Landsat Missions
Impulse noise is a general term for single-pixel bright or dark spots that are not authentic imagery. This artifact can have several different causes, each with a slightly different appearance.
Bit-Flip Noise
Bit-flip noise is a specialized form of impulse noise that causes single-pixel shifts in the data that are set powers of 2—a pixel may be 128, 64, or 32 DN higher or lower than its actual value. (Smaller shifts occur—16, 8, 4, and 2 DN—but are generally too small to see.) This artifact arises from some digital source, often a transmission error or a problem in the ground processing systems. Unlike most sources of impulse noise, bit-flip noise is often correctable.
Figure 1. Example of bit-flip noise in Landsat 5 TM band 3 data.
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Transcription Artifacts
Noise pixels appearing in a regular pattern throughout the scene often indicates a ground processing or transcription problem. Reprocessing the scene will usually eliminate the artifact.
Figure 2. Example of a transcription artifact that resembles impulse noise. Landsat 2 WBVT data.
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Fire Noise
Figure 3. Example of “Fire Noise” artifacts in Landsat 7 ETM+ data, bands 5 and 7.
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If the impulse noise pixels appear only in bands 5, 6, or 7, it might not be impulse noise—it might be "fire noise." Fire noise is a common ground phenomenon, not an artifact. It mimics impulse noise and occurs when small fires on the ground appear as very bright pixels in the near-IR and thermal bands. These “fire noise” signals are found only in inhabited areas, generally rural areas, during certain seasons, and in weather conditions that are favorable for burning brush and refuse. They appear as bright points in near-IR and thermal bands (bands 5, 6, and 7) and are often saturated or oversaturated in bands 5 and 7. They do not generally appear in visible band data, but large fires may have visible smoke plumes in high-resolution imagery.
Fire noise is common in Landsat 7 ETM+ imagery because the high spatial resolution of the instrument allows it to see smaller fires. While fire noise does occur in Landsat 5 TM imagery, it is more rare and only very large fires can cause the artifact.
Although "fire noise"is often mistaken for impulse noise or single event upset (SEU), it is not a true artifact. Fire noise is an accurate observation of the ground. However, if these ground fires are bright enough, they can cause oversaturation artifacts in bands 5 and 7.
Other types of Impulse Noise
Different forms of impulse noise have slightly different appearances, and other names for it include "Salt and Pepper noise" and "Low Saturation noise." If any form of impulse noise pixels appears only in certain detectors, it could be a warning of an imminent dead or flat detector problem. This artifact is not correctable and might indicate future problems.
If the impulse noise only occurs in dark regions and has a value of 0 DN, it is called low saturation noise. This artifact happens in instruments with low dark currents and high coherent noise—Landsats 5 and earlier, usually in bands 5 and 7. Low saturation noise is the result of expected amounts of random noise plus expected amounts of coherent noise reinforcing each other to occasionally low saturate the detector at 0 DN. Low saturation noise is uncorrectable and is not a cause for concern.
If the impulse noise appears in groups of several pixels, and the scene was acquired over South America or in polar regions, it may not be impulse noise—it may be caused by single event upsets (SEU).
If the pixels are evenly distributed across the scene and have no apparent pattern or set value, further analysis is required to determine the cause and nature of the noise.
See Also – SEUs, Oversaturation
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Page Last Modified: May 13, 2008
