The three Gimbaled X-band Antennae (GXA) on Landsat 7 downlink data to the ground stations. During normal operations, these antennae move slowly, tracking the ground station below.
However, in the first year of Landsat 7 operation, the GXA quickly slewed from one ground station to another several times. The GXA quick slew causes a vibration that affects the Enhanced Thematic Mapper Plus (ETM+) scan mirror, causing what is known as a GXA Anomaly — a region of distorted scan lines with oscillating lengths. These anomalies often occur in areas between two ground stations, such as central Europe, eastern Asia, or parts of South America. The severity of these anomalies varies depending on the GXA used.
GXA Anomalies are visible in the browse imagery as an undulation along the right edge of the image. In Level-0 Reformatted (L0R) data products, the GXA Anomaly causes scan line lengths to change rapidly. These distortions are present in all ETM+ bands with roughly equal magnitude, and they can affect the data throughout the width of the image.
Figure 1. Example of GXA Anomaly in Level-0 Landsat 7 ETM+ data.
Click to view larger image. - .gif (475 KB)
Fortunately, GXA distortions can usually be removed in Level-1 Geometrically Corrected (L1G) processing. In most cases, the L1G imagery shows no sign of the anomaly. In some cases, scan dropouts are visible at the edges of the imagery.
In rare instances, the magnitude of the GXA distortion is so large that the scan mirror loses sync with the free-flying calibration shutter. When this large distortion occurs, the anomaly is accompanied by a period of shutter intrusion into the image, usually on the left side. The data lost to shutter intrusion cannot be recovered, and shutter intrusion is usually not visible in browse images. However, the extremely large GXA Anomaly that causes shutter intrusion will be visible in browse imagery.
U.S. Geological Survey (USGS) and NASA analysis of GXA anomalies prompted a theory that the frequency of the GXA stepper motor (37 Hz) is too close to the frequency of the scan mirror harmonic (34.7 Hz). Through this analysis, they determined that decreasing the acceleration rate of the GXA stepper motors would reduce or eliminate the severe GXA disturbance. On April 26, 2000, the Flight Operations Team (FOT) decreased GXA slew acceleration rate by 28 percent. This fix successfully eliminated major GXA disturbances. GXA anomalies still occur in Landsat 7 ETM+ imagery, but they are minor and easily corrected in L1G processing. No major GXA anomalies were reported after this change.