Coherent Noise
Landsat data are systematic, geometric, radiometric, and terrain corrected to provide the highest quality data to the user communities. Occasionally, anomalies occur and artifacts are discovered that require research and monitoring.
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Coherent Noise (CN) appears as a repeating pattern in satellite imagery. CN is most visible over dark homogenous regions. These patterns may appear in only one band or in several bands and may or may not be phase-locked to the instrument scan time. Although CN can be corrected, this correction often degrades other parts of the image, so correction is performed only for very high magnitude noise sources.
Currently, no CN correction is performed for Landsat 5 Thematic Mapper (TM) or Landsat 7 Enhanced Thematic Mapper Plus (ETM+) Level-1 products, except for Nyquist Noise (described below).
CN can arise from many electrical systems onboard the satellite, including the power supply, the detector circuitry, and every electrical system in between. All satellites have some CN sources that appear at a certain frequency and magnitude. These noise sources are often discovered before launch, but they may change over the life of an instrument. If significant changes occur, or if new CN patterns appear that were previously unseen, they may be cause for concern and further analysis.
Nyquist frequency CN is a specialized type of CN that is phase-locked to the start of the mirror scan and has a frequency exactly twice the sampling rate of the pixels of the instrument. This common artifact arises in the analog/digital conversion circuits of an instrument. Nyquist Noise is easily removed and is usually not seen in Level-1 images. Nyquist Noise is a known, correctable artifact and is not a cause for concern.
Anomalous Coherent Noise appears when a CN source changes in magnitude across a single instrument scan. It has been seen in Bands 1 and 8 on Landsat 7 ETM+. It only affects a few detectors in each band. The Landsat 7 ETM+ anomalous CN is largest in the Internal Calibrator (IC) region and at the edges of a scene. It drops to zero magnitude in the center of a scene.
The Anomalous Coherent Noise sources on Landsat 7 appear at 20 kHz, which is the same operating frequency as the Scan Line Corrector (SLC). As a result, some link between the anomalous noises and the SLC was postulated. This theory appears to have been confirmed with the failure of the SLC in June 2003. After returning to normal operations, it was observed that the 20 kHz noises in Bands 1 and 8 have decreased in magnitude, and the anomalous nature of this noise has been greatly reduced.
Anomalous Coherent Noise is a known problem on Landsat 7. Even though it is uncorrectable, it is not a cause for concern. However, new Anomalous Coherent Noise sources require further analysis.