Image from TERRA
Sat, 10 Nov 2018 11:10 EST

Within a day, the fire had consumed 70,000 acres of land.

Image from TERRA
Tue, 30 Oct 2018 13:30 EDT

This summer and early fall, beachgoers and residents along Florida’s central Gulf Coast endured an unpleasant and, at worst, debilitating aquatic annoyance: a dangerous red tide caused by the harmful algae Karenia brevis.

Image from TERRA
Wed, 10 Oct 2018 13:03 EDT

More than 400 miles above Earth, a satellite the size of a school bus is earning its frequent flyer miles. On Oct. 6, NASA’s Terra completed 100,000 orbits around Earth. Terra, which launched Dec. 18, 1999, is projected to continue operation into the 2020s.

Tag: MODIS

MODIS News and Events

Sensor Degradation Leads to Calibration Improvements

Changes in the MODIS sensor, not dark ice led scientists to incorrectly determine that the Greenland ice Sheet in the Arctic was darkening. These results and others led to improved calibration corrections in the Collection 6 reprocessing of Moderate Imaging Spectroradiometer (MODIS) data.

A recent study suggested that sensor degradation of the MODIS instrument on NASA’s Terra satellite was responsible for the incorrect data. Ironically, the high quality of the MODIS sensor is what initially led Arctic scientists to look for a subtle change in ice brightness. The data collected by MODIS led researchers to consider that the Greenland ice sheet was darkening from an increase in dark aerosol deposition on the snow. However, ground based measurements showed that there had been little if any change in the color of snow.

Ice darkening could have had big implications for climate researchers. Just like black asphalt absorbs radiation from the sun, resulting in a warmer surface temperature, the darkening of the ice sheet could warm the surface temperature around the snow, increasing snowmelt. This could result a shrinking Greenland ice sheet and have implications on sea level rise.

The combination of a well-understood sensor, high-quality analysis of the satellite data, and coordinated measurements at the surface led to the conclusion that a sensor artifact, a change to an image from defects or degradation of optics, was the culprit this time.

Satellites, like many scientific instruments, need to be calibrated to be able to maintain accuracy. If you’ve ever stepped on a scale and gotten a number that was far too high or too low to be accurate, you’ve been a witness to the degradation of an instrument. Scales can be fixed easily by taring, or resetting the instrument to zero when nothing is being weighed.

Fixing satellite sensor degradation, isn’t quite as easy, but is common practice. The complexity of an instrument like MODIS makes the calibration process much more difficult. Often, there are small instrument effects that are not obvious until many years of data from multiple applications indicate an issue. The dark ice example is one such case.

The vigilance of the MODIS scientists and those working with the instrument calibration led to five previous reprocessings with the sixth currently underway.. Each time the data is reprocessed, degradation of the instruments are taken into account and the data is calibrated based on data collected from other sensors on other satellites as well as from data collected on Earth. MODIS is on-board both Terra and Aqua, when data from these sensors don’t match up the data can be corrected based on data gathered on Earth.

While it may be easy to assume that an aging instrument may be less accurate, in reality the longer the data record that it collects, the more accurate the data can be through reprocessing. Just think, if Terra had only lasted it’s six year design life, instead of going on 16, this error in the data may never have been caught and accounted for in newer data collections.

Thus, while the dark ice example could be considered by some as an indication of a flaw in how satellite data are used, it is, in reality, a success story for how a community of scientists working with groups like the MODIS Characterization and Support Team are striving to get to the right answer.

References:

Sun, J., X. Xiong, A. Angal, H. Chen, A. Wu, and X. Geng, (2014) Time-Dependent Response Versus Scan Angle for MODIS Reflective Solar Bands,IEEE Transactions on Geoscience and Remote Sensing, 52 (6), 3159-3174.

Polashenski, C. M., J. E. Dibb, M. G. Flanner, J. Y. Chen, Z. R. Courville, A. M. Lai, J. J. Schauer, M. M. Shafer, and M. Bergin (2015), Neither dust nor black carbon causing apparent albedo decline in Greenland’s dry snow zone: Implications for MODIS C5 surface reflectance, Geophysical Research Letters, 42.

Atmospheric science: Arctic snow is not becoming dirtier. (2015, 29 October). Nature Research Highlights, Accessed November 25, 2015.

Flooding in Iraq

20 November, 2015

turkey_Iraq_tmo_2015315

NASA Earth Observatory images by Joshua Stevens and Adam Voiland, using MODIS data from LANCE/EOSDIS Rapid Response and Landsat data from the U.S. Geological Survey.

Torrential rains in late October 2015 resulted in flooding in Iraq, prompting authorities to declare a state of emergency. Widespread power outages, overflowing sewers, and flooded streets displaced almost 84,000 people, according to news reports. The image from the Moderate Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, featured on NASA’s Earth Observatory, shows the flooding.

To see before and after images and read the whole article visit NASA’s Earth Observatory.

 

Early Winter Weather Across North America

19 November, 2015

canada_amo_2015316 The Moderate Imaging Spectroradiometer on NASA’s Terra satellite acquired this image of Canada and the western United States in mid-November, 2015.  Snow and cold rain doused the region, bringing early winter weather and making researchers ask if this is the beginning of a wet El Niño winter. The image shows a band of snow stretching across southern Alberta and Saskatchewan.

Read the whole article on NASA’s Earth Observatory.

Dense Fog Over Europe

15 November, 2015

Europe_tmo_2015305The Moderate Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this image of dense fog in Europe. The fog appears brighter than typical clouds because the water droplets in the fog are smaller and more efficiently scatter light. The fog disrupted air travel in London and central Europe in early November 2015. The image was featured on NASA’s Earth Observatory Image of the Day.

Hofsjökull Ice Cap Gains Mass for the First Time in Twenty Years

18 November, 2015

iceland_tmo_2015313The Moderate Imaging Spectroradiometer on NASA’s Terra satellite captured a view of the Nordic island, Hofsjökull, on November 9, 2015. The island is experiencing its first gain in mass. Since 1995, the ice caps in Iceland have been retreating and losing volume, however in October 2015, ground based measurements showed that the Hofsjökull ice cap gained mass.

Read the full story on NASA’s Earth Observatory