Shutter Synchronization Anomalies
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.
"Shutter synchronization anomalies" is a catchall term used for any anomaly that causes the Landsat Internal Calibrator (IC) shutters to be visible in the imagery due to a failure in the synchronization between the shutter and the primary scan mirror. These anomalies can affect large portions of an image or even several Landsat scenes, and are usually first discovered in browse images.
Scan Mirror Anomalies (SMA)
An SMA occurs when the primary scan mirror deviates from its normal scan time, causing it to briefly lose synchronization with the IC shutter. The usual cause is a single-event upset in the scan mirror electronics, causing a spurious end of scan signal. There is often a dropped major frame at the start of the SMA, followed by several scans where the IC shutter is visible in the imagery. This loss of synchronization may be brief or may continue for up to 20 seconds.
SMAs are especially common over the South Atlantic Anomaly region, where radiation can cause glitches in the instrument electronics. They are also common in older instruments with worn mirror bumpers, where even small perturbations of the mirror behavior can cause problems with shutter synchronization. They have been extensively studied in ETM+ data and are estimated to affect one in every 2,000 scenes, or about 0.05 percent.
Late Start Anomalies (LSA)
The LSA is a shutter synchronization problem directly related to the amount of bumper wear on the instrument. LSAs resemble SMAs, but LSAs occur only at the beginning of a subinterval just after the instrument is powered on.
When LSAs occur early in the lifetime of an instrument, it can be a sign that bumpers are too thick, causing short scan travel times that confuse the scan mirror electronics and cause a brief lockup of the primary scan mirror. This often causes several dropped scans, then an imaging period where the IC shutter is not synchronized to the primary scan mirror. This artifact was observed early in the lifetime of Landsat 7, usually during the On-orbit Initialization and Verification (OIV) period when the temperature of the instrument was coldest. No early-lifetime LSAs were observed in Landsat 7 ETM+ data after 2001.
LSAs also occur when the scan mirror bumpers are so worn that the electronics require time to achieve synchronization with the IC shutter. This happens late in an instrument’s lifetime and is often related to temperature, so these anomalies can be reduced by increasing the warm-up time of the instrument. When the bumpers are so worn that mirror-shutter synchronization cannot be achieved, LSAs become caterpillar track anomalies, as described below.
Late-lifetime LSAs usually only appear in the partial WRS scene that is acquired before normal imaging, and are thus unseen by the public. In early 2006, LSAs began appearing in L7 ETM+ data, some of which were large enough to extend into the publicly available full scene. A decision was made in February 2006 to increase the warm-up time of the ETM+ to eliminate the LSAs from normal acquisitions. The warm-up time of the ETM+ was increased several times until early 2007, when it was determined that synchronization was no longer practical. The ETM+ was switched to bumper mode on April 1, 2007. LSAs cannot occur in bumper mode and are not expected to be seen again in L7 ETM+ imagery.
When the synchronization of the primary scan mirror and the IC shutter fails completely, the resulting artifact is known as caterpillar tracks. The entire IC shutter is visible in the scene and remains visible while the instrument is operating. The IC shutter may slowly come back into synch with the scan mirror over dozens of WRS scenes. Caterpillar Tracks occur at the end of lifetime for an instrument, when the scan mirror bumpers are worn down to the point that the mirror’s scan time is so long that the timing electronics cannot synchronize the mirror and the shutter. This is also known as "loss of synch," and it indicates the end of normal mirror operations for the instrument. Imagery can still be collected in bumper mode, where the primary scan mirror’s scan time is left unregulated. L7 ETM+ is currently operating in bumper mode.