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The Total Ozone Mapping Spectrometer, launched in July 1996 onboard an Earth Probe Satellite (TOMS/EP), continues NASA's long term daily mapping of the global distribution of the Earth's atmospheric ozone. TOMS/EP will again take high-resolution measurements of the total column amount of ozone from space that began with NASA's Nimbus-7 satellite in 1978 and continued with the TOMS aboard a Russian Meteor-3 satellite until the instrument stopped working in December 1994. This NASA-developed instrument, measures ozone indirectly by mapping ultraviolet light emitted by the Sun to that scattered from the Earth's atmosphere back to the satellite. The TOMS instrument has mapped in detail the global ozone distribution as well as the Antarctic "ozone hole," which forms September through November of each year.
In addition to ozone, TOMS measures sulfur-dioxide released in volcanic eruptions. The U.S. Federal Aviation Administration (FAA) is studying ways to use these measurements to detect volcanic ash clouds that are hazardous to commercial aviation.
Ozone, a molecule made up of three oxygen atoms, shields life on Earth from the harmful effects of the ultraviolet radiation of the Sun. The increased amounts of ultraviolet radiation that would reach the Earth's surface because of ozone depletion could increase the incidence of skin cancer and cataracts in humans, harm crops and interfere with marine life.
Researchers face two crucial problems in ozone studies --, finding a slow, long-term trend among a variety of short-term trends, and ascertaining how much of the change in global ozone is due to human activities and how much is attributable to natural atmospheric processes. In order to separate these factors, scientists must record data over at least a complete solar cycle, 11 years. TOMS instruments aboard Nimbus-7 and Meteor-3 satellites have proved invaluable in meeting this requirement.
Prior to the development of TOMS and its predecessor, the Nimbus-4 Backscatter Ultraviolet (BUV) instrument, ozone measurements were obtained via ground-based Dobson Spectrophotometer stations. These data, while reliable, are limited because the stations only measure ozone in the atmosphere directly above them so data are limited to over land regions. From the launch of the first TOMS until late 1994, the NASA TOMS program provided daily global views of ozone. TOMS is the primary source of high-resolution global maps of the total ozone content of the atmosphere. TOMS/Nimbus-7, the first TOMS instrument, provided reliable, high-resolution maps of global ozone amounts on a daily basis from October 1978 to May 1993.
During its lifetime on Nimbus-7, TOMS helped make "ozone" a household word through false-color images of the Antarctic Ozone Hole. Even after 14 years of operations, TOMS was testing new concepts such as "now-casting" of winds at flight altitudes and volcanic ash clouds. Now-casting is real-time ozone mapping that occurs when satellite data is processed and displayed on a computer screen simultaneously as the satellite passes overhead. In April 1993, during an early morning pass of NASA's Nimbus-7, scientists were able to take real-time ozone readings from TOMS.
The Meteor-3/TOMS, the first significant U.S. science instrument to fly on a Russian spacecraft, was one of the main sources of ozone data until it was deactivated in 1994. Meteor-3/TOMS was launched in Aug. 15, 1991 from Plesetsk on a cyclone launch vehicle in what was then the Soviet Union.
The launch vehicle for TOMS/EP was an expendable Pegasus-XL rocket, released at an altitude of 38,000 feet (11.6 kilometers) from an L-1011 jet. Following separation, an onboard hydrazine propulsion system then will circularize the spacecraft in its final 310-mile (500-kilometer) sun-synchronous orbit. The final orbit was reached approximately 21 days after launch. Data collection began at approximately launch plus 30 days. TOMS/EP, built for NASA by TRW Space & Electronics Group, Redondo Beach, Calif., has a minimum operating requirement of two years.
The TOMS instrument is a second-generation backscatter ultraviolet ozone sounder. TOMS can measure "total column ozone"—the total amount of ozone in a "column" of air from the Earth's surface to the top of the atmosphere—under all daytime observing and geophysical conditions. TOMS observations cover the near ultraviolet region of the electromagnetic spectrum, where sunlight is absorbed only partially by ozone.
TOMS/EP measures total ozone by observing both incoming solar energy and backscattered ultraviolet (UV) radiation at six wavelengths. "Backscattered" radiation is solar radiation that has penetrated to the Earth's lower atmosphere and is then scattered by air molecules and clouds back through the stratosphere to the satellite sensors. Along that path, a fraction of the UV is absorbed by ozone. By comparing the amount of backscattered radiation to observations of incoming solar energy at identical wavelengths, scientists can calculate the Earth's albedo, the ratio of light reflected by Earth compared to that it receives. Changes in albedo at the selected wavelengths can be used to derive the amount of ozone above the surface.
TOMS makes 35 measurements every 8 seconds, each covering 30 to 125 miles (50 to 200 kilometers) wide on the ground, strung along a line perpendicular to the motion of the satellite. Almost 200,000 daily measurements cover every single spot on the Earth except areas near one of the poles, where the Sun remains close to or below the horizon during the entire 24-hour period.
In addition to various "housekeeping" sensors to monitor the well-being of the instrument, TOMS uses several in-flight calibration modes to assess system performance.
The extremely high quality of TOMS ozone data has also helped scientists in detecting a small but steady long-term damage to the ozone layer over several parts of the globe, including most of the heavily populated areas in the northern mid-latitudes. This discovery led to the curtailment of the production of ozone-depleting chemicals through an international treaty signed in Montreal in the 1980s. To ensure that ozone data will be available throughout the next decade, NASA will continue the TOMS program using U.S. and foreign launches. The Japanese Advanced Earth Observations Satellite (ADEOS) will carry a fourth TOMS into orbit when it launches later this year. It will complement the Earth Probe instrument by collecting data at a 800 km (497-mile) orbit while the Earth Probe will be taking measurements at a 500 km (310-miles) orbit. The two instruments working at different altitudes will enhance data collection by affording a more reliable separation of stratospheric ozone from tropoospheric ozone. It will also improve detection of urban pollution and other small-scalle features and allow comparison of ozone data from cloudy and clear atmospheres. A fifth TOMS Instrument has been assembled for flight in 2000 on a Russian Meteor-3M satellite.
The TOMS program is managed by NASA's Goddard Space Flight Center, Greenbelt, Md., for the Office of Mission to Planet Earth, Washington, D.C.
TOMS is part is NASA's Mission to Planet Earth, a long term, coordinated research effort to study the Earth as a global environmental system. Using the unique perspective available from space, NASA will observe, monitor and assess large-scale environmental processes, focusing on climate change. MTPE satellite data, complemented by aircraft and ground data, will allow humans to better understand natural environmental changes and to distinguish natural changes from human induced changes. MTPE data, which NASA will distribute to researchers worldwide, is essential to humans making informed decisions about their environment.
September 22, 1997