This article focuses on instruments aboard an orbiting satellite and high-flying aircraft study grass fires that straddle a continent. NASA designed its $1.3 billion Terra to be the flagship in a new series of Earth-observing satellites that will study phenomena affecting the climate. The instruments carried by Terra that were most active during the Safari 2000 field experiment were Moderate-Resolution Imaging Spectro-Radiometer (MODIS), Multi-Angle Imaging Spectro-Radiometer (MISR), and Measurements of Pollution in the Troposphere (MOPITT). MOPITT accomplishes its mission by using gas correlation spectroscopy to measure rising and reflected infrared radiance in three absorption bands of carbon monoxide and methane. The Terra’s Safari 2000 observations were augmented by measurements taken by instruments aboard several aircraft, including the high-altitude Lockheed-Martin ER-2 that NASA flew from Pietersburg, South Africa, as part of the African field experiment. The South African Weather Bureau contributed two Aerocommander 690A aircraft to Safari 2000. One of the twin-engine, turboprop planes was used for aerosol research, while the other one helped validate the carbon monoxide measurements obtained by MOPITT.
Each year, fires ravage nearly a half-million square miles of grasslands in subequatorial Africa. This year, for the first time, the impact of these conflagrations on the world’s climate and the region’s environment were studied in a six-week field experiment that was supported by the National Aeronautics and Space Administration.
This experiment was part of a three-year scientific study called the Southern African Regional Science Initiative, or Safari 2000. An integral part of this year’s field experiment was data gathered by instruments borne on NASA’s Terra satellite and on airplanes, including the agency’s high-flying ER-2 reconnaissance aircraft that tracked and analyzed the movement of fires and smoke.
The fires that scorch Africa south of the equator are concentrated in the moist, subtropical belt that crosses from Angola on the Atlantic coast, through southern Congo and through Zambia, to northern Mozambique, and southern Tanzania on the Indian Ocean. The blaze sweeps across an area larger than Idaho, Montana, Wyoming, North Dakota, and South Dakota combined. About three-quarters of the fires are started by humans for land management or agriculture.
This year’s season was expected to be especially destructive, because heavy rains earlier in the year fostered the growth of plant biomass that becomes fuel for the grass fires during the dry season. A similar scenario played out in 1994 after a heavy rainy season, according to Robert Scholes, an ecologist affiliated with CSIR Environmentek, a South African research organization. Scholes is also one of the organizers of the Safari 2000 project. He said that the burgeoning biomass could cause twice as much acreage to burn this year. Predicting this, and evaluating its effect on the region and the world is a major goal for Safari 2000.
The American space agency has had a long-standing professional collaboration with South African scientists like Scholes, a partnership dating back to 1992, according to Michael King, an aerospace scientist and senior project scientist of NASA’s Earth Observation System, which includes the Terra satellite used in Safari 2000.
The burning season in subequatorial Africa consumes some 500,000 square miles of grasslands from the Atlantic Ocean to Indian Ocean seaboards. It was studied from space for the first time in August and September.
“Eight years ago, we conducted Safari 92, using aircraft to test whether the high levels of tropospheric ozone inferred front satellites over the South Atlantic were a result of biomass burning in Africa,” King said. “More recently, several South African scientists were visiting their colleagues at the University of Virginia and suggested NASA take part in studying the grass fires in Africa, simultaneously taking advantage of the new satellite capability of Terra along with the unique meteorology of southern Africa.”
NASA agreed, and the science initiative now known as Safari 2000 was born. The major field experiment of the initiative was scheduled to run during the dry season from Aug. 13 to Sept. 25, coinciding with the peak burning season in southern Africa.
The $1.3 billion Terra satellite is NASA’s most advanced Earth-observing satellite. It was launched from Vandenberg Air Force Base, Calif., in December 1999, to conduct a six-year mission observing how the land, oceans, and atmosphere interact. The school bus-size satellite orbits 438 miles above Earth, collecting spectral data of the entire globe, ranging from visible light to the infrared. Terra compiles 850 gigabytes of measurements daily, the equivalent of 100,000 encyclopedia volumes.
