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      Isolated locations of many volcanoes around the world demonstrate the important role and need that global satellite remote sensing techniques play in terms of maintaining a high-level of aviation safety. The synoptic perspective and  repetitive coverage afforded by these satellites provides time series data sets (temporal response patterns) necessary for the detection and recognition of changes in volcanic activity levels in otherwise inaccessible areas of the world. Unique sensor types aboard earth orbiting satellites that travel in either a geosynchronous or a polar sun-synchronous orbit,  have the ability to accurately monitor this environmental hazard that volcanic activity presents to commercial transport aviation. They do so by observing changes in terms of thermal activity, in terms of development and movement of ash clouds and in terms of SO2 gas emissions as these plumes drift thousands of miles away from an eruption site to various heights and various directions of the atmosphere which could pose a significant threat to oncoming commercial air  traffic.

GOES

     Geostationary Operational Environmental Satellites (GOES) beginning with the GOES 8 in 1994 have provided an excellent platform for tracking hazardous volcanic ash clouds. Visible and thermal infrared detectors aboard these satellites are considered to be by far the most useful volcanic cloud detectors for two reasons: 1) they detect volcanic ash directly and 2) GOES gives nearly global coverage very frequently (about every 15-60 minutes). It has been acknowledged at the 10th Conference on Aviation, Range, and Aerospace Meteorology that the results of an impact study show "that there will most likely be some degradation of ash detection capability, especially at night, but that analysts armed with animated imagery will still be able to identify and track ash clouds, and issue timely advisories for aircraft avoidance". (above:  GOES 10 composite image showing lower left to upper right track of Cleveland volcano ash plume )
    
NOAA AVHRR

Another means of detection is the Advanced Very High Resolution Radiometer (AVHRR) sensor onboard polar-orbiting NOAA satellites which can distinguish between volcanic clouds and meteorological ones using two-band data in the thermal infrared . According to the Alaska Volcano Observatory (AVO), this NOAA-14 AVHRR Band 3 image below was received at 1451 UT (0651 AKDT), and features a thermal anomaly extending 9-10 km northwest from the summit area of Pavlof Volcano on September 23, 1996. AVO reports that the hot spot corresponds with eyewitness accounts of glowing flows on the northwest flank of the volcano on the same day. They also note that the colors represent radiometric temperature; red is hot, blue is cold and that several of the hot pixels were saturated at about 48 degrees C. In addition, adjacent to the hot pixels are three anomalously cold pixels, called sensor recovery pixels. These are caused by the sensor being overwhelmed with radiation from what has been termed the upscan heat source and that the presence of these in the image indicates the presence of incandescent lava at the surface.
     Clearly, the data received by both the GOES and the NOAA satellites are very useful in tracking volcanic ash plume direction because they both provide a very large area of coverage. Yet, a major drawback to both of these systems is that this detection begins after the eruption has occurred; they are lacking in fine resolution abilities to measure volcanoes in their pre-eruption stages (pixels 1000-1100m) according to a website publication titled An Exploration of the Use of Satellite Remote Sensing to Predict Extrusive Events on Colima Volcano  (link to read more about this in depth study by Bruce Wahle). One way to offset this deficiency was discovered with the use of the Total Ozone Mapping Spectrometer. In the UV spectrum, (TOMS) detected  SO2 gas and collects volcanic cloud position data approximately once each day during daylight hours.
    
TOMS

According to a USGS publication dated Jan.30,2001, "the first TOMS sensors aboard the Nimbus-7 and Meteor-3 satellites detected 55 out of 350 known eruptions between 1979 and 1992, and also from several eruptions not known from ground studies". However, USGS acknowledged there was still a problem in that early TOMS instruments could only measure SO2 gas from moderate to large eruptions that spread clouds over enormous areas. Addressing these deficiencies they also note that improved TOMS instruments on a later ADEOS satellite launched in 1997 and placed in lower orbits were now able to detect gases from smaller eruptions and from the passive degassing of some volcanoes. (The interesting feature of the above right image is the side by side contrast of  the same volcanic eruption taken by AVHRR and that taken by TOMS.) However ADEOS failed shortly after launch. Replacing TOMS will be a new sun synchronous polar orbiting satellite known as EOS AURA, scheduled for launch in June of this year; it will have many functions and one of them will be to monitor SO2 gases with onboard sensors known as TES ,MLS, and OMI. (sensor details found in EOS AURA link)