The Landsat satellites were designed to detect landscape changes over time. However, sometimes what Landsat needs to see on the Earth’s surface is no change at all.
This is the story of a key Landsat calibration test site that is being retired as a test site because of the extensive land use change taking place in that area.
Engineers use test sites around the world to make sure Landsat imagery is consistent and accurate. These calibration test sites are regions on the Earth that remain stable over long periods of time. They do not change year to year or season to season. They are typically deserts or dry lakebeds with very low humidity, low atmospheric changes, and little to no human intervention.
These sites are described as pseudo-invariant. They are not completely invariant because no site can be perfectly stable over time and varies to some degree. So engineers refer to the sites as Pseudo-Invariant Calibration Sites, or PICS.
The idea of studying these test sites is simple. Engineers look at a spot of land that is expected to remain consistent. The reflectance at different wavelengths of light in the Landsat data should stay stable. When changes occur in these reflectance values, it may mean the sensor’s response has shifted.
Examples of other test sites are the remote desert sands of Libya, the Railroad Valley Playa lakebed in Nevada, and even the straight bridge that crosses Lake Pontchartrain, Louisiana. (The span of the bridge happens to be nearly aligned with the Landsat ground track.)
One of the best test sites, named Egypt-2, was steadily reliable until around the late 1990s. In a vast expanse of Sahara Desert in southern Egypt, center pivot irrigation fields began appearing where there had only been sand just a few years before, changing the reflectance.
Precipitation is extremely rare here, so how do they get water for all these crops?
The region, called Sharq El Owainat, sits atop the Nubian Sandstone Aquifer System. Groundwater from the aquifer is pumped through wells to the surface and spread to the field via center-pivot irrigation sprinklers. The region now produces about a third of Egypt’s wheat crop, which is exported by way of the Sharq El Owainat airport.
With all of that land use change happening in the region, Egypt-2 could not continue as a test site.
The Sharq El Owainat airport is visible on the right side of these images. The runway appears in the 1997 image.
Landsat’s accuracy is key to other systems as well. Satellite sensors launched by other countries, cubesats launched by private companies, and sensors on NASA Earth-observing satellites benefit from the accurate Landsat calibration.
Calibration work may seem esoteric, but it makes the science application work possible. The work that solves problems couldn’t be done without the accuracy of the data. Landsat images are simply pretty pictures—until we calibrate them; then they become accurate datasets.
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