|
Celebrating a Mission That Changed How We Use Radar Oct. 11, 2019,
marks the 25th anniversary of the end of a space mission that transformed the
way we use radar to observe large-scale environmental processes on our home
planet. The Spaceborne Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X-SAR)
mission made available to people worldwide the scientific data used to this day
to inform decisions to slow and mitigate climate change.
The SIR-C
instrument, built by NASA'S Jet Propulsion Laborator in Pasadena, California, and
the X-SAR instrument, built by the German Aerospace Center (DLR), constituted the
most advanced imaging radar system ever used in air or space. During hundreds
of orbits on two flights aboard the Space Shuttle Endeavour, in April and
October 1994, the radar system made multiple passes over 19 "supersites"
- areas of scientific interest in such locations as the Sahara, Brazil, the Alps
and the Gulf Stream. It also imaged events occurring during the flights, such as
as a volcano erupting in Russia.
"The many innovationsof SIR-C/X-SAR have been used in virtually
every air- and spaceborne radar mission since, starting with NASA's Shuttle
Radar Topography Mission, which mapped 80% of the Earth in 2000," said Tony
Freeman, now manager of JPL's Innovation Foundry, who led end-to-end
calibration of SIR-C. "DLR's TerraSAR-X and TanDEM-X missions have since
filled remaining gaps."
Radar imaging
of Earth has never been the same since SIR-C/X-SAR's demonstration of what's
known as simultaneous multifrequency, fully polarized, repeat-pass interferometric
SAR. To unpack that sizable trunk of terminology, let's start with "synthetic
aperture radar": Since the late 1970s, NASA has been imaging Earth with
radar - in darkness, under cloud cover or vegetation, even underground - using
the movements of a host airplane or spacecraft to "synthesize" an "aperture"
much larger than the antenna itself. The larger the aperture, the greater the image
resolution. Indeed, SIR-C's predecessors, SIR-A and SIR-B, were synthetic
aperture radar missions.
However, unlike
SIR-C/X-SAR, neither predecessor made radar observations simultaneously in
three frequencies - C-, L- and X-band - using three adjacent antennas combined
into a massive, 12-by-4-meter, 11.5-ton structure. That advance, analogous to the
leap from black-and-white to color film, allowed the mission to collect data in
different scales, providing a crisp snapshot of each targeted feature,
unmuddied by possible changes over time.
Blazing a Trail
In addition to
multiple frequencies, some observations were made in multiple "polarizations."
Radio frequency waves can be either horizontal (in a wavy plane parallel to the
ground) or vertical (in a plane perpendicular to the ground). The C- and L-band
antennas could send and receive waves of both horizontal and vertical
polarization. Using this "fully polarized" data, scientists can separate
out the scattering of radar waves to distinguish, for example, vegetated from unvegetated
terrain.
SIR-C/X-SAR wanted
to capture changes over time; that's why it flew on shuttle flights six months
apart. To observe the same supersites during both flights and to make
consistent daily passes over them, the shuttle crew used sophisticated
algorithms to navigate the spacecraft in precise orbits as close as 10 meters
apart. And they did this flying upside down, since the cargo bay holding the
instruments was on top of the shuttle. While the X-band antenna had a hinge,
the C- and L-band antennas were fixed at a particular angle, but they had "electronic
steering" that allowed them to "see" to either side of what was
right in front of them.
Those repeated,
slightly offset passes over the same terrain were essential for the
data-processing technique of interferometry. Combining views, interferometry creates
detailed, 3D topographical images of a target at the moment of simultaneous
observations. And it can reveal even minute changes in the target between
successive observations - like the gradual creep of an earthquake fault or the
movement of an ice sheet.
These ancient river channels, invisible to the human eye beneath the deep, dry sand of the Sahara Desert, were revealed for the first time by SIR-C/X-SAR instruments during their second shuttle flight in October 1994.
Credit: NASA/JPL-Caltech
Full image and caption
The SIR-C/X-SAR
dataset proved immediately useful, revealing, for instance, ancient riverbeds beneath
the Sahara - an artifact of preindustrial climate change - and remains in high
demand.
"SIR-C/X-SAR was the path opener for multiple U.S. and international missions
that followed," said Charles Elachi, the mission's principal investigator
before he became director of JPL. "Imaging of subsurface river channels in
the Eastern Sahara enabled new understanding of the environmental history of
that and other arid regions. Using multiple frequencies enabled for the first
time 'color' radar images that have been used extensively to map vegetation and
forests and extract their vegetation content. Using repeat-pass interferometry
enabled us for the first time to map surface motion at the centimeter level.
This technique is now commonly used to map motions resulting from earthquakes,
volcanic eruptions and other natural disasters."
Freeman agrees: "SIR-C/X-SAR was innovative on so many
fronts: We knew what we were working on was something special, but we didn't
know at the time how many firsts the mission would rack up".
The NASA
Image and Video Library makes mission data available to researchers worldwide. The
University of Michigan hosts a search tool for accessing its own vast SIR-C/X-SAR
database. And in its MapReady
tool the University of Alaska Satellite Facility has processed the data for
compatibility with multiple computer platforms.
Missions using
technologies pioneered by SIR-C/X-SAR have revealed changes in Earth's natural
features over increasingly meaningful periods, informing long-term policy to
prevent and mitigate climate change. At the same time, they reveal the immediate
effects of natural disasters rapidly enough to advise first responders.
Using the technique of interferometric SAR first demonstrated on SIR-C/X-SAR, four international space agencies cooperated to combine many years' worth of radar observations over the Greenland ice sheet to map its depth and accelerating speed of loss.
Credit: Courtesy of NASA/GSFC/Jefferson Beck
Larger view
SIR-C/X-SAR was
a collaboration of NASA, DLR and the Italian Space Agency (ASI), which contributed
to the ground segment for X-SAR observations. JPL managed the mission for NASA.
DLR was responsible for calibration, operations and data processing for X-SAR.
|