“Terra is the flagship of NASA’s planned series of several Earth-observing satellites, which will be continued by the National Polar-orbiting Operational Environmental Satellite System (NPOESS) project that includes NOAA, the Department of Defense, and NASA. The NPOESS project will replace the current polar orbiting NOAA and DOD weather satellites and will be based on a more sophisticated series based on NASA’s experience,” explained Yoram Kaufman, an atmospheric scientist who was project scientist of Terra for four years. Terra observes a variety of phenomena, including fires, that may affect climate.
The instruments carried by Terra that were most active during the Safari 2000 field experiment were MODIS, MISR, and MOPITT. MODIS stands for Moderate-Resolution Imaging Spectro-Radiometer. This device measures cloud characteristics, vegetation, fires, ocean color (caused by sediment or plankton), ocean currents, snow cover, land and ocean surface temperatures, and aerosol, or particle, concentrations and properties, at moderate resolutions.
MODIS was built by Hughes Corp.’s remote sensing division based in Santa Barbara, Calif., and is a whisk broom scanning/ imaging radiometer. It consists of a cross-track scan mirror, collecting optics, and a set of linear arrays with spectral interference filters in four focal planes.
During its operation, MODIS views a swath of land 2,330 kilometers wide and provides radiometric images of solar radiation reflected in the daytime, and both day and nighttime thermal emissions from around the world.
“MODIS is the most multidisciplinary unit on Terra, and provides the big, global perspective,” King said. In Safari 2000, MODIS monitored the location and intensity of the African fires, aerosol concentrations in the atmosphere, and vegetation health, and studied burn scars the fires left.
“Scientists also used MODIS to study the optical properties of clouds off the Namibian coast to calculate cloud droplet size and optical thickness,” said King. In addition, MODIS measured the solar reflectance of different local ecosystems, including Etosha National Park, which is the largest salt pan in Africa; the Okavango Delta and Sowa Pan in Botswana, and Kruger National Park in South Africa.
The Terra’s Multi-Angle Imaging Spectro-Radiometer, or MISR, was built by NASA’s Jet Propulsion Laboratory in Pasadena, Calif., to study the sunlight scattered by the Earth’s surface, clouds, and atmospheric particles. Unlike most satellite-borne instruments that look straight down or toward the edge of the planet, the MISR views the Earth with a camera aimed at nine separate angles simultaneously to enable researchers to determine how sunlight is scattered into different directions.
“This is because the land reflects solar radiance at different magnitudes and at different angles,” King said.
The knowledge can be used to discover how the climate may be changing.
One MISR camera is aimed toward nadir—straight down to the Earth; others are aimed to the Earth’s surface fore and aft of Terra at 26.1 degrees, 45.6 degrees, 60 degrees, and 70.5 degrees. As the Terra orbits the Earth, the nine MISR cameras view a 360-kilometer-wide arc over a period of seven minutes in each of four wavelengths: blue, green, red, and near-infrared. In addition to studying the behavior of sunlight on Earth, the MISR measures the concentration and type of aerosol particles in the atmosphere resulting from fires; the amount, height, and type of clouds; and the distribution of vegetation.
“In the African research project, MISR’s multiple-look angle images were used in concert with ground-based observational towers to study aerosol properties of smoke generated by fires and industrial emissions,” said King.
The Terra carries a Measurements of Pollution in the Troposphere (MOPITT) system that was provided by the Canadian Space Agency, and designed by Com Dev Ltd. of Cambridge, Ontario. This scanning radiometer discovers the sources and distribution of carbon monoxide and methane in the troposphere, from ground level to 10 miles up.
