Life of a Pixel: How Sunshine Becomes Satellite Data

We've all seen satellite imagery like this. Expansive. Breathtaking. But every one of those images is made of millions of individual data points, each with its own story. Let's hear the tale of remote sensing's smallest hero: the pixel.

Light emitted from the sun hits the Earth in many wavelengths. Some wavelengths penetrate the atmosphere and reach the surface. The surface, depending on the type of land cover, reflects and absorbs different wavelengths. Only certain wavelengths are visible to human eyes. Landsat satellites capture this visible light, as well as wavelengths that are invisible to the human eye. And from that captured light, the building blocks of pixels are created.

Each pixel represents the land cover at a specific location on the ground, one small piece of a much larger image. The Operational Land Imager aboard Landsat records a 185 km wide swath of imagery across 9 wavelengths, from visible light to infrared, including wavelengths that can reveal healthy vegetation or spot the hot edge of an active fire. The Thermal Infrared Sensor adds two more wavelengths that detect heat, helping us track everything from water use in fields of crops to heat radiating from an active volcano. Readings from the sensors are routed to Landsat's recording electronics along with calibration and orientation metadata useful for processing. Combined, Landsat 8 and 9 record over 30 million pixels per second when actively capturing land surface imagery.

Landsat is considered a medium resolution Earth observation mission. Most Landsat pixels represent a 30-meter square on the ground, roughly the size of a baseball diamond. Millions of them together form a single Landsat scene.

Each Landsat satellite's polar orbit, synchronized to the sun, revisits the same path on Earth every 16 days. The orbit positions Landsats to capture fresh imagery over most of the planet and ensures regular passes over ground stations located around the world. Ground stations command the satellites to send down data. Pixels are transmitted by Landsat and received by giant antenna dishes. Once data is safely on the ground, Landsat’s recorders are cleared, making room for the newest imagery.

In addition to maintaining the Landsat archive, the U.S. Geological Survey operates the primary ground station at the EROS Center in South Dakota, that can send commands to Landsat satellites as well as receive data from them. When data are not directly downloaded by EROS, they can be delivered through other ground stations and added to the archive. Every day, teams at the USGS process, calibrate, validate, and store an enormous volume of data. At this point, the pixels have been safely delivered home to the archive. But that’s not the end of the story.

Once Landsat imagery is calibrated, validated, and stored, the USGS processes the data into additional science products: measuring things like the temperature of the earth’s surface, where surface water is expanding or shrinking, and the burned area of a fire. The generation of science products is an important part of turning raw Landsat data into information that helps communities manage water, land, and natural resources.

Over 54 years of Landsat data are stored at the USGS EROS Center in South Dakota. The archive currently holds more than 12 million Landsat products, and every single pixel is available at no cost through tools like EarthExplorer, Landsat in the Cloud, and the ESPA on-demand interface. Communities and scientists around the world rely on Landsat products for things like tracking water levels in the Great Salt Lake, measuring agricultural water use in Colorado and Nebraska, and monitoring wildfire impacts in the Badlands.

The pixel, one small unit of reflected light. More than fifty years of them, billions per day, building an uninterrupted record of our changing planet.