One of the recurring anomalies in Landsat 7 occurs when the scanning mirror and associated scanning mechanisms lose synchronization, causing visible artifacts to appear in the image(s) taken during the event. These events produce impacts to the imagery ranging from relatively minor to quite severe.

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On November 24th at 00:37:37Z, an incorrect command to the Landsat 7 spacecraft caused the ETM+ to switch gain states for Band 6.  As the only band that uses both formats, Band 6 Low Gain data normally processes through Format 1, while Band 6 High Gain data normally processes through Format 2.

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A Single Event Upset (SEU) occurs when an energetic particle travels through a transistor substrate and causes electrical signals within the transistor.  This is a known phenomenon that usually occurs in near earth orbit to spacecraft passing through the Van Allen belts, especially the northern and southern auroral zones and the south Atlantic anomaly.  SEUs have been witnessed on satellite systems as far back as Explorer 1 in 1958, which discovered the presence of the Van Allen belts.

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ETM+ Band 6 ( thermal infrared band) Level 1R (high and low gain) and Level 1G (high gain only) images produced prior to November 23, 1999 have a high degree of unevenness or 'striping' throughout the scene. This is a result only of ground processing - the basic data were never affected.

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A new artifact was discovered in ETM+ data, in Detector 9 of Band 1 and Detector 14 of Band 8. Both of these detectors already exhibit a 20 kHz coherent noise source, and they now appear to suffer from a rare oscillating instability of unknown origin.

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There are coherent noise sources in ETM+ imagery. At least one of these sources is of great enough magnitude to be detected with the human eye in homogenous regions. However, the Band 3 coherent noise source is of small enough magnitude that it is invisible in most terrain.

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There are three Gimbaled X-Band Antennae on the Landsat 7 spacecraft used for downlinking data to the ground stations. In normal operation, these antennae move slowly, tracking the ground station below.

However, there are unavoidable times when the GXA antennae must quickly slew from one ground station to another to insure that all data downlinked correctly. When a GXA quick slew occurs during imaging, the antenna sets up a vibration that affects the ETM+ scan mirror, causing what is known as a GXA Anomaly -- a region of distorted scan lines with oscillating lengths.

 

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Oversaturation is the term used here to describe an artifact that affects most TM-like instruments in one way or another, including the Landsat 7 ETM+.  It manifests itself over very bright targets. The comet-like artifacts are caused by very bright objects on the ground that exceed the dynamic range of the instrument.  Here, the effect is observed in Band 1 on Path 204 Row 23.

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