by Michael Abrams, Jet Propulsion Laboratory, Pasadena, Calif.
The El Niño phenomena and its effects are with us all too frequently. However, it was only with the development of remote sensing instruments operating from satellites that the cause for this global disruption of weather patterns was discovered. Remote sensing technology uses satellite instruments to view the Earth in wavelengths extending from the visible part of the electromagnetic spectrum through the thermal infrared to the microwave region. Satellite instruments are mostly passive, which means they record reflected or emitted radiation from the surface. Passive sensors more frequently operate only during the day and cannot penetrate clouds. Active sensors provide their own sources of illumination. One common type of active sensor that operates in the microwave region is called radar. Radar instruments can operate in any type of weather and day or night.
"Visible" instruments are like cameras. They record data either on film or convert light to electrical signals that are recorded digitally. Making observations in the light bandwiths beyond the visible part of the spectrum requires the use of detectors rather than film. These detectors are sensitive to reflected or emitted electromagnetic radiation in other parts of the spectrum and usually record the received signals digitally for transmission back to receivers on the Earth. Data from the various spectral regions can be used to complement one another and give a fuller, more accurate picture of Earth from space.
All matter emits energy at thermal infrared wavelengths. This fact was recognized early by the military for reconnaissance applications in the 1950s, and has attained a high degree of sophistication, as witnessed by the startling images we saw from aircraft cockpits during the Desert Storm War. Ghostlike images of soldiers running from glowing tanks and radiant missiles homing in on buildings and vehicles are examples of how this phenomenon is exploited for military purposes.
In the civilian sector, satellite acquisition of thermal infrared images began in 1960 with the U.S. meteorological Television Infrared Operational Satellite (TIROS). The wide area coverage and coarse spatial resolution of these weather satellite instruments are ideal for monitoring cloud patterns and ocean temperatures. Their routine use for weather prediction dramatically affects our everyday lives, and makes it less likely that we will leave the house without an umbrella.
We are all familiar with the animated satellite cloud images shown on our local evening news programs. These images are delivered by several operational environmental satellites. Improvements in the tracking and monitoring of hurricanes by remote sensing has dramatically reduced loss of life when a hurricane comes on land, though the loss of property is still frightful. These instruments have spatial resolutions of 1-4 km (the size of the smallest features they can detect) and cover large swath widths, up to an entire hemisphere (see figure 1).
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| Figure 1: GEOS satellite image of the western hemisphere in the thermal wavelength region. Warm areas are dark, and colder areas--such as clouds--are bright. |
These instruments provide three thermal infrared channels and acquire 1-km spatial resolution images, covering a 2700 km swath width, obtaining images over the same area several times each day. They are used routinely by the science community to monitor the ocean and land surface of our planet.
Using the multispectral capability of AVHRR, volcanologists are able to detect volcanic ash plumes and distinguish them from meteorological clouds. An operational system has been developed to warn aircraft about potentially hazardous flying conditions. Before this warning system, several jumbo jets unknowingly flew into ash plumes, which disabled their engines. It was only by the skill of the pilots and a large measure of luck that total disasters were avoided.
Subsurface coal fires and forest fires can also be monitored with AVHRR data. Because coal fires heat the ground, their thermal signatures are detectable in infrared images. In visible images, the fires are invisible. Forest fires are a major cause for concern, destroying resources and often threatening urban areas. Thermal infrared wavelengths can penetrate smoke and show fires or areas of higher temperatures, and scientists are using the images to monitor the clearing of the rain forests by slash-and-burn agricultural practices. Evidence from the thermal images is being used to pressure governments to develop plans to more effectively manage the rain forests and develop alternatives to destructive agriculture methods.
Thermal infrared images of the oceans have dramatically increased our knowledge and understanding of ocean currents, heat exchange between the ocean and atmosphere, and the interplay between sea-surface temperature and the weather. The El Niño phenomenon was recognized long ago, but the triggering mechanism was poorly understood until infrared satellites vividly showed the increase in ocean temperature in the eastern Pacific. We now have a method for predicting the onset of future El Niños, which allows scientists to warn the public months in advance. Municipal authorities can better prepare for above-average rainfall by cleaning out flood control channels and catchment basins. Homeowners can fix leaky roofs and shore up hillsides in anticipation of heavy precipitation, instead of being caught off-guard.
