Figure 1. Example of Single Event Upsets (SEUs) in Level-0 Landsat 7 Enhanced Thematic Mapper Plus (ETM+) Red Green Blue (RGB) imagery, Bands 3, 2, 1.
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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 known phenomenon often occurs in spacecraft passing through the Van Allen belts, especially the northern and southern auroral zones (> 60 degrees north or south latitude) and over the South Atlantic Anomaly. SEUs have been observed on almost every low-earth orbit satellite system, even as far back as Explorer 1 in 1958, which discovered the presence of the Van Allen belts.
On Landsat 7 Enhanced Thematic Mapper Plus (ETM+) scenes, SEUs appear as bright spots—often larger than one pixel and sometimes the width of a detector array—that appear in only one band at a time. They may saturate or oversaturate the detectors. After the initial bright spike, there is usually a single dark pixel as the affected detectors recoil in bright target recovery. These dark pixels often saturate low (recording a value of zero); detection of low saturated pixels is the primary means of discovering these artifacts within the data.
Figure 2. Example of SEU showing the detector array
Examination of the Image Assessment System (IAS) trending database shows that SEUs are correlated to the regions in Earth's orbit with high particle flux. The vast majority of SEUs found have occurred over the South Atlantic Anomaly, an anomaly in Earth’s magnetic field that causes particle storms at low orbital altitudes. A geographic plot from the trending database of low saturations in Bands 2-4 shows the observed geographic extent of these artifacts. Other effects, such as Oversaturation and Detector Ringing events, may be included in the data set displayed here, which may explain many of the high latitude cases.
Figure 3.Global occurrence of SEUs found in the IAS database.
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SEUs have been discovered in all bands except Band 6, but mining the IAS trending database for SEUs is complicated by the presence of other low saturating artifacts, especially in Bands 1, 5, and 7. Bands 5 and 7 were excluded from this map because of the frequent occurrence of Oversaturation in those bands over small fires around the world. Band 1 was excluded because of low saturation caused by Detector Ringing events.
SEUs appear similar to and can be mistaken for Impulse Noise or Fire Noise. SEUs have a multi-detector shape, whereas Impulse Noise usually affects only a single pixel. Unlike Fire Noise, each SEU event only occurs in a single band.
Generally, SEUs are not a cause for concern. They do not pose a threat to the health of any properly designed instrument. The imagery directly underneath the SEU is uncorrectable in the band in which it appears.