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Since the Soviet Union launched Sputnik-1 in October 1957, satellites have become part of the infrastructure of essential services on which much of the developed world depends. The core satellite-dependent activities are telecommunications, navigation, meteorology and Earth observation (EO). Although the first three applications have large, economically viable markets, EO has been the slowest to become financially sustainable.

EO applications are becoming increasingly important for monitoring Earth's rapidly changing environment. Although EO hasn't yet achieved its full financial heights, there are plenty of EO satellites in orbit, operating a wide range of active and passive sensors, with many different resolutions and spectral bands. Moreover, for 35 years the U.S. government's Landsat series has been the mainstay of remote sensing and generated a strong awareness of the value of satellite imagery for mapping the changing nature of our planet. Now high-resolution satellites from DigitalGlobe (www.digitalglobe.com) and GeoEye (www.geoeye.com), partially funded by U.S. government surveillance contracts, also stimulate the growing Internet-wired population to peer with wonder at the world via Google Earth and to satisfy their curiosity about their neighbors' backyards.

Opening Commercial Markets
A key reason for the slow development of EO applications is that most of the commercial markets, such as precision agriculture, need frequent data to build an operational service. EO applications have had to fit in with the capabilities of whatever satellites were readily available, which means remote sensing applications tend to use high-quality sensors with a lot of specialized spectral bands on big expensive satellites that only revisit an area of interest every couple of weeks.

To satisfy customers who want to monitor fast-changing assets such as crops, satellites must be able to view large areas of the world reliably every day at an appropriate level of detail. Farmers, for example, don't care about the limitations of orbital dynamics, but they know exactly which day was clear and sunny last week. They don't see why satellites can't always provide a timely picture that details how their crop is growing in each part of their field.

Daily revisit imaging, with enough resolution to be useful, can only be supplied by a constellation of several satellites. But the cost of multiple satellites has been, until now, unaffordable—even to the industrialized nations. Fortunately, developments in small satellites now make it economically feasible to launch constellations of many EO satellites at a base cost within the reach of individual companies.

To obtain images with a useful resolution, a satellite must be in a low Earth orbit—about 600-800 kilometers above the planet, orbiting the world 14 times per day. Typical EO satellites are in a sun-synchronous polar orbit that allows a satellite to cover the whole planet and capture images with a constant angle of illumination. However, the imaging system's resolution determines the width of the image that can be obtained, and this in turn determines how many days it takes to revisit a target area. Landsat 5, for example, with a swath width of 185 kilometers,

DMC has proved its worth in rapidly responding to disasters, but the biggest impact has been to stimulate new commercial applications

has a revisit of 16 days, which has limited the development of widespread operational applications in rapidly changing markets such as agriculture. This limitation has been removed by the development of a new generation of low-cost EO satellites by Surrey Satellite Technology Ltd. (www.sstl.co.uk), which has enabled the launch of an operational constellation designed for daily repeat imaging.

The First EO Constellation
The international Disaster Monitoring Constellation (DMC) provides a new EO paradigm, with four satellites carrying the same wide-swath optical sensor and coordinated in orbit to achieve a daily revisit capability anywhere in the world.

The first DMC satellites, each with a mass of less than 100 kilograms, carry a six-lens imager array that provides 650-kilometer-wide swath images in three spectral bands at 32-meter ground sample distance (GSD). Four satellites working together in phased orbit can achieve daily revisit rates almost anywhere in the world. The latest version has increased capacity and carries a 4-meter panchromatic imager with agile off-pointing capability.

Each satellite is owned and operated by a different nation. By working together in space each DMC Consortium Member gains a national EO capability and benefits from the strength of participating in a constellation, using the extra capacity on each satellite for commercial applications.

The constellation is coordinated by DMC International Imaging Ltd. (DMCii), which runs commercial imaging campaigns and provides a free disaster response service to the world. Through DMCii, the DMC Consortium has joined major space agencies in the International Charter (Space & Major Disasters) to provide a unified system of space data acquisition and delivery to those affected by natural or man-made disasters. DMCii regularly takes its turn as Emergency On-Call Officer (ECO), coordinating the DMC satellites as well as those of Europe, Canada, India, Argentina, the United States, Japan and China.

New Opportunities for Time-Sensitive Applications
One of the most challenging commercial applications, precision agriculture, uses the DMC to image large agricultural areas at specific growth points. With daily repeat capability users can have reasonable confidence of obtaining mainly cloud-free data, despite the short imaging windows required. The 32-meter GSD imagery enables detailed measurement of leaf area index within each field and the specification of precise fertilizer application patterns for each crop. Precision agriculture is also driving the development of RapidEye (www.rapideye.de), the first commercial agricultural constellation, which was launched in August 2008.

The success of coordinated constellations also can be seen in DMCii's four years of repeat contracts for monitoring deforestation across the entire Amazon Basin for Brazil. Mapping such cloudy regions demonstrates the potential to more regularly monitor other vast areas of tropical rainforest in Africa and Indonesia, which are under serious threats from logging, agriculture and expanding human settlements. Cross-border monitoring of precious natural resources provides the objective information on which good governance can be developed.

The next few years will prove interesting as the business potential of constellations for remote sensing is fully realized

Recently the DMC scored a new first when it successfully imaged 38 countries in Europe within tight time constraints during 2007. This was achieved within the different imaging windows specified by each country, resulting in the first high-resolution image set of Europe for the Global Monitoring for Environment and Security (GMES) fast-track Land Monitoring service. Achieving this in the wet cloudy summer of 2007 was only possible by coordinating the constellation's daily revisit capability.

With Europe's GMES program kicking off in earnest in 2009, the DMC aims to contribute its rapid-response, wide-area monitoring capability to support services ranging from land cover mapping to emergency response and food security in other parts of the world.

Constellations Gain Momentum
The five-satellite DMC, which altogether cost just $100 million in orbit, paved the way for a new paradigm in remote sensing. DMC has proved its worth in rapidly responding to disasters, but the biggest impact has been to stimulate new commercial applications in the remote sensing industry by providing reliable daily revisit imaging, and to show that constellations can be affordable and functional.

The success of Surrey Satellite Technology's low-cost small satellites has enabled Germany's RapidEye GmbH to commission the five-satellite commercial RapidEye constellation as a core part of its business plan to deliver precision agriculture services. The next few years will prove interesting as the business potential of constellations for remote sensing is fully realized. In coming years, it's likely we'll look back and wonder how we managed without satellite constellations to keep our precious planet under daily scrutiny.

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