The Landsat program represents the longest running series of satellite missions dedicated to documenting the surface of the Earth from space. For over three decades, Landsat satellites have collected imagery recording land cover conditions over a majority of the globe. With each mission, the National Aeronautics and Space Administration (NASA) and the U.S. Geological Survey (USGS) have made advances in technology, improving the quality of imagery acquired, increasing the durability of the satellite platforms and sensors, and expanding the science involved in analyzing the imagery.
The SouthFACT project is primarily designed to analyze satellite imagery from the two active Landsat missions - Landsat 7 (L7) and Landsat 8 (L8). However, custom requests can be submitted through the application that include Landsat 5 (L5) data. Therefore, the possible analysis window for the SouthFACT application(s) is from 1984 to present day. Below are brief descriptions of L5, L7, and L8 satellites with information relevant to the SouthFACT project. More detailed information about the Landsat program can be found on the USGS Landsat website.
Landsat 5 was launched in 1984 and set the stage for the future of low orbit Earth monitoring satellites. Designed with a minimum life expectancy of three years, L5 exceeded all expectations - providing high quality imagery for 29 years before being decommissioned in 2013. Satellite imagery acquired by L5 allowed scientists to further explore and develop methods of remotely monitoring the surface of the Earth. These methods represent the building blocks of techniques still common today.
The SouthFACT project is currently using L5 only for custom requests. However, future products provided in the Forest Change Viewer may utilize L5 to produce standard decadal / annual / seasonal change going back to the mid-1980's.
Landsat 7 was launched in 1999, incorporating technological enhancements and knowledge gained from the L5 mission. Imagery captured by L7 from launch until May 2003 offered scientists another source of data to further develop remote sensing analysis methods. In May 2003, an equipment malfunction impacted the ability of the L7 satellite to capture continuous data of the Earth’s surface. While L7 imagery collected after the Scan Line Corrector (SLC) failure occurred provide incomplete snapshots of the Earth's surface, a majority of each clear image still represents useable data.
The SouthFACT project is currently using L7 data to produce vegetation change products for the Southern US. Portions of the imagery impacted by the SLC failure are "masked" to reduce confusion when users interpret the analysis products.
Launched in 2013, Landsat 8 is the most recent mission and includes several technological advancements, providing opportunities for scientists to analyze Landsat imagery in new and exciting ways. One advancement is the ability of the sensors to interpret readings of the Earth within a larger range of values. Historically, values recorded by Landsat satellites were expressed on a scale of 0 to 255. L8 can record values on a scale of 0 to 65,535. This increase in the range of values equates to an increase in sensitivity, which can help scientists identify and monitor events that have historically been difficult to detect.
The SouthFACT project is also using L8 to produce vegetation change products for the Southern US. By analyzing imagery across satellite platforms / sensors, the project can potentially detect changes in vegetation occurring within an eight day window.
Sensors onboard each Landsat satellite (platform) have been designed to measure electromagnetic energy emitted by the Sun and reflected off of the Earth. These sensors are designed to measure energy within various wavelength ranges (or spectral bands) and the values recorded are indicative of the amount of energy reflected (or absorbed) within each wavelength range. A much more detailed description of the electromagnetic spectrum can be found on NASA’s website.
Landsat sensor technology has changed over time and as a result, L5, L7, and L8 satellites provide slightly different data. The table (right) provides a listing of the Landsat missions used in the SouthFACT project, the sensor technology, wavelength range (band) names, the portion of the electromagnetic spectrum measured by each, and the spatial resolution of each band (i.e., horizontal ground units) [Source: USGS].
Additional information about Landsat measured wavelengths, including a visualization tool (Spectral Characteristics Viewer) to compare reflectance across platforms, can be found on the Landsat website.