Remote sensing has also shown us that the Gulf Stream is like a river in the Atlantic Ocean, flowing from the Caribbean, northward along the east coast of North America, across the Atlantic, and then southward along the west coast of Europe. Satellite images show eddies, loops, and splits in the current, which manifest as differences in water temperature. The fishing industry uses these images to look for fish: operational systems have been developed to broadcast satellite images directly to fishing vessels to help guide them to the most favorable areas for fishing.
Thermal infrared images acquired by satellite systems like AVHRR compile regional maps of ice concentration in polar regions, and are especially useful during periods of winter darkness. On these images, open water is warmer than ice. Ice insulates the relatively warm water beneath it; larger amounts of energy are transmitted to the surface through thin ice, and smaller amounts through thick ice. In this way, an estimate of ice thickness can be obtained. This information is used by ships navigating polar regions, such as off-shore petroleum exploration vessels.
Satellite thermal infrared images are also used to monitor oil spills and oil slicks. Thin layers of oil on the ocean surface change the apparent thermal temperature of the water, since oil has a lower emissivity than water, and absorbs some of the emitted thermal radiation. Therefore, the brightness-or radiant-temperature of the water below an oil slick is lower even though the kinetic temperature is the same. So an oil slick appears colder than the surrounding water, and can be detected thermally.
What does the future hold for thermal infrared sensing from space? In June 1998 NASA will launch the Earth Observing System AM-1 satellite platform, the first in a series of missions to measure the health of the Earth. Included on the first mission are two imaging instruments with infrared capability: the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). MODIS is an improved AVHRR, with more, higher spectral-resolution channels and about the same 1 kilometer spatial resolution. ASTER is a high spatial resolution imaging device, the first ever put into space with multiple spectral bands. With its 90-meter spatial resolution, ASTER thermal infrared data will be used in many applications: to study degradation of wetlands areas, investigate cloud properties, monitor changes in temperate glaciers, map surface rock types (see figure (2)), explore mineral and petroleum deposits, monitor volcanoes by observing changes in high temperature phenomena such as lava lakes and changes in gas emissions, and study the urban heat island effect and its ramifications for human impact on urban environments.
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| Figure 2: From Geophysical Monograph 92, 1995: Mauna Loa Revealed: Structure, Composition, History and Hazards; Chapter on "Remote Sensing of Mauna Loa" by Kahle et al., page 149. Multispectral thermal infrared color composite of lava flows on the flank of Mauna Loa, Hawaii. Different colors can be related to the relative ages of the flows; green areas are vegatated. |
(1) GOES satellite image of the western hemisphere in the thermal infrared wavelength region. Warm areas are dark, and colder areas-such as clouds-are bright.
(2) From Geophysical Monograph 92, 1995: Mauna Loa Revealed: Structure, Composition, History and Hazards; Chapter on "Remote Sensing of Mauna Loa" by Kahle et al., page 149. Multispectral thermal infrared color composite of lava flows on the flank of Mauna Loa, Hawaii. Different colors can be related to the relative ages of the flows; green areas are vegetated.
| A Few Words From the Author | |
I was born in 1948 in Cleveland, Ohio. In 1970 I earned a bachelor's degree in biology, and in 1973 a master's degree in geology, both from the California Institute of Technology. In 1973 I went to work for the Jet Propulsion Laboratory, where I worked on the first Earth Observing Satellite, ERTS-1. Since then I have done research on varied applications of remote sensing for geologic problems, including stratigraphic |
mapping, mineral exploration, and for the last 4 years, on volcanic hazards mitigation and mapping, mainly in the Trans Mexican Volcanic Belt. My interest in geology started at an early age with fossil collecting in fields near my home. The degree in biology was an unplanned detour; long hours in basement labs made it clear that a career in an outdoor, natural science was far more attractive. |