Landsat is the longest-running, continuously operating record of Earth observations, and it’s the gold standard reference point that other civil and commercial satellite programs trust. If a researcher is studying multiple Landsat images of the same spot on Earth, and there is something in those images that suggests a change, that researcher needs to have the confidence that that change is a real change on the landscape and not because of something that changed or degraded in the sensor. Think of it this way. We are using a system to quantify changes on the Earth—we need to make sure the system itself is not changing.