Landsat 7
Landsat 7 before launch
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| Mission type | Earth imaging |
|---|---|
| Operator | USGS / NASA |
| COSPAR ID | 1999-020A |
| SATCAT № | 25682 |
| Mission duration | Elapsed: 17 years, 9 months and 25 days |
| Spacecraft properties | |
| Bus | TIROS-N |
| Manufacturer | Lockheed Martin Space Systems |
| Dry mass | 2,200 kilograms (4,900 lb) |
| Power | 1550 W |
| Start of mission | |
| Launch date | 15 April 1999, 18:32:00 UTC |
| Rocket | Delta II 7920 |
| Launch site | Vandenberg SLC-2W |
| Contractor | Boeing |
| Orbital parameters | |
| Reference system | Geocentric |
| Regime | Sun-synchronous |
| Perigee | 701 kilometers (436 mi) |
| Apogee | 703 kilometers (437 mi) |
| Inclination | 98.2126 degrees |
| Period | 98.83 minutes |
| RAAN | 234.2660 degrees |
| Argument of perigee | 78.1806 degrees |
| Mean anomaly | 281.9541 degrees |
| Mean motion | 14.57108304 |
| Repeat interval | 16 days |
| Epoch | 11 June 2016, 18:59:03 UTC |
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Landsat 7 is the seventh satellite of the Landsat program. Launched on April 15, 1999, Landsat 7's primary goal is to refresh the global archive of satellite photos, providing up-to-date and cloud-free images. The Landsat Program is managed and operated by the USGS, and data from Landsat 7 is collected and distributed by the USGS. The NASA World Wind project allows 3D images from Landsat 7 and other sources to be freely navigated and viewed from any angle. The satellite's companion, Earth Observing-1, trails by one minute and follows the same orbital characteristics. Landsat 7 was built by Lockheed Martin Space Systems Company.
Landsat 7 was designed to last for five years, and has the capacity to collect and transmit up to 532 images per day. It is in a polar, sun-synchronous orbit, meaning it scans across the entire earth's surface. With an altitude of 705 kilometers +/- 5 kilometers, it takes 232 orbits, or 16 days, to do so. The satellite weighs 1973 kg, is 4.04 m long, and 2.74 m in diameter. Unlike its predecessors, Landsat 7 has a solid state memory of 378 gigabits (roughly 100 images). The main instrument on board Landsat 7 is the Enhanced Thematic Mapper Plus (ETM+).
On May 31, 2003 the Scan Line Corrector (SLC) in the ETM+ instrument failed. The SLC consists of a pair of small mirrors that rotate about an axis in tandem with the motion of the main ETM+ scan mirror. The purpose of the SLC is to compensate for the forward motion (along-track) of the spacecraft so that the resulting scans are aligned parallel to each other. Without the effects of the SLC, the instrument images the Earth in a "zig-zag" fashion, resulting in some areas that are imaged twice and others that are not imaged at all. The net effect is that approximately 22% of the data in a Landsat 7 scene is missing when acquired without a functional SLC.
Following the SLC failure, an Anomaly Response Team (ART) was assembled, consisting of representatives from the USGS, NASA, and Hughes Santa Barbara Remote Sensing (the manufacturer of the ETM+ instrument). The team assembled a list of possible failure scenarios, most of which pointed at a mechanical problem with the SLC itself. Since there is no backup SLC, a mechanical failure would indicate that the problem was permanent. However, the team was unable to rule out the possibility of an electrical failure, though such a possibility was deemed remote. Nevertheless, on September 3, 2003, USGS director Charles G. Groat authorized the Landsat project to reconfigure the ETM+ instrument and various other subsystems on board Landsat 7 to use the spacecraft's redundant ("Side-B") electrical harness.
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