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Interferometry—Cutting Edge
European ERS satellites at cutting edge of imagery applications
By Kevin P. Corbley Generating digital elevation models (DEM) from synthetic aperture radar imagery using a process called interferometry is one of the hottest trends in mapping right now. Much of the interest in this new technology has been fueled by the availability of inexpensive image pairs from the vast archive of Europe's ERS satellites.
DEMs created with this new technique already have been successfully applied in the oil & gas and telecommunications industries, and they are showing exceptional promise for civil engineering and seismological applications as well.
The use of ERS imagery as primary data sets in so many commercial projects may come as a surprise to some experienced remote sensing users, especially in North America, who remember the European Space Agency (ESA) launching its first radar satellite, ERS-1, as a research mission in 1991. "In just a few years, ERS radar imagery had proven its value in hundreds of scientific projects and was routinely used in commercial applications around the world," said Giorgio Apponi, business development director for Eurimage, the private Italian company responsible for ESA data distribution.
In North America, where ERS has not been promoted as heavily as in Europe, demand for its synthetic aperture radar (SAR) imagery has begun to grow for use in interferometric DEM generation, fusion with optical satellite imagery and other traditional remote sensing applications. Those uses will continue to increase as customers in the United States and Canada soon find ERS data easier to obtain and use.
With the launch of ESA's next-generation SAR satellite, Envisat, scheduled for late next year, Eurimage has initiated a major campaign to expand the North American market. A 10,000 square kilometer ERS SAR scene costs approximately U.S.$1,000 - or about 10¢ per square kilometer. The combined archive of the ERS-1 and -2 missions contains nearly 2 million images covering almost the entire Earth land surface dating back to 1991. There are at least 125,000 image pairs in the archive available for DEM generation. These are true tandem pairs acquired by ERS-1 and -2 only 25 hours apart.
Flexibility is the other primary selling point of ERS data. In addition to being ideally suited for interferometry and data fusion -- two of the most popular image processing techniques in use today -- imagery from the European satellites also performs well when used alone in traditional SAR applications, such as mapping deforestation, wetlands, and topographic structure. Applying Interferometric Techniques
Interferometry is a computer-intensive technique that requires two SAR satellite images acquired over the exact same area on different dates. Based on slight phase differences in the radar signals of the two images, the computer calculates terrain elevation for the terrain. This DEM information is usually more accurate and less expensive than elevation data extracted from traditional stereo-pair methods.
"The ERS program is very well suited to doing interferometry and extracting DEMs," said Dennis Nazarenko, Vice President of Business Development for Atlantis Scientific, a SAR value-added company based in Nepean, Ontario, Canada, that has refined the interferometry technique to an operational level.
Nazarenko explained that the two ERS satellites can be programmed to work in tandem, acquiring images over the same spot within 25 hours of each other. This is extremely advantageous in interferometry because any surface changes that occur on the ground between acquisitions detracts from DEM accuracy. A high degree of accuracy is virtually assured with tandem ERS images because variations in vegetative cover or soil moisture are minimal over a one-day period.
"At times when one ERS satellite is available for data acquisition, the repeat cycle is only 35 days, which is still very good for interferometry in many environments," said Nazarenko.
One of the key client groups purchasing Atlantis Scientific DEMs is the telecommunications industry. Wireless telephone companies use the elevation data to forecast how radio frequency signals carrying cellular phone calls will travel across the terrain of a service area. Topographic peaks and valleys can disrupt the cell signal, dropping calls and creating static.
Accurate mapping of elevation points enables the companies to precisely locate cellular relay antennas so that transmission quality is maintained throughout the cell site area. Atlantis Scientific routinely provides its telecommunications customers with ERS-derived DEMs having five to 15-meter vertical accuracy.
"The availability of a global ERS archive has enabled us to respond very quickly to telecomm requests for DEMs in South America and Asia," said Nazarenko. "And the relatively low cost of the raw data allows us to sell completed DEMs at only $2-$3 per square kilometer. That's very competitive."
Atlantis Scientific has also taken the lead in perfecting a technique called differential interferometry which is showing considerable promise for measuring very subtle surface elevation changes. Differential interferometry involves the comparison of changes in the radar signal between two or more pairs of SAR images separated in times by days, months or years. The difference in the signal properties can be correlated to changes in the Earth's surface to within millimeters.
"One of the areas where this technology is receiving significant interest is among our petroleum industry clients where it is being applied to studies of oil field subsidence," said Nazarenko. "In areas where subsidence is substantial, there can be impacts on production, property and surface drainage, all of which are undesirable and can prove costly to remediate."