MOPITT accomplishes its mission by using gas correlation spectroscopy to measure rising and reflected infrared radiance in three absorption bands of carbon monoxide and methane. This involves changing the length or pressure of the reference gas sample in the optical path of the instrument, thereby modulating the sample’s density.
The pollution measuring system has a resolution of 22 km at nadir and can observe swaths 640 km wide, measuring carbon monoxide to within 10 percent accuracy, and measuring the methane column to better than one percent accuracy. MOPITT operates continuously both day and night, and calibrates itself by means of on-board black bodies and a space look; that is, looking out at deep space, where it is a constant 4 kelvin.
Smoke and Mirrors
Two other Terra instruments contributed to Safari 2000. The Advanced Spaceborne Thermal Emission and Reflection Radiometer, or ASTER, instrumentation takes high-resolution images of the Earth—15 to 90 meters— in the visible near-infrared, short-wave infrared, and thermal infrared regions of the spectrum. These findings are used to derive data products for cloud and soil properties, surface mineralogy, surface temperature, topography, and an index for vegetation.
During Safari 2000, ASTER was the high-resolution magnifying glass that enabled the most detailed space-based views of the Earth’s surface before, during, and after burning of the vegetation.
NASA worked with the Japanese Ministry of International Trade and Industry to design ASTER, which consists of three distinct telescope subsystems, each subsystem working in its own spectral region. For example, NEC Corp. of Tokyo built ASTER’s visible near-infrared subsystem. The VNIR subsystem features two telescopes, one looking backward at the satellite’s track, and the other aimed at nadir. The latter is a Schmidt reflecting/ refracting telescope that works with its backward-looking counterpart to produce stereo images.
The VNIR provides 15-meter resolution over a 60-km swath. The telescopes can track over 24 degrees to revisit a location of particular interest, such as the African fires. Two halogen lamps periodically help calibrate the telescopes.
Tokyo-based Mitsubishi Electric Co. built the short-wave infrared, or SWIR, subsystem in ASTER. The SW1R points a mirror from plus or minus 8.54 degrees from nadir during Terra’s orbit. The mirror operates in six short-wave infrared channels possessing 30-meter resolution over a 60km swath. The SWIR mirror does double duty by periodically directing light from either of the two calibration lamps into the SWIR’s fixed, refracting telescope.
Fujitsu Ltd., also headquartered in Tokyo, designed SWIR’s thermal infrared, or TIR, subsystem around a scanning mirror that operates in five thermal infrared channels when pointing plus or minus 8.54 degrees from nadir. The light from the scanning mirror is reflected into a Newtonian, catadioptric telescope system with a primary mirror and a lens to correct aberrations. Similar to the SWIR, the TIR scanning mirror periodically views an onboard black body for calibration.
During Safari 2000, the Clouds and the Earth’s Radiant Energy System, or CERES, measured the Earth’s radiation budget; that is, the difference between the solar energy sent to the Earth and the amount radiating back into space. The density of vegetation cover, atmospheric changes, cloud formations, and aerosols affected by, or generated by, the grass fires all affect the radiation budget.
The CERES was built by TRW in Redondo Beach, Calif., and consists of two broadband scanning radiometers. One radiometer scans in a cross-track mode as the other scans in a biaxial mode. The former will continue NASA’s Earth Radiation Budget Experiment and Tropical Rainfall Measuring Mission study, while the latter will provide fresh angular flux information. That data will be used to enhance the accuracy of angular models researchers use to calculate the Earth’s radiation budget.
Each of the CERES radiometers is a three-channel device. One channel measures reflected sunlight, another measures thermal radiation emitted by the Earth in the 8- to 12-micrometer wavelength, and a third channel measures the total radiation, both solar and infrared. Scientists subtract solar radiation from the total channel to obtain the emitted infrared radiation.
TRW equipped its radiometers with a solar diffuser, a tungsten lamp system with stability monitor, and a pair of black body sources to calibrate the radiometers during their operation.
