In a remote part of the Sahara Desert in Egypt, center pivot irrigation fields fill the landscape. Thanks to the Landsat archive, we can see how dramatically this area has changed over the last 35 years. But until the growth in irrigation, this part of the world hadn't changed much from a satellite's perspective. That's why Landsat calibration engineers originally chose it as a test site to verify the accuracy of incoming surface reflectance data. Changes observed at calibration sites often indicate a shift in sensor response aboard Landsat. In this case, the change was on the land, making the area unsuitable for future calibration efforts. Fortunately, other reliable test sites exist, and engineers continue to rely on them, maintaining Landsat's accuracy, which is crucial in determining the precision of other Earth-observing satellites.
Examples of these test sites are available in the EROS Cal/Val Center of Excellence (ECCOE) test site catalog.