Subsidence monitoring with ERS data has not been limited to oil fields. Atlantis Scientific has formed an alliance with a Canadian civil engineering firm to develop the technology for urban and transportation applications where slope stability is an issue. Similar projects are ongoing in Europe, confirmed Andrea Celentano, Eurimage ERS Help Desk Officer.
"Land movement related to sink holes, earthquakes and even volcanoes can be detected with ERS interferometric techniques," he said.
The National Council of Research in Italy, for instance, has been measuring subtle changes in the landscape around the Mount Etna and Mount Vesuvius volcanoes to predict future eruptions. In theory, the volcano cone should expand and contract prior to erupting. Seismologists are feeding ERS DEMs into a GIS modeling package to correlate ground movement with volcanic events. Merging SAR with Optical
Digitally fusing an ERS SAR image with a multispectral scene from an optical satellite such as Landsat, SPOT or IRS highlights the strengths of each data set, providing the end user with a complete picture of the surface terrain and land cover in the study area.
"We have had tremendous results merging ERS imagery with Landsat data for an oil & gas client in Brazil and a mining company in Colorado," said Mark Lucas, Chief Technical Officer for ImageLinks, Inc., an image processing firm in Melbourne, Fla. "In these cases, the fused product provided a more comprehensive understanding of the area's geology."
SAR and optical data complement each other in geologic applications. The Active Microwave Imaging sensors aboard ERS operate with a 23 degree incidence angle which means the radar signal strikes the ground obliquely, illuminating the peaks, valleys, folds, lineaments and faults that an off-nadir optical sensor might not see. On the other hand, however, the multispectral imagery identifies the mineral content, soil condition and rock composition of the surface geology, characteristics not typically discernible in SAR data.
"Together, the terrain features and surface lithology provide a much better visualization of the subsurface geology than either image data set could by itself," said Lucas. "Our clients in mining and oil exploration use this information to prioritize where to conduct soil sampling or test drilling in the search for oil, gas and ore deposits."
Another reason ERS data appeals to ImageLinks in these projects is the accuracy of the data. The ERS satellites are known for traveling in very precise orbits and providing mission controllers with extremely exact location data, referred to as ephemeris. This information is critical to accurately georectifying satellite imagery when ground control points are not available.
"Many of our projects are in South America where there are no maps for ground control, but we are able to rectify our ERS images to within 100 meters using just the ephemeris," said Lucas.
Change detection related to post-disaster monitoring is another application area where ImageLinks finds ERS and optical data equally useful.
"An archived optical image acquired on a clear day provides a good basemap for detecting damage that might result from a flood, hurricane or tornado," said Lucas. "An ERS image acquired after the event can be quickly overlaid on the optical image to provide a visual reference of the change."
SAR imagery provides the most reliable post-disaster assessment because of its guaranteed acquisition. The clouds, rain and smoke that often accompany floods, hurricanes and forest fires do not impede image collection by the SAR sensors aboard the ERS satellites.
ImageLinks recently initiated a project in cooperation with the University of Central Florida to create a databank of scenes for use in natural disaster situations. The company plans to develop operational techniques to compare archived optical images with post-event ERS scenes to help direct emergency relief efforts and clean up in the wake of calamities. New Applications Coming
Although North American clients have embraced ERS for use in cutting edge projects involving interferometry and data fusion, imagery from the European satellite has proven its value in what are considered traditional SAR applications, said Luciana Di Domenico, ERS Business Manager at Eurimage.
"Forestry, wetlands mapping, environmental monitoring and petroleum exploration are the disciplines where ERS has built a significant clientele," she said.
The Joint Research Centre of the European Union and several private firms have utilized ERS data to map forest stands and to track deforestation around the world. Major oil companies rely on ERS data to spot oil slicks that could indicate damaged subsea pipelines or valuable hydrocarbon reserves. And the Food and Agriculture Organization of the United Nations routinely obtains ERS scenes to monitor land development worldwide and ensure there will be enough arable lands to sustain future generations.
Eurimage says the list of ERS SAR applications will grow as additional remote sensing customers become more familiar with the data and utilize it in new ways. Through Global Marketing Insights Inc., of Cleveland, Ohio, Eurimage plans to develop an applications provider network of value-added services companies in North America to assist in developing new operational uses for the data and implement their U.S.-based marketing plan.
"In many ways, we have just scratched the surface of what can be accomplished with ERS SAR data," said Di Domenico. "We expect the uses of radar imagery will expand even further when ESA launches Envisat next year. About the Author:
Kevin Corbley is a consultant and freelance writer specializing in remote sensing, GIS and GPS. He is located in Denver, Colorado, and may be reached at 303-722-0312 or by email at [email protected]. Back |
