MODIS Today: About this website

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 that flies aboard the satellites in NASA’s Earth Observing System, Terra and Aqua. Traveling north to south, Terra loops over the equator in the morning. Aqua moves in the opposite direction, passing south to north over the equator in the afternoon. Both satellites carry the Moderate Resolution Imaging Spectroradiometer (MODIS). Together, the Terra MODIS and Aqua MODIS provide a complete view of the Earth every one to two days across a wide spectrum of energy.

During a given day, both Terra and Aqua will make two to four passes each over the continental USA during daylight hours, and another two to four during the night. The MODIS Today United States composite features the latest image available from either Terra or Aqua, 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.

In addition to the images available on the website, we also provide a daily link to a KML (Keyhole Markup Language) file which can loaded into the freely available Google Earth application. When the daily KML file for Terra or Aqua is loaded into Google Earth, you can browse the entire continental USA (depending on the satellite pass positions for that particular day) at up to 250-meter resolution. The images for Google Earth are tiled into small subsets so they download quickly to your desktop.

More information on the instrument and data processing

The MODIS instruments are able to capture images at a maximum resolution of 250 meters per pixel at the Earth’s surface. This website features MODIS 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. Several processing steps are needed to create the images from the raw data collected by the MODIS instrument. First, the raw digital counts acquired by MODIS 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 Terra and Aqua 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 MODIS Rapid Response System. What we are calling "false color" is referred to as a "Band 7-2-1 Combination".

Missing areas in images

Even after we have processed all of the data for the day to generate a MODIS image over the continental US, sometimes a portion of a MODIS image displayed on our website will be missing (example: 2009-Jan-24 southwest part of the US).

All the data you see on MODIS 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 NASA Goddard Space Flight Center, University of South Florida, University of Texas-Austin, University of New Mexico, or the US Forest Service Salt Lake City. 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, Terra and Aqua, both of which have MODIS imaging instruments on board. These satellite 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 NASA.

When there are gaps in the MODIS Today images, it could be due to:

  1. the satellite transmission was interrupted, usually because the transmitter on the spacecraft was switched to a "playback" mode rather than a "broadcast" mode, and less often on the Terra satellite to avoid interfering with the NASA Deep Space Network antenna at Goldstone, CA
  2. imperfect reception of the satellite signal at the ground station (e.g., our own antenna at SSEC)
  3. problems in the automated processing of the received data.

So while there may be some gaps in the data from the Terra satellite on certain days i.e.,

2009-Jan-24 (Terra)

there may be better coverage from the Aqua satellite for that day:

2009-Jan-24 (Aqua)

and in fact the Terra and Aqua satellite are in different orbits for just this purpose; to give more complete coverage. Each satellite actually takes a different ground track every day, and they only repeat their ground track every 16 days.

Image time

Because the US composite images consist of multiple Aqua or Terra passes that occur several times a day, there is no single time associated with a MODIS Today composite image. Click on either "Today's Terra Passes" or "Today's Aqua Passes" at the top of the page when viewing a particular composite image in order to see which Terra or Aqua 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.