Laboratories in Flight
The Terra’s Safari 2000 observations were augmented by measurements taken by instruments aboard a number of aircraft, including the high-altitude Lockheed-Martin ER-2 that NASA flew from Pietersburg, South Africa, as part of the African field experiment. The ER-2 is a descendant of the U.S. Air Force’s U-2 that routinely flies at altitudes between 65,000 and 70,000 feet on research missions at cruising speeds of 470 miles per hour.
During typical six-hour missions, the ER-2 covers 2,500 miles, although the aircraft can fly for more than eight hours at ranges exceeding 3,400 miles. The plane carries a payload of scientific instruments up to 2,600 pounds in several locations: its nose bay, the main equipment bay behind the cockpit, two wing-mounted superpods, and smaller underbody pod. NASA’s Airborne Science Directorate operates two ER-2 jets at the space agency’s Dry-den Flight Research Center in Edwards, Calif.
For the Safari 2000 project, the ER-2 carried a MODIS Airborne Simulator, built by Daedalus Enterprises Inc. in Ann Arbor, Mich. “This instrument is a 50-channel spectrometer that operates within the 0.47-micrometer to 14.1-micrometer range. NASA Ames developed the logarithms to adapt the MODIS Airborne Simulator for use in the high-altitude ER-2 aircraft environment,” King said.
During the Safari 2000 field experiment, the airborne device monitored 17 of the 36 wavelengths available on Terra’s MODIS instrument to determine sea surface temperatures, fire location and temperatures, vegetation properties, and both aerosol and cloud properties.
The ER-2 also carried an airborne version of the MISR to complement its orbiting counterpart. “Think of the Air MISR as the 10th telescope of Terra’s nine-telescope system, which serves to validate the others’ findings,” suggested King. The Air MISR’s single telescope is equipped with an electric motor to aim it at the nine different angles the satellite MISR observes simultaneously.
The Canadian Space Agency funded, and the University of Toronto built, an airborne version of MOPITT for the ER-2. This version looks only at nadir, and hence does not image a swath beneath the aircraft as does MOPITT on board the Terra satellite.
In addition, the ER-2 carried two solar flux radiometers, one installed on top of the aircraft, and the other beneath it. “These devices were designed and built by NASA’s Ames Research Center to measure and give spectral significance to both the sunlight reaching Earth, and the radiance reflected up from the atmosphere,” King explained.
The other aircraft that took part in Safari 2000 included a Convair-580 owned by the University of Washington in Seattle. That plane carried an array of instruments to study aerosols, cloud properties, atmospheric chemistry, and meteorology. The CV-580 flew out of Pietersburg for the first four weeks of the Safari 2000 dry season campaign, and was based in Walvis Bay, Namibia, for the final two weeks to study the stratus clouds that form off Namibia.
The South African Weather Bureau contributed two Aerocommander 690A aircraft to Safari 2000. One of the twin-engine, turboprop planes was used for aerosol research, while the other one helped validate the carbon monoxide measurements obtained by MOPITT.
The United Kingdom Meteorological Office flew a C-130 out of Windhoek, Namibia, from Sept. 2 to 19 to investigate aerosols and radiation in the western portion of the Safari 2000 area of study.
Although King said it is early to draw specific conclusions on the Safari 2000 satellite and aircraft data, he stressed that “we obtained valuable information and spectacular imagery of the fires that was presented to the Deputy President of South Africa and cabinet on Sept. 13.”
While the field experiment was short-lived at six weeks, the Terra satellite will continue to collect observations over southern Africa and the rest of the globe for years. An enduring goal of the experiment was to establish a data center in southern Africa where scientists and students could come for training and have access to the latest tools for working with the aircraft, surface, and satellite data.
According to King, “Through this partnership, the scientific understanding of the ecosystem and environment of southern Africa will be enhanced, and a valuable resource has been established for use by the people of southern Africa for years to come.”