By Ian Dowman, professor emeritus, University College London, and author of High-Resolution Optical Satellite Imagery, available from Whittles Publishing (www.whittlespublishing.com) and in North America from CRC Press (www.crcpress.com).
The acquisition of high-resolution images from satellites has developed at a rapid pace since GeoEye launched Ikonos in 1999. The major drivers for such advancements have been improved technology and government policy. Technology has made it possible to make more agile, smaller satellites, and government policy has encouraged the development of high-resolution sensors for defense applications, allowing the data to be used commercially and encouraging the development of space programs to achieve national policy objectives.
Although improved resolution and greater accuracy in positioning are desirable, they’re expensive, and currently all government funding is carefully scrutinized. Because of the nature of the industry, the future of high-resolution satellite imagery is linked to government funding and may suffer as a result of cutbacks.
So what’s the current status of high-resolution commercial sensors? In short, resolution is slowly increasing, and the time between which images can be acquired is decreasing. Also, there’s a rapidly developing small-satellite market, with sensors moving toward resolution that’s similar to larger systems but with limited imaging capacity.
The market for high-resolution imagery is expanding. Although current systems are able to satisfy the demand, the use of high-resolution commercial systems is limited by restrictions imposed by the U.S. government. Today, Earth observation systems, including the ground stations, can be ordered almost off the shelf. The price of components is lower these days, and the components are becoming more compact. All this reduces the total cost.
Nevertheless, large mirror telescopes are required for a ground resolution of 1 meter or less. Increased imaging capacity requires sufficient on-board storage capacity, as well as considerable satellite agility. Downlink capacity is still the bottleneck, requiring highly developed image-compression techniques.
However, such limitations don’t seem to affect the demand for satellites for national and regional use, and there are an increasing number of systems designed and operated to serve national requirements. Commercial operators also use small satellites, such as the RapidEye constellation. Resolution is increasing, with Surrey Satellite Technology Ltd. (SSTL) now producing a small satellite with 1-meter ground sample distance.
The use of constellations is also enabling shorter revisit times to almost any point on the planet. There are also some exciting radar developments, with systems such as RADARSAT-2, TerraSAR-X, TanDEM-X and COSMO-SkyMed already launched and small radar sensors being developed by companies such as SSTL.
Image processing software is well developed and operational within most digital photogrammetric software packages, requiring minimal understanding by operators. Ortho-images and digital elevation models (DEMs) can be produced easily because of good matching software and the ability to overlay an image onto a DEM. Several companies offer value-added products such as orthoimages, and many offer services such as agricultural monitoring and change detection.
Although high-resolution satellite imagery has proved valuable for many applications, including mapping, monitoring and surveillance, such imagery is vital for disaster management. As demonstrated many times in recent years, the timely, accurate, high-resolution data provided by satellite imagery operators for disaster response have saved countless lives. Also impressive is how quickly satellite operators can work together to acquire and deliver such valuable data.
Future Missions and Sensors
There are several satellites in various planning stages. For example, GeoEye’s GeoEye-2 and DigitalGlobe’s WorldView-3 will be similar to their predecessors but with higher resolution, greater coverage and additional sensors. The satellites are also more agile than ever. Pléiades, recently launched by Astrium, can image a 100-km × 200-km area in a single pass, viewing about 10 targets with its sensors pointed less than 20 degrees off track; image location accuracy will be 3 meters without ground control. India will continue its program with Cartosat-3, with a planned 0.33-meter GSD, as will South Korea with KOMPSAT-3 and the Japan Aerospace Exploration Agency with ALOS-3.
The growing market in small satellites is likely to provide more satellites in the future. SSTL offers a range of satellites.
Although the company is primarily concerned with building and operating satellites, images are sold through DMC International Imaging, which operates satellites owned by China, Nigeria, Algeria and the United Kingdom. SSTL is able to keep the price down by producing systems based on experience, as well as using off-the-shelf components without offering, for example, high positioning accuracy.
SSTL is planning a new constellation that will be operated by a single organization and will include optical and radar sensors. One possible scenario for such a system is for wide-swath optical sensors with a morning overpass to image possible targets and follow up in the afternoon with a radar sensor, with a possible 3-meter resolution, to provide high-resolution imagery independent of cloud cover.
Organizational and Administrative Issues
There are limits on hardware development and limits imposed by government regulation. Companies in the United States can’t distribute products with a GSD less than 0.5 meters. Although sensors are collecting data at higher resolutions, such data are only available to government agencies. Other countries, such as India, restrict distribution of high-resolution data of their country. If resolution was to improve further, additional restrictions could be applied.
Governments also influence the cost of data. U.S. companies are contracted to provide data, often as a priority, for defense purposes; this acts as a subsidy, which enables the development of better sensors. For example, the cost of absolute positioning to better than 15 meters is high, but this is possible with the U.S. commercial sensors, as well as with RapidEye, Pléiades,SPOT-6 andSPOT-7. There’s already competition between satellite imagery and aerial imagery at 0.5-meter to 1-meter GSD, and the competition would increase if higher resolution satellite data were to become available.