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17 December 2011

First Global Image from VIIRS

From its vantage 824 kilometers (512 miles) above Earth, the Visible Infrared Imager Radiometer Suite (VIIRS) on the NPOESS Preparatory Project (NPP) satellite gets a complete view of our planet every day. This image from November 24, 2011, is the first complete global image from VIIRS.

The NPP satellite launched on October 28, 2011, and VIIRS acquired its first measurements on November 21. To date, the images are preliminary, used to gauge the health of the sensor as engineers continue to power it up for full operation.

Rising from the south and setting in the north on the daylight side of Earth, VIIRS images the surface in long wedges measuring 3,000 kilometers (1,900 miles) across. The swaths from each successive orbit overlap one another, so that at the end of the day, the sensor has a complete view of the globe. The Arctic is missing because it is too dark to view in visible light during the winter.

The NPP satellite was placed in a Sun-synchronous orbit, a unique path that takes the satellite over the equator at the same local (ground) time in every orbit. So, when NPP flies over Kenya, it is about 1:30 p.m. on the ground. When NPP reaches Gabon—about 3,000 kilometers to the west—on the next orbit, it is close to 1:30 p.m. on the ground. This orbit allows the satellite to maintain the same angle between the Earth and the Sun so that all images have similar lighting.
The consistent lighting is evident in the daily global image. Stripes of sunlight (sunglint) reflect off the ocean in the same place on the left side of every swath. The consistent angle is important because it allows scientists to compare images from year to year without worrying about extreme changes in shadows and lighting.

The image also shows a band of haze along the right side of every orbit swath. When light travels through the atmosphere, it bounces off particles or scatters, making the atmosphere look hazy. The scattering effect is most pronounced along the edge of the swath, where the sensor is looking at an angle through more of the atmosphere. Scientists can correct for this scattering effect, but need measurements from a range of wavelengths to do so. The degree to which light scatters depends partly on the wavelength of the light. Blue light scatters more than red light, for example, which is why the sky is blue. VIIRS measures 22 different wavelengths of light, but not all of the sensor’s detectors are operating at peak performance yet. Those measuring thermal infrared light are not yet cold enough to collect reliable measurements.

Once VIIRS begins full operations, it will produce a range of measurements from ocean temperature to clouds to the locations of fires. These measurements will help extend the record from earlier sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS). VIIRS is very similar to MODIS, but flies at a higher altitude to measure the whole planet without gaps. (MODIS daily measurements have gaps at the equator. See the MODIS image from November 24.) VIIRS also sees the Earth in less detail, 375 meters per pixel, compared to 250 meters per pixel for MODIS.

Image by NASA’s NPP Land Product Evaluation and Testing Element. Caption by Holli Riebeek.