Sentinel Gets Its Start

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To assure Sentinel’s usefulness, especially to fire and emergency services teams across Australia, the group needed to handle a stumbling block that had been discovered not long after the Australians began receiving MODIS direct broadcast data at the Alice Springs location. The Terra and Aqua satellites sometimes passed over Australia as many as four times each day, in theory providing plenty of opportunity for coverage of the area, but there always seemed to be bits and pieces of the data stream missing. These apparently random periods of data loss, or blackouts, marred an otherwise seamless flow of key fire observations to state agencies.

Among the first users to notice the problem was research scientist Grant Allan from Australia’s Northern Territory Bushfires Council. In addition to NOAA hotspot detections that they had been using since 1998, Allan’s group had been using MODIS hotspot data provided by the Western Australia Department of Land Administration (DOLA) from the Perth receiving station throughout 2002. DOLA provided simple image maps, listings of fires by geographic location, or data files that could be imported into users’ GIS-mapping software applications. DOLA did not provide MODIS true-color imagery, however. As a complement to the MODIS and NOAA AVHRR hotspots from DOLA, Allan had been using the MODIS imagery that the Alice Springs facility was providing from its Web site as a precursor to the Sentinel Hotspots project. “The first MODIS images we had acquired, beginning in April 2002, were unaffected by the data drop outs,” Allan says. “At the time, we were able to collect images every second day to delineate the changing perimeters of active fires.”


Photographs of Smoke and Burn Scars in the Outback


According to Allan, the mapping of fire perimeters and burned areas with MODIS imagery provided more information to the Bushfires Council than just the hotspots. “Although we have found that a sequence of hotspots is invaluable to monitor the spread of fires,” says Allan, “it is not always possible to compile an accurate picture of the area burned using that information by itself. By April 2002, we were regularly collecting MODIS direct broadcast images from the Alice Springs receiving station, and they had become a significant component of our program in a short amount of time. As our fire season continued across the southern region of the Northern Territory, the unpredictability both of the timing of the drop outs and what area would be affected became frustrating.”

  Fires are impossible to track from the ground, or even by aircraft, across the wide open spaces of Australia’s Northern Territory. At left a pillar of smoke rises above a fast-moving bushfire. At right, dark red burn scars reveal the complexity of fire patterns across the grassland. In the center and upper left, isolated patches of unburned vegetation remain green.(Photographs courtesy Grant Allan, Northern Territory Bushfires Council)

Image of Burn Scars from Satellite


Help from the States
In September 2002, Allan emailed Justice, asking for an explanation of the MODIS blackouts and suggestions for overcoming the problem. “MODIS on Terra regularly stops its direct broadcast as it passes over central Australia, usually for an area extending from 21 to 25 deegrees [south latitude]. This causes a major problem for us,” he wrote. “We are in the middle of a big fire season and we have come to rely on MODIS images to delineate fire perimeters. If we can’t get around the problem in Australia, can we get around it with your help? Is there an opportunity for your system to provide suitable image products for central Australia?”

Feeding MODIS Rapid Response images produced at Goddard Space Flight Center (GSFC) to Allan’s group and other fire agencies in Australia was possible, but not ideal. Because the MODIS Rapid Response project is a global fire detection effort, it can’t rely on the real-time observations provided by the direct broadcast signal because those can only be collected when the satellite passes over the receiving station. The direct broadcast receiving station at GSFC in Maryland usually picks up real-time observations of the Atlantic Coast of the U.S. and sometimes gets images from as far inland as the Mississippi River. But once the satellite gets out of range, the signal is lost.

The MODIS Rapid Response system instead uses the standard MODIS data feed, in which the satellite stores several hours worth of observations and then passes them off to a nearby geostationary satellite, which relays them to a ground station in the New Mexico desert. This relay yields global coverage, but creates a two- to four-hour lag in posting the images to the web. Justice knew it would be better to find a way to reduce or eliminate the direct broadcast blackout periods when fire-monitoring activities were underway in Australia.

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Timely satellite data allowed Northern Territory Bushfires Council scientist Grant Allan to track and map fires in the wilderness. Burn scars appear as dark reddish smudges against the brown and green grasslands. Faint gray smoke rises above some actively burning fires. This true-color image was acquired on October 16, 2002. (Image courtesy MODIS Land Rapid Response Team at NASA GSFC)

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