 | |
| Launch | |
|---|---|
| Date | April 15, 1999 |
| Vehicle | Delta II |
| Site | Vandenberg AFB |
| Orbit characteristics | |
| Reference system | WRS-2 |
| Type | Sun-synchronous, near-polar |
| Altitude | 705 km (438 mi) |
| Inclination | 98.2° |
| Repeat cycle | 16 days |
| Swath width | 185 km (115 mi) |
| Equatorial crossing time | 10:00 AM +/- 15 minutes |
History of Landsat 7
The government-owned Landsat 7 was successfully launched on April 15, 1999 from the Western Test Range of Vandenberg Air Force Base, California, on a Delta-II expendable launch vehicle. The Earth observing instrument on Landsat 7, the Enhanced Thematic Mapper Plus (ETM+), replicates the capabilities of the highly successful Thematic Mapper instruments on Landsats 4 and 5.
The ETM+ also includes additional features that make it a more versatile and efficient instrument for global change studies, land cover monitoring and assessment, and large area mapping than its design forebears.
These features are:
- a panchromatic band with 15m spatial resolution
- on-board, full aperture, 5% absolute radiometric calibration
- a thermal IR channel with 60m spatial resolution
- an on-board data recorder
Landsat 7 is the most accurately calibrated Earth-observing satellite, i.e., its measurements are extremely accurate when compared to the same measurements made on the ground. Landsat 7’s sensor has been called “the most stable, best characterized Earth observation instrument ever placed in orbit.” Landsat 7’s rigorous calibration standards have made it the validation choice for many coarse-resolution sensors.
The excellent data quality, consistent global archiving scheme, and reduced pricing ($600) of Landsat 7 led to a large increase of Landsat data users. Considered a calibration-triumph, the Landsat 7 mission went flawlessly until May 2003 when a hardware component failure left wedge-shaped spaces of missing data on either side of Landsat 7’s images.
Six weeks after suffering the loss of its scan line corrector (SLC), the ETM+ resumed its global land survey mission resulting in only a short suspension of its imagery acquisitions for the U.S. archive. However, the malfunction has impacted the imagery of Landsat 7.
Specifically, the ETM+ optics contain the Scan Mirror and Scan Line Corrector assembly among other components. The Scan Mirror provides the across-track motion for the imaging, while the forward velocity of the spacecraft provides the along-track motion. The Scan Line Corrector (SLC) assembly is used to remove the "zigzag" motion of the imaging field of view produced by the combination of the along- and across-track motion. Without an operating SLC, the ETM+ line of sight now traces a zigzag pattern across the satellite ground track.
In this SLC-Off mode, the ETM+ still acquires approximately 75 percent of the data for any given scene. The gaps in data form alternating wedges that increase in width from the center to the edge of a scene.
The remainder of the ETM+ sensor, including the primary mirror, continues to operate, radiometrically and geometrically, at the same high-level of accuracy and precision as it did before the anomaly; therefore, image pixels are still accurately geolocated and calibrated.
To fulfill the expectations of the user community for full coverage single scenes, data from multiple acquisitions are being merged to resolve the SLC-off data gaps. In all cases, a binary bit mask is provided so that the user can determine where the data for any given pixel originated. The USGS is continuing to research other methods of providing better merged data products, and will continue to provide information resulting from this work as it becomes available.
References:
- NASA, Landsat 7.
- Wikipedia, Landsat 7.
