When data are missing from a digital image, an artifact known as Data Loss occurs. The missing data will often be substituted with telemetry data onboard the satellite and will thus have a random appearance. Alternately, the missing data may be replaced by null values, or filled with a designated fill pattern by the ground processing systems.
“Christmas Tree” Anomaly
Figure 1. Example of a “Christmas Tree” anomaly in Level-1 data. Landsat 5 Thematic Mapper (TM) Bands 3,2, and 1.
Click to view larger image. - .gif (164 KB)
Data Loss is sometimes referred to as a “Christmas Tree” anomaly when the loss occurs in all bands. When the artifact is viewed in any 3-band Red Green Blue (RGB) combination, the lost data appear as bright red, green, and blue artifacts, often next to or overlapping each other. This colorful artifact is usually caused by telemetry data erroneously being included in the imagery.
Figure 2. Example of a Dropped Scan in Level-0 data. Landsat 7 Enhanced Thematic Mapper Plus (ETM+) Band 7.
Click to view larger image. - .gif (1.01 MB)
Sometimes the data over an entire scan line are lost. This is usually due to either a temporary problem in the main scan mirror or transmission problems between the satellite and the receiving ground station. A dropped scan is also sometimes known as a dropped major frame.
Because Landsat 7 data are downlinked and processed in two different data format streams, transmission-related data loss will usually only affect one format—either Bands 1-5 and Band 6 low gain, or Bands 7-8 and Band 6 high gain.
Sun Glint Anomaly
Figure 3. Examples of Sun Glint Anomaly in Landsat 5 TM browse imagery.
Figure 4. Examples of Sun Glint Anomaly in Landsat 5 TM browse imagery.
A known problem on Landsat 5 causes severe Data Loss in a repeatable and predictable cycle. The Sun Glint Anomaly occurs when sunlight reflects off a part of the spacecraft in a way that interferes with the omnidirectional downlink antenna, causing data to be lost in transmission. For data downlinked to the Earth Resources Observation and Science (EROS) Center, these losses occurred over paths 31 and 32, rows 29 to 31, from day 100 to day 228 of each year.
Changes in the ground station hardware in late 2006 mitigated the transmission problem. Data downlinked to EROS should no longer have Sun Glint Anomalies, but other ground stations may experience these losses in other locations and other times of year. This Data Loss is unavoidable but easily identified in browse imagery.
In all forms, Data Loss is uncorrectable in the scene in which it appears, but some transmission and ground processing problems may be corrected with reprocessing.