite.
Spring snow melt often determines the annual runoff cycle and the
resulting water supply, ground water and reservoir replenishment
rates. For many areas, long-term or ground-based snow cover data
do not exist, and radar data is the only way to collect this
information. SIR-C/X-SAR acquired snow cover data over Mammoth
Lakes, Calif., the Austrian Alps and northwest China. The
shorter wavelength X-band data is useful to scientists for
determining snow type, while the longer wavelengths of L-band and
C-band helps them estimate snow volume. These data will help
communities determine how much water will be available for human
and agriculture use. In October, the emphasis will shift to the
Patagonian district in Southern Chile, which contains the largest
modern glaciers and ice fields in South America.
Wetlands are often highly desirable places for humans to
live and work, and they are also home to delicate ecosystems
that are especially vulnerable to changes introduced by humans.
Wetlands are the source of many trace gases that play an
important part in the global atmospheric cycle. SIR-C/X-SAR will
be able to determine the extent and limits of selected wetlands
areas because radar is extremely sensitive to the presence of
standing water, even under vegetation cover. Data from the
multiple flights of SIR-C/X-SAR will help scientists monitor how
the wetlands conditions are changing.
Oceanography
The Gulf Stream (mid-Atlantic region)
East-North Atlantic Ocean
Southern Ocean
Oceanographers study how waves move through the ocean and
how the air and sea interact. The ocean is a reservoir for heat
and energy, and the air-sea interaction moves this heat and
energy around the globe regulating the Earth's climate. The Gulf
Stream off the East Coast of North America is a major ocean
current that transports heat from the equator toward the poles.
The relatively low altitude of the shuttle is particularly
advantageous for oceanography investigations since the SIR-C/X-
SAR radars are more sensitive to ocean features than satellites
in higher orbits. Oceanographers are using data from SIR-C/X-SAR
to study surface and internal waves and wave/current nteractions.
In addition, extensive wave energy information was collected over
the Southern Ocean by an associated experiment provided by the
Johns Hopkins Applied Physics Lab. These data will help
scientists study how the Earth's climate is moderated by the
ocean.
Geology
Galapagos Islands
Sahara Desert
Death Valley, Calif.
Andes Mountains, Chile
Geologists study the present surface of the Earth. By
observing older rocks they can determine how an area came to be
and what it may have looked liked in the past. SIR-C/X-SAR is
mapping geologic structures and variations in rock types over
large areas, as well as areas of volcanic activity and erosion.
These data are especially useful in areas of heavy vegetation and
continuous cloud cover, where field work is often difficult.
The longer L-band radar wavelength are particularly useful
for looking beneath surfaces. SIR-C/X-SAR obtained penetration
data of the Sahara desert that show braided channels beneath an
old river valley. On the ground and in optical photographs, this
big valley and the channels in it are invisible because they are
entirely covered by windblown sand. Some of these same channels
were observed in SIR-A images in 1981. It is hypothesized that
the large valley was carved by one of several ancient predecessor
rivers that crossed this part of North Africa, flowing westward,
tens of millions of years before the Nile River existed. The
Nile flows north about 300 kilometers (200 miles) to the east of
the area observed by the radar.
The existence of hidden river channels indicates that
portions of the Sahara have undergone significant climate change
and have evolved from an area of flowing streams to what is now
an arid desert. SIR-C/X-SAR is also studying other geologic
features that record past climate changes. In areas of Death
Valley, Calif., western China and the Patagonia region of the
southern Andes, the radar mapped alluvial fans. Alluvial fans
are gravel deposits that wash down from the mountains. They are
found throughout the semi-arid deserts of the world in areas
where there is a significant amount of tectonic activity. The
gravel builds up at the base of the mountains during periods of
overall wetter climate. The radar is sensitive to these rocky
and rough surfaces that allows scientists to study the history of
past climate and the relative age of surfaces. As an area ages,
it is exposed to weathering. This changes its roughness
characteristics. Mapping areas of past climate change will give
scientists a stronger base from which to monitor and predict
future climate changes.
During STS-59, SIR-C/X-SAR took radar images of several
volcanoes, including Mt. Pinatubo and the volcanoes of the
Galapagos Islands. These radar images are helping scientists
identify the different types of lava flows and their ages. A key
objective of STS-68 will be to obtain a second image of Mt.
Pinatubo during the summer monsoon season that is likely to
produce new mudflows and to evaluate whatever short-term changes
may have occurred.
Calibration
Flevoland, The Netherlands
Kerang, Australia
Oberpfaffenhofen, Germany
Western Pacific Ocean
Ground equipment consisting of calibration devices called
corner reflectors and receivers were deployed in southern
Germany, The Netherlands, Australia and Death Valley, Calif. to
measure the amount of radar energy obtained at the ground during
the flight. This information is being used calibrate the radar
data.
