Coherent Noise Storm
Figure 1. Coherent Noise Storm in Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) Band 1 data at the moment of Scan Line Corrector (SLC) Failure.
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A Coherent Noise Storm is a symptom of a serious, sudden event on the satellite or instrument. As a system onboard the satellite fails, electrical anomalies can create sudden and brief bursts of noise that exhibit coherent frequency.
Only one Coherent Noise Storm has been observed in Landsat imagery, at the moment of failure for the Landat 7 ETM+ Scan Line Corrector (SLC) on May 29, 2003. The scan lines were properly aligned at the top of the image but were misaligned at the bottom (see Figure 1). In the middle of the image, the SLC failed catastrophically. This failure was accompanied by noise of very high magnitude—up to 200 DN—with a frequency of 20 kHz that lasted for about eight scans before disappearing. This noise was observed in most bands but was highest in Bands 1 and 8, where Detector Ringing events had been seen previously.
Coherent Noise Storms are cause for immediate concern. They are signs that a sudden electrical change has occurred onboard the satellite or instrument. Even after the event has passed, detectors involved in the noise storm may exhibit a change in their noise characteristics.