Landsat 7

Landsat 7 is an Earth observation satellite launched as part of the long-running Landsat program, a joint initiative between NASA and the USGS. Its primary mission is to provide systematic, global, and repetitive imagery of the Earth's surface to monitor changes in land cover and support applications in agriculture, forestry, geology, and hydrology. The satellite continues the legacy of its predecessors, offering a crucial data stream for environmental science and resource management.

Overview

Landsat 7 was designed to extend the continuous, calibrated data record of Earth's land surfaces begun by Landsat 1 in 1972. It operates in a Sun-synchronous orbit, passing over the equator at approximately 10:00 AM local time to ensure consistent illumination for its optical sensors. The mission represents a collaboration between NASA, responsible for spacecraft development and launch, and the USGS, which handles mission operations and data archiving at the EROS Center. Its data policy, establishing a precedent for open access to remote sensing information, has been instrumental for global scientific research.

Mission and launch

The primary mission objective was to refresh the global archive of Landsat imagery with higher-quality data and ensure continuity from previous satellites like Landsat 5. NASA managed the development and launch, with the spacecraft built by Lockheed Martin based on the TIROS-N bus heritage. Landsat 7 launched successfully on April 15, 1999, aboard a Delta II rocket from SLC-2W at Vandenberg Air Force Base in California. This specific launch site was chosen to achieve the required Sun-synchronous orbit for optimal land imaging conditions.

Spacecraft and instruments

The spacecraft bus, provided by Lockheed Martin, is based on the proven TIROS-N design, providing power, attitude control, and communications. Its single primary instrument is the Enhanced Thematic Mapper Plus (ETM+), a significant upgrade from the Thematic Mapper on Landsat 4 and Landsat 5. The ETM+ collects data in eight spectral bands, including visible, near-infrared, shortwave infrared, and a panchromatic band with 15-meter resolution. A key improvement was the inclusion of a Scan Line Corrector to compensate for the forward motion of the satellite, ensuring geometric fidelity in the images.

Data and applications

Landsat 7 data is distributed freely by the USGS from the EROS Center, a policy that revolutionized the use of remote sensing for science and policy. Its imagery is fundamental for monitoring deforestation in the Amazon rainforest, tracking urban sprawl in cities like Las Vegas, assessing crop health for the USDA, and managing water resources in regions like the Colorado River Basin. Scientists also use its data for glaciology studies in Antarctica and Greenland, and for responding to natural disasters like the 2004 Indian Ocean earthquake and tsunami.

Operational history

Following its successful launch and checkout, Landsat 7 began nominal operations, providing high-quality data that seamlessly joined the archive from Landsat 5. However, on May 31, 2003, a permanent hardware failure in the Scan Line Corrector mechanism occurred, causing systematic data gaps in all subsequent images. Despite this anomaly, the satellite remains operational in a "sweet-spot" mode, and the data, though flawed, is still widely used with specialized correction algorithms. It far outlived its design life, providing a critical bridge until the launch of its successor, Landsat 8, in 2013, and continues to collect data in tandem with newer satellites like Landsat 9.

Category:Earth observation satellites of the United States Category:NASA space probes Category:Spacecraft launched in 1999