Apr 15, 2014

Space Veteran Landsat 7 Marks 15 Years of Observing Earth

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Launched on April 15, 1999, the Landsat 7 satellite has now been observing Earth from outer space for 15 years. The Landsat program is a decades-long NASA and U.S. Geological Survey (USGS) partnership that has provided a continuous, unbiased record of change across the earth land surface since 1972.

As illustrated in this collection of Landsat 7 images, Landsat 7 provides a worldwide audience with objective views, both current and historical, of events and trends across the global landscape. Landsat data can be used to detect and monitor urban growth, forestry practices, the extent of floods, wildfire burn acreage, major natural or human-caused disasters, and many other important changes in land-surface conditions.

Landsat 7’s remarkable longevity has been vital to the majority of Landsat data users who require frequent imaging of specific areas for land and resource management. For example, water resource managers in western U.S. states need Landsat’s unique combination of thermal and vegetation condition readings at field scale to estimate water use more efficiently for crop irrigation ­— typically the major source of water consumption in these arid regions

Continuous data and more of it

Combined with Landsat 8, Landsat 7 ensures the collection of images across the entire U.S. every eight days (clouds permitting) and enables the collection of critical global imagery sets on a seasonal basis. Working in tandem, Landsat 7 and Landsat 8 together collect nearly 1000 images daily, almost double the amount of data collected when Landsat 5 and 7 were operating together. This increased data collection benefits all Landsat applications, especially in persistently cloudy areas (e.g. humid tropics and high latitudes) where multiple imaging attempts are essential.

A mechanical failure onboard Landsat 7 in 2003 reduces the amount of data in each scene by 22%. However, this loss does not affect the quality or usefulness of the remaining data. Many users simply treat the blank areas of each scene as if they were obscured by clouds. Landsat customers, especially those with agricultural interests, who require 8-day repeat data collection have a strong motivation to use Landsat 7 data even with the 22% loss per scene.

Barring the failure of any key spacecraft component, the remaining fuel on Landsat 7 is expected to permit imaging operations through 2017. NASA and the USGS are working together on a plan to ensure long-term continuity of land imaging operations while also addressing the near-term need to replace Landsat 7.

Seeing the world’s forests and the trees

An outstanding example of the scientific value of Landsat 7 imagery is the research published in 2013 by a team of scientists led by the University of Maryland. This team analyzed datafrom Landsat 7 to map changes in forests from 2000 to 2012 around the world at local to global scales.

Published in the journal Science, the study comprehensively described changes in the world’s forests from the beginning of this century, tracking forest loss and gain at the spatial granularity of an area covered by a baseball diamond (30-meter resolution). The uniform data obtained from more than 650,000 scenes taken by Landsat 7 ensured a consistent global perspective across time, national boundaries, and regional ecosystems.

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