Lisa Jo Rudy, Science Editor
Terra is a multi-national, multi-disciplinary satellite mission involving partnerships with the aerospace agencies of Canada and Japan. Managed by NASA’s Goddard Space Flight Center, the mission also receives key contributions from the Jet Propulsion Laboratory and Langley Research Center. The Terra spacecraft houses five major instruments; three of these – ASTER, MODIS and MISR – have been actively supporting the process of visualizing the disaster in Japan. Together with commercial and international satellite images, Terra instruments are measuring and visualizing earthquake and tsunami damage, and may even provide visualization of heat created by the damaged Fukushima Daiichi nuclear reactors.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) obtains high-resolution (15 to 90 square meters per pixel) images of the Earth in 14 different wavelengths of the electromagnetic spectrum, ranging from visible to thermal infrared light. ASTER data provide researchers with the tools to create detailed maps of land surface temperature, emissivity, reflectance, and elevation. ASTER is the only Terra instrument capable of being directed to a specific location, and while its imaging systems cover a swath of only 60 kilometers, its high resolution provides visualization of objects as small as an individual house.
ASTER has, from its inception, been sponsored by both the U.S. and the Japanese Ministry of Economy, Trade and Industry (METI), so collaboration between the two nations regarding its use and findings is straightforward and productive. Says Marc Imhoff, Terra Project Scientist, “We have been working with the Japanese as much as possible given power outages and evacuations. Through ASTER, we’re able to use optical data to assess how much and where the damage is. We can actually evaluate the coastlines of the entire country.” Imhoff explains that ASTER’s relatively fine resolution is ideal for covering significant swaths of coastline with sufficient resolution to make policy decisions: “Space viewing lets you get a good overview of the damage – not down to the level of individual homes, but a general feel for inundation and severe damage.”
The Japanese have scheduled ASTER’s next pass along the eastern coast, over Fukashima. It will be able to detect excess heat from damaged nuclear power plants. Of course, says Imhoff, “Clouds are a problem. And, naturally, Terra doesn’t offer continuous viewing – so if an event is ephemeral you may miss it.”
The Multi-angle Imaging Spectroradiometer (MISR) views the sunlit Earth simultaneously at nine widely spaced angles, providing ongoing global coverage with accurate measures of brightness, contrast, and color of reflected sunlight. Of particular importance for the Japan event, MISR’s multiple camera angles provide stereoscopic images, allowing viewers to distinguish and measure the height of plumes of smoke and aerosols.
MISR covers a swath of about 400 kilometers during each pole-to-pole orbit, allowing the instrument to visualize Japan approximately twice a week. Says David Diner of JPL, Principal Investigator for MISR, “The most relevant way to use MISR involves stereoscopic views provided by its nine cameras. We’re able to visualize and determine smoke and ash plume injection heights, and researchers can use numerical models to understand dispersal of the airborne particles.”
Diner also notes that MISR’s ability to distinguish among surfaces has been valuable in visualizing tsunami-related inundation. “On [the Saturday after the tsunami] we observed the scene using false color imaging, using the near-infrared to compare the relative locations of vegetation and water. In addition, one of the angles looking forward observed glint reflection off the water, enhancing the brightness of water-covered areas significantly. Using these data we quickly saw that areas that had been land were now inundated, and inhabited areas were now washed out. It was stunning to see the extent of devastation from space.”
Moderate Resolution Imaging Spectroradiometer (MODIS) is housed on the Terra satellite, and an identical instrument is housed aboard the Aqua platform. Terra’s orbit around the Earth is timed so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS view the entire Earth’s surface every 1 to 2 days, acquiring data in 36 spectral bands.One
MODIS is the instrument on Terra that is most often used for situations like this because it images once daily, providing 250 meter to 1 kilometer broad scale mapping. Explains Jeff Schmaltz of the MODIS Rapid Response Team, “MODIS does provide daily views, so rapid response imagery is called upon a lot for things like weather, oil spills, and earthquakes. The rapid response mechanism was set up early on in the mission; both Terra and Aqua are part of rapid response. We collect info twice a day, once at night, from each satellite (though night data sets are not usually funneled through rapid response).”
Together, the three Terra-based instruments create a full, multi-layered, time-sensitive picture of the Japanese situation. MODIS provides daily imaging of inundated areas and plumes with relatively low resolution. By using daily images from MODIS, researchers can appropriately identify areas for additional study with MISR’s nine-camera stereoscopic and multiangular capabilities. MODIS also provides views of the entire coastline, while ASTER focuses in with higher resolution on the more critical areas. Terra images also are the source of derived products that provide additional information. Satellite data can, for example, provide pre and post-event imagery that supports the development of flood maps or maps showing the progress of aerosols from fires.
Global Differential GPS (GDGPS)
In addition to satellite-based imagery, NASA is also developing computer models using real-time GPS data to predict and manage events such as earthquakes and tsunamis. Y. Tony Song and his colleagues at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., in collaboration with scientists from University of Nevada at Reno, are now in the process of completing a prototype system to predict the movement and size of a tsunami. The Global Differential GPS system, managed by JPL, combines global and regional real-time data from a network of hundreds of GPS sites and estimates their positions every second. According to a JPL press release, “It can detect ground motions as small as a few centimeters…. [Following last year’s Chilean quake], researchers used real-time data from the agency’s Global Differential GPS (GDGPS) network to successfully predict the size of the resulting tsunami.”
Song explains how the system works: “If you know how fast the land is moving, you know how fast the water is moving because the water is pushed by the land. Using this information, we can estimate the energy transferred to the ocean and scale the size of the resulting tsunami. We tested the system successfully in the Chile earthquake last year and we tested it this time, but our real-time GPS data was damaged during the earthquake. We have two real-time sites; both near Tokyo and mounted on tall buildings. Both sites experienced nearly simultaneous intermittent short data gaps (of a few seconds). The data gaps caused the positioning solution to be inaccurate. It should be pointed out that these prototype sites were not initially designed for the purpose of tsunami detection. However, we are fortunate that other GPS instruments, belonging to Japan but not in our system, recorded the earthquake. We are using those after-event data to retest our system.”
Song explains that GPS data near an epicenter directly measure ground motion using a system that is completely separate from the usual Richter system used to measure earthquake magnitude. The system’s extreme sensitivity – it can sense a 1 centimeter shift in the Earth – allows users to infer how much the ocean will move, directly measuring tsunami size. “Japan has the best tsunami warning system,” says Song, “With the GPS system, we can make the warning system even better.”
In the future, Song and his colleagues believe a combination of satellite and ground sensing will be the key to detecting tsunami formation early enough for advanced warnings and give people more time to move to higher ground. In addition, Terra images provide information about the coastal topography, which can be used to improve inundation maps for effective evacuation. With GPS and related land surface instruments and ocean buoys, researchers also will have the tools in hand to predict when and where a tsunami will strike – and how significant the wave will be when it arrives.