- Branża: Aerospace
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The Executive Branch agency of the United States government, responsible for the nation's civilian space program and aeronautics and aerospace research.
Mean solar time of the meridian of Greenwich, England (known as the prime meridian, longitude 0°), used by most navigators and adopted as the prime basis of standard time throughout the world. Abbreviated GMT.
Industry:Aerospace
Measure of the energy radiated by an object. In general, radiance is a function of viewing angle and spectral wavelength and is expressed as energy per solid angle.
Industry:Aerospace
Non-uniform detector-to-detector variations that are due to different spectral responses of the adjacent detectors. Spectral striping is rarely considered a problem within a single band of an instrument because filters that cover all the detectors of a band are chosen for their homogeneity. However, spectral striping is a serious problem when trying to cross-calibrate two imagers because there is not a way to remove the differences with a single radiometric correction if there are real differences in the spectral bands and real differences in the target, including the atmosphere. Also see cross-calibration and relative spectral response.
Industry:Aerospace
NDVI is the most commonly used vegetation index for satellite imagery. The difference in reflectance from the near infrared and red bands is divided by the sum of the two reflectances. This compensates for different amounts of incoming light and produces a number between 0 and 1. The typical range of actual values is about 0.1 for bare soils to 0.9 for dense vegetation.
Industry:Aerospace
Observations made on the ground at a site that is being imaged from space for the purpose of verifying either the absolute radiometric and/or geometric calibration of the imagery or the classified product from the image. These data which are acquired from field checks, high-resolution remote sensing data, or other sources of "known" data are used as the basis for making decisions on training areas and evaluating classification results. Instrument ground truthing made during field trips is often called vicarious calibration when experimental measurements are made of such factors as solar irradiance, atmospheric transmittance, and reflectance of either natural or calibrated homogeneous or gridded targets. Classification ground truthing can be done (1) by visiting the sites to identify what is on the ground, (2) by referring to classification of the area from other sources such as thematic maps, or (3) by classifying higher spatial resolution imagery from satellites or aircraft into classes that can be observed in the coarser resolution imagery. Also see ground data and vicarious calibration.
Industry:Aerospace
On May 31, 2003, the Scan Line Corrector (SLC), which compensates for the forward motion of Landsat 7, failed. Subsequent efforts to recover the SLC were not successful, and the failure appears to be permanent. Without an operating SLC, the Enhanced Thematic Mapper Plus (ETM+) line of sight now traces a zig-zag pattern along the satellite ground track. As a result, imaged area is duplicated, with width that increases toward the scene edge.
The Landsat 7 ETM+ is still capable of acquiring useful image data with the SLC turned off, particularly within the central part of any given scene. The Landsat 7 ETM+ therefore continues to acquire image data in the "SLC-off" mode. All Landsat 7 SLC-off data are of the same high radiometric and geometric quality as data collected prior to the SLC failure. The SLC-off effects are most pronounced along the edge of the scene and gradually diminish toward the center of the scene (Figure 2). The middle of the scene, approximately 22 kilometers wide on a Level 1 (L1G, L1Gt, L1T) product, contains very little duplication or data loss, and this region of each image is very similar in quality to previous ("SLC-on") Landsat 7 image data. Landsat 7 data collected after May 31, 2003 is know as "SLC-off data."
Industry:Aerospace
One of the three colors, either additive (blue, green, and red) or subtractive (cyan, yellow, and magenta) that may be combined to produce the full range of colors.
Industry:Aerospace
One of the two Earth-sensing payloads carried aboard Landsat 4 and 5, the Multispectral Scanner being the other on 4 and 5. It is a non-photographic imaging system, which utilizes an oscillating mirror and seven arrays of detectors, which sense electromagnetic radiation in seven different spectral bands. The Thematic Mapper is a derivative of the Multispectral Scanner generation of sensors, but it achieves much greater ground resolution, better spectral separation, improved geometric fidelity, and greater radiometric accuracy and resolution.
Industry:Aerospace
One of two on-board external solar calibrators on Landsat 7 ETM+. The FASC consists of a nearly white panel, which is extended in front of the full aperture of the ETM+ as it ascends over the North Pole from the night side so the Sun’s irradiance can be reflected into the aperture. FASC measurements are typically made once a month. The gain or responsivity of the detectors can be calculated from factors that include, known solar spectral irradiance, measured solar zenith and azimuth angles relative to the solar panel, known distance from the Sun, and pre-launch measurements of the nearly diffuse bidirectional reflectance factor of the solar panel. Also see PASC and IC.
Industry:Aerospace
One of two on-board external solar calibrators on Landsat 7 ETM+. The PASC consists of structure in front of the ETM+ aperture that has four independent pinholes for reflecting solar irradiance on a presumably parallel path to the optical axis of ETM+ as it comes over the North Pole and into sunlight. As compared to FASC measurements, PASC irradiance from a single pinhole at a time only fills a small part of the defining aperture. PASC measurements were made approximately once a day prior to failure of the Scan-Line-Corrector (SLC), and once every other day subsequently.
The SLC-failure also necessitated rewriting the software for PASC processing by the Image Assessment System (IAS) at EROS Data Center in Sioux Falls, SD. Unfortunately, there are two major problems with PASC data: 1) the elliptical PASC images are saturated in several bands, and 2) an as-yet unknown cyclic variation has made them unusable in an absolute sense. The relative gain or responsivity of the detectors can be calculated from factors that include, known solar spectral irradiance, measured solar zenith and azimuth angles. Also see full aperture solar calibrator and internal calibrator.
Industry:Aerospace