Rain Experiment
Data were also taken during STS-59 to support the two SIR-
C/X-SAR experiments designed to image rain. These experiments
took place over the Western Pacific Ocean, an area scientists
call the "rainiest place on Earth."
Although radar can penetrate clouds, it is important to
understand how rain can change conditions on the ground and thus
change the radar image. At the shorter wavelengths of X-band and
C-band, rain may reduce the strength of the radar or scatter the
signals significantly.
The rain experiments offer a unique challenge to the
operation of the radar during flight. All the other experiments
can be reasonably tied to a specific area, while the rain
experiments only require that a "deep" rainstorm be in progress.
Weather targets are transitory in both space and time and cannot
be scheduled, so finding a good target of opportunity is a
gamble. Scientists chose the western Pacific because there is a
high probability that it will be raining there when the shuttle
passes over it.
Interferometry -- A New Technology
One of the bonuses of flying SIR-C/X-SAR for a second time,
is the opportunity to demonstrate a new technology from the
shuttle platform, called interferometry. Scientists will
conduct the experiment during the last three days of the flight
using subsequent passes of SIR-C/X-SAR over the same areas on the
Earth. Investigators hope it will be possible to generate
digital elevation models (topography) of the Earth's surface if
the shuttle orbit can be repeated with sufficient precision. Once
topography is determined, a third interferometric pass can be
used to determine what, if any, topographic change has occurred
in the intervening time between radar overpasses. This
demonstration will be carried out in preparation for TOPSAT, a
mission planned for later this decade to measure global
topography and centimeter-scale topographic change using laser
and radar Interferometry. The focus of these experiments is to
improve our assessments of natural hazards such as flooding,
subsidence, mudflows and volcanic eruptions. For example, the
topography of mountain glaciers is important because it directly
reflects ice-flow dynamics and is closely linked to global
climate and sea level change. Monitoring mountain glaciers on a
global basis will give scientists important information on the
rate of global warming. In addition, topographic data can also
be used to reduce the risk of natural disasters by monitoring
lava flows, flooding and earthquake faults.
Spaceborne Imaging Radar C/X-Band Synthetic Aperture Radar (SIR-
C/X-SAR)
The main payload for the STS-68 Space Radar Laboratory is
the Spaceborne Imaging Radar C/X-Band Synthetic Aperture Radar
(SIR-C/X-SAR), a sophisticated set of radars that fills nearly
all of Endeavour's cargo bay. This is the second flight for SIR-
C/X-SAR. The first flight took place in April 1994.
SIR-C, built by the Jet Propulsion Laboratory and the Ball
Communications Systems Division for NASA, is a two-frequency
radar including L-band (23-cm wavelength) and C-band (6-cm
wavelength). SIR-C is the first spaceborne radar with the
ability to transmit and receive horizontally and vertically
polarized waves at both frequencies. The multi-frequency,
multi-polarization capability creates a new and more powerful
tool for studying the world. A good way to understand this is to
think of visual images: Color pictures have more information
about a subject than do black and white pictures, just like
multi-frequency, multi-polarization radar images contain more
information about the surface than single frequency, single
polarization radar images.
The SIR-C antenna is the most massive piece of flight
hardware ever built at JPL. Its mass is 10,500 kg (23,100 lbs)
and it measures 12 meters by 4 meters (39.4 feet by 13.1 feet).
The instrument is composed of several subsystems: the antenna
array, the transmitter, the receivers, the data-handling
subsystem and the ground processor. The antenna consists of
three leaves, and each is divided into four subpanels.
Unlike previous SIR missions, the SIR-C radar beam is formed
from hundreds of small transmitters embedded in the surface of
the radar antenna. By properly adjusting the energy from these
transmitters, the beam can be electronically steered without
physically moving the large radar antenna. This feature,
combined with the roll and yaw maneuvers of the shuttle, will
allow images to be acquired from 15-to 55- degree angles of
incidence. Advancements in radar technology will allow SIR-C to
acquire simultaneous images at L-band and C-band frequencies with
HH, VV, HV, and VH polarizations.
Polarization describes how the radar wave travels in space.
For example, when data is acquired with HH polarization, the wave
is transmitted from the antenna in the horizontal plane and the
antenna receives the backscattered radiation in the
horizontal plane. With HV polarization, the wave is transmitted
horizontally, but is received by the antenna in the vertical
plane. It is the interaction between the transmitted waves and
the Earth's surface that determines the polarization of the waves
received by the antenna. Multi-polarization data contain more
specific information about surface conditions than single
polarization data. Multi-polarization data are particularly
useful to scientists studying vegetation because the data allow
them to see different types of crops and to measure the volume of
trees contained under the canopy of a forest.
