Satellite images provide a unique, dramatic perspective of our Earth and its atmosphere. The images used to piece together this view of the United States were generated using data gathered from an advanced sensor called the Visible Infrared Imaging Radiometer Suite (VIIRS), flying aboard the NASA/NOAA Joint Polar Satellite System (JPSS) series, currently consisting of Suomi-NPP and NOAA-20. Both satellites orbit the earth from pole to pole 14 times per day, with Suomi-NPP preceding NOAA-20 by about 50 minutes. The instruments onboard S-NPP and NOAA-20 include the Visible Infrared Imaging Radiometer Suite (VIIRS). Each VIIRS instrument provides a complete view of the Earth every day across a wide spectrum of energy.
During a given day, both Suomi-NPP and NOAA-20 will make three to four passes each over the continental USA during daylight hours, and another two to four during the night. The VIIRS Today United States composite features the latest image composite available from either Suomi-NPP or NOAA-20, allowing users to toggle between satellites. You will occasionally notice distinct boundaries in the composite because the images that comprise the composite originate from different passes. By combining the images acquired from the blue, red and green portions of the energy spectrum, SSEC generates a “true” or “natural” representation of the Earth’s land, ocean, and atmosphere.
More information on the instrument and data processing
The VIIRS instruments are able to capture images at a maximum resolution of 375 meters per pixel at the Earth’s surface. This website features VIIRS images at 4000 meter resolution over the entire continental United States as well as 2000-, 1000-, and 250-meter resolution images for eight subset regions. The 250-meter resolution is higher than the actual instrument can resolve, to make it consistent with the MODIS Today website. Several processing steps are needed to create the “True Color” and “False Color” images from the raw data collected by the VIIRS instrument. First, the raw digital counts acquired by VIIRS are calibrated to physical units known as reflectance. Next, the images are processed to remove some (but not all) of the image features introduced by the atmosphere, such as haze. Then the images are transformed from the projection geometry of the instrument (which includes distortions due to phenomena such as earth curvature) into a standard map projection, known as the equirectangular projection. Finally, the images are enhanced using contrast adjustment and image sharpening techniques. As new images become available from Suomi-NPP and NOAA-20 throughout the day, they automatically appear on the website. The images are usually available within 60 minutes of the time they are acquired onboard the spacecraft.
True color and false color images
To better understand our "true color" and "false color" images, please refer to this page on "What do the different band combinations mean?" from the NASA Worldview Snapshots page. What we are calling "false color" is referred to as a "VIIRS Bands M11, I2, I1 Combination".
Day/Night Band (DNB) images
The VIIRS instruments include a unique band that is sensitive to very small amounts of visible light over a broad spectral region. This allows it to detect visible light even at night, including reflected moonlight and terrestrial and atmospheric sources that emit visible light. Features that can be seen at night using this band includes clouds and bright land surfaces, as well as city lights, fires and even lightning strikes. How much of each of these features can be seen is highly dependent on the phase and lunar rise and set times for the given day. At or near full moon phase, the DNB images appear very much like a standard visible reflectance image; while at or near new moon phase there is little lunar reflectance contribution, so all that can be seen are features that emit visible light.
This website features VIIRS Day/Night Band images at 4000 meter resolution over the entire continental United States as well as 2000-, 1000-, and 250-meter resolution images for eight subset regions. The 250-meter resolution is higher than the actual instrument can resolve, to make it consistent with the MODIS Today website.
The Day/Night band NOAA-20 Image quality previous to 2 July 2020 may show stray light artifacts along the rightmost portion of each orbit. To alleviate this problem, elements 3900-4064 for each line of data have been trimmed starting on 2 July 2020.
Missing areas in images
Even after we have processed all of the data for the day to generate a VIIRS image over the continental US, sometimes a portion of a VIIRS image displayed on our website will be missing.
All the data you see on VIIRS Today is acquired in "real-time" by satellite ground stations in the continental United States. SSEC at UW-Madison is the primary site, but we also process data received by ground stations at Hampton University in Virginia and at the National Weather Service Forecast Office in Monterey, California. The antennas at these stations receive data directly from the satellite as it is acquired, as long as the satellite can be seen by the antenna (i.e., it must be above the horizon). We acquire data from two satellites, Suomi-NPP and NOAA-20, both of which have VIIRS imaging instruments on board. These satellites are civilian, unclassified, and the data are "in the clear" (that is, they are not encrypted like DirectTV). The data can be received by anyone with the right kind of antenna. Furthermore, the data are processed using open source software which has been made available by NOAA.
When there are gaps in the VIIRS Today images, it could be due to:
- imperfect reception of the satellite signal at the ground station (e.g., our own antenna at SSEC)
- problems in the automated processing of the received data.
- the satellites are performing a maneuver (e.g., drag makekup maneuver).
When choosing the "Show All Available Images" button on the main VIIRS Today page, thumbnails are available starting on 8 July 2020.
Because the US composite images consist of multiple Suomi-NPP or NOAA-20 passes that occur several times a day, there is no single time associated with a VIIRS Today composite image. Click on either "Today's SNPP Passes" or "Today's NOAA-20 Passes" at the top of the page when viewing a particular composite image in order to see which Suomi-NPP or NOAA-20 passes (and their associated times) may have been used to create that composite. Individual sectors of the US may or may not use more than one pass, depending on where the sector lies with respect to the overpasses for that day.