High-resolution satellite imagery, previously restricted to intelligence organizations, is about to make a commercial debut. In the next two years, U.S. companies plan to launch satellites carrying sensors capable of recording and transmitting images of the earth's surface with one-meter resolution. Fully processed images will be precise enough to meet U.S. accuracy standards for 1:2400 scale mapping (1.5-meter positional accuracy) -- detailed enough to spot a motorcycle from 400 miles out in space.
Before the end of the decade, providers will downlink, process, and distribute black and white, color, infrared and stereo pairs to customers via the Internet, all within hours or days of an order, depending on the product requested. Both special and off-the-shelf imagery will be compatible with GIS, CAD and desktop mapping systems, and will run on PCs having sufficient memory and disk space to handle map files.
Customers will have the option of buying fully processed images ready for GIS/CAD integration or processing the raw data on their own. Knowledgeable buyers with established accounts will shop for satellite imagery at vendors' Web sites by simply entering the coordinates of an area and providing accompanying information.
Before the end of the decade, affordable, high-resolution, space-based imagery will undoubtedly find application in nearly all levels of city and county government, as well as across the entire spectrum of U.S. commerce. The combination of domestic and foreign competition may put the cost of high-resolution imagery well below that of aerial photography.
Until recently, the same circumstances that blocked commercial development of space-based imagery in the United States effectively limited the image resolution of satellites owned by foreign companies. As of this writing no image-recording satellites are owned by U.S. companies, nor can any commercially available satellite imagery match the spatial resolution and positional accuracy of aerial photography. All that, however, is soon to change.
Spot Image, S.A., of Toulouse, France, began producing 10-meter resolution imagery from its own satellites as early as 1986, and today is a major supplier in this market. Other foreign companies quickly followed -- the Japanese JERS satellite, providing 20-meter resolution; Europe's remote sensing ERS-1 and ERS-2 satellites offering 10-meter resolution; and India's IRS-1C, with 5-meter resolution -- all with black & white imagery.
With even Russia selling 5- and 8-meter resolution from their KFA-1000 and MK-4 satellites, respectively -- and on very rare occasions, letting go of 2-meter military images -- security concerns began falling through the cracks. In 1992, Congress passed the Land Remote Sensing Act, allowing private companies to launch satellites with sensors capable of producing 3-meter resolution. In 1994, the Clinton administration bumped that to 1-meter resolution.
Before looking at currently available satellite imagery, it might help to review some concepts and typical applications associated with this technology.
Specific wavelengths of light recorded by orbiting sensors not only provide different levels of detail but different information. Panchromatic, or pan, (black and white) has a much higher resolution than multispectral color imagery. Pan sensors capture a broad spectrum of light in a single measurement in which each pixel (smallest picture element that can be individually addressed) is assigned a specific gray-scale value. In 1-meter resolution imagery, each pixel represents an area of about three feet.
Multispectral imagery is made up of three or more bands -- blue, green, red, infrared, and parts of the thermal spectrum. Each band is separately recorded, processed and integrated into a composite color image. Since pan delivers considerably higher resolution, it generally costs more than color.
Ten-meter pan imagery, presently a commonly available product, has applications in regional, county and urban planning; updating road maps, looking at subdivisions, individual houses or buildings, mapping street centerlines and evaluating site proposals. To make map information more understandable as well as attractive, pan imagery is often overlaid with color. The process causes only a slight reduction of the higher resolution of the pan image.
The applications for multispectral imagery depend to some extent on the bands the sensor is directed to measure:
Spectral modes of green, red and infrared are effective for determining the health and vigor of vegetation, the chlorophyll content, and the early detection in trees and forests of such diseases as pitch canker and Dutch elm disease.
Measurements in the near- and middle-infrared range reveal moisture, which in turn indicates the cell structure of the leaf or plant.
"Almost anything affecting vegetation health and vigor," said EOSAT director of Strategic Marketing, Tina Cary, "can be monitored and analyzed using multispectral data."
Some satellites have pan sensors that can be directed to record an image from two slightly different angles to provide stereo pairs. With one of the higher-quality processing software packages such as ERDAS Imagine, PowerScene, or TNTmips, users can view the pairs in 3D.
Applications include land-cover analysis and terrain modeling, providing elevations, sighting and routing pipelines and roads, and determining slopes, angles and inclines. The same software packages enable users to do virtual "fly-overs," viewing features and structures of the terrain model from different angles.
Satellite radar has specialized applications. Unlike optical sensors which passively measure light energy reflecting from objects, radar transmits microwave energy, a portion of which is reflected back from objects, conveying information about their size, depth, texture and reflectivity. Since radar is not limited to daylight or to clear skies, it is highly effective in applications such as tracking oil spills at night, monitoring ice in shipping lanes, detecting solid surfaces beneath ground cover such as sand or vegetation, or resource mapping typically cloud-covered tropical regions to determine the status of forests.
Methods used for processing radar imagery are different from those used for optical data. According to Coleen Hanley of Spot Image, the cost of radar imagery varies with the provider, and from the European Space Agency(ESA), it is less than most optical imagery.
Satellite imagery from the U.S. government and from France, India, Japan, Russia, and other European countries are currently available through U.S. subsidiaries, distributors, aerial photography companies and former aerospace firms. A cross-section includes Atterbury Consultants; EarthWatch; EOSAT; Hammon, Jensen, Wallen & Associates; SPOT Image; and UGC Consultants.
EOSAT, the U.S. firm that acquired rights to operate NASA Landsat satellites in 1986, produces 30-meter multispectral products from the ThematicMapper sensor. Since Landsat sensors measure bands from blue through infrared and into thermal wavelengths, ThematicMapper data is used in monitoring a wide variety of conditions associated with vegetation.
India's IRS-1C satellite produces 5.8-meter resolution pan imagery suitable for community level applications. The same satellite carries a 23-meter multispectral sensor. EOSAT is the exclusive provider of IRS data outside India, and the data is available through the firm's distributor, Atterbury Consultants. "IRS products are very reasonable in price," said Atterbury's Bob Wright. The company recently began taking orders for IRS products.
Russian imagery is produced from scanned film. The resolution is 8-meters for the MK-4 satellite, and 5-meters for the KFA-1000. Some MK-4 imagery is in color. The processing for film-based images is more involved and slightly more expensive than for digital imagery. The problem with Russian satellite imagery, Wright said, is getting it. "It's available on a catch-as-catch-can basis." Atterbury has 1995 Russian imagery of California, Nevada, New Mexico, Texas and Oklahoma. "That is about the most recent for North America; it's mostly archived KFA-1000 imagery, comparable to India's IRS-1C, and slightly better than SPOT's 10-meter imagery."
SPOT Image Corp., the U.S. subsidiary of SPOT, distributes panchromatic and multispectral data with varying degrees of image processing for a variety of applications. The highest-resolution imagery SPOT produces is 10-meter panchromatic. Products include SPOTView, a highly processed, geocoded, digital orthoimagery; and SPOT MetroView, pre-packaged 15x15-minute image sets of selected U.S. cities. MetroView is formatted for most GIS and desktop mapping systems, and is available in five map projections. Pan imagery of any entire U.S. state goes for 36