X-SAR is built by the Dornier and Alenia Spazio companies
for the German space agency, Deutsche Agentur fuer
Raumfahrtangelegenheiten (DARA), and the Italian space agency,
Agenzia Spaziale Italiana (ASI). It is a single-polarization
radar operating at X-band (3-cm wavelength).
X-SAR uses a slotted-waveguide antenna, which is finely
tuned to produce a narrow, pencil-thin beam of energy. The X-SAR
antenna is mounted on a supporting structure that is tilted
mechanically to align the X-band beam with the L-band and C-band
beams. X-SAR will provide VV polarization images.
Both SIR-C and X-SAR can be operated as either stand-alone
radars or in conjunction with each other. The width of the
ground swath varies from 15 to 90 kilometers (9 to 56 miles),
depending on the orientation of the antenna beams. The
resolution of the radars can be varied from 10 to 200 meters (33
to 656 feet.)
Previous Radar Missions
Since the late 1970s a variety of NASA satellite missions
have used imaging radar to study Earth and our planetary
neighbors. Perhaps the most familiar example of NASA's success
using imaging radar is the Magellan mission to Venus. Magellan's
radar pierced the dense clouds covering Venus to map the entire
surface of the planet, revealing a world that had been hidden to
humans for centuries.
SIR-C is the latest in a series of Earth observing imaging
radar missions that began in June 1978 with the launch of Seasat
SAR and continued with SIR-A in November 1981 and with SIR-B in
October 1984. Both the SIR-A and SIR-B sensors were derived from
the Seasat SAR, and all three were capable of transmitting and
receiving horizontally polarized radiation at L-band frequency.
The major difference between the Seasat and SIR-A sensors
was the orientation of the radar's antenna with respect to
Earth's surface. Microwave radiation transmitted by Seasat
struck the surface at a fixed angle of approximately 23 degrees
from the local zenith direction. SIR-A was designed to view the
surface at a fixed 50 degree angle.
SIR-B improved upon both those missions because its antenna
could be mechanically tilted. This allowed SIR-B to obtain
multiple radar images of a given target at different angles
during successive shuttle orbits.
The X-SAR antenna is a follow-on to Germany's Microwave
Remote Sensing Experiment (MRSE), which was flown aboard the
first shuttle Spacelab mission in 1983.
These early missions had a tremendous impact on the
international remote sensing community when SIR-A discovered
ancient river beds hidden beneath the sands of the Sahara, and
SIR-B data led explorers to the Lost City of Ubar in Oman.
Data Acquisition Plans for STS-68
Portions of data will be downlinked to the ground in near-
real time via NASA's Tracking and Data Relay Satellite System
(TDRSS). However, only one channel of data can be downlinked or
played back at a time. This is not a problem for X-SAR since it
only has one channel of data. SIR-C has up to four channels of
data, and each channel must be played back separately.
Historically, processing SAR data has required a great deal
of computer time on special-purpose computer systems.
SIR-C/X-SAR scientists will benefit, however, from rapid advances
in computer technology that make it possible to process the
images with a standard parallel super computer. Yet even with
these advances, it will still take five months to produce survey
images from the large volume of data acquired. Detailed
processing will take another nine months to complete. Data will
be exchanged among Italy, Germany and the United States to meet
the needs of the science investigators.
NASA/JPL will attempt to release some radar images to the
press during the shuttle flight. The images will be processed at
JPL and sent electronically via Internet to the Johnson Space
Center, where the image will be released on NASA Select
Television. Hard copy prints will be released simultaneously to
the wire services at JPL.
Science Team
An international team of 49 science investigators and three
associates will conduct the SIR-C/X-SAR experiments. Thirteen
nations are represented, including: Australia, Austria, Brazil,
Canada, China, England, France, Germany, Italy, Japan, Mexico,
Saudi Arabia and the United States.
Dr. Diane Evans of the Jet Propulsion Laboratory is the U.S.
project scientist. Dr. Herwig Ottl of DLR is the German project
scientist and Prof. Mario Calamia of the University of Florence
is the Italian project scientist.
Management
The SIR-C mission is managed by the Jet Propulsion
Laboratory for NASA's Office of Mission to Planet Earth. Michael
Sander is the JPL project manager.
X-SAR is managed by the Joint Project Office (JPO) located
near Bonn, Germany. Rolf Werninghaus of DARA is the project
manager and Dr. Paolo Ammendola of ASI is the deputy project
manager.
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