Fullscreen HTML5Med Res (??MB)HTML4

Terra: Mission to Planet Earth Terra was launched at 1:57 pm EST on December 18, 1999 from Vandenberg Air Force Base, California. After reaching orbit, the satellite successfully deployed its solar array and its high gain antenna to enable communication with other satellites. 30  Silicon Chip www.siliconchip.com.au By SAMMY ISREB The recent NSW bushfire crisis has been one of the worst such events in Australia’s history. The intense media coverage included astounding NASA satellite photos showing the enormity of the situation. Space it seemed, gave the ultimate testimony of the ravages of the fires. So how exactly does the amazing tech­nology behind these images work and what benefits does it pro­vide? S INCE ITS BEGINNINGS in 1958, NASA has focused not just on space exploration but also on the furthering of scientific knowl­ edge regarding planet Earth. With so much unknown about the factors effecting the climate on Earth, NASA embraced Earth System Science, a field encompassing meteorology, oceanography, biology and atmospheric sciences. In the early 1990s, NASA commenced what is known as the Earth Science Enterprise, a detailed study into the Earth as an environmental system. The Earth Science Enterprise is composed of three main components: a series of satellites, an advanced data storage and processing system and various teams of scientists working to analyse the data. On the 18th of December 1999, the first of the Earth Observ­ing System (EOS) series of satellites was launched. The Terra satellite, formally known as the EOS AM-1, was built at a cost of over US$1.3 billion by Lockheed Martin Missiles and Space. The 5190kg Terra circles the Earth at an altitude of around 705 kilometres, with a polar orbit of inclination of 98°. This orbit is specifically designed to descend southwards over the equator at 10:30am (local time), at which time cloud cover is usually at its daily low. Terra orbits the Earth once every 99 minutes. During the 6-year initial duration of Terra, its orbit will be periodically adjusted to maintain its integrity (once per orbit). Five state-of-the-art instruments make up the scientific payload of Terra. These are used to generate an integrated snapshot of the Earth which is far superior to anything available from previous orbital remote sensing techniques. Dr Grassem Asrarm, NASA’s Associate Earth Science Director, commented that Terra “has nearly unlimited potential to improve scientific understanding of global climate change”. www.siliconchip.com.au This map shows the abundance of airborne particulates, or aerosols, over Southern Africa during the period August 14 - September 29, 2000. Low particle concentrations are shown in shades of blue, high concentrations in shades of red. These results were generated from MISR imagery acquired over this time period and processed using MISR’s automated software system. The approach for deriving aerosol amount makes use of the variation of scene brightness and contrast as a function of observation angle. Black areas over the land area correspond to places where a result was not obtained; eg, due to the presence of clouds. Extensive burning of grass and shrubland for land management and agriculture comprises a principal source of these aerosols. Vegetation availability increases northward, hence the greater abundance of haze and smoke in Angola and southern Zaire. The lower aerosol abundance around Lesotho and southeastern South Africa is consistent with the higher terrain elevations near the Drakensberg Mountains. Let’s now have a look at the five different measurement systems on Terra. CERES – measuring radiation balance The two “Clouds and the Earth’s Radiant Energy System” (CERES) instruments aboard Terra provide the most accurate global radiation measuring system ever, allowing in-depth insight into the Earth’s radiation balance. Everything, from plants, animals and even the Earth as a whole, emits energy, some of which leaves the Earth. This provides a balance against the incoming energy from the Sun, maintaining a consistent climate. Know­ledge of this radiation balance and the factors which affect it, is essential for understanding and modelling the Earth’s climate, both now and into the future. The CERES units are essentially scanning radiometers oper­ating at three discrete channels. The first of these is a short­ wave channel for measuring reflected sunlight, ranging from 300nm - 5µm in wavelength. The second, longwave, channel, is designed to measure Earth-emitted thermal radiation in the 8-12µm region. The third channel provides a total radiation measurement between waveMarch 2002  31 SYDNEY Smoking! – this MODIS image shows the December 2001-January 2002 bushfires to the north, south and west of Sydney. lengths of 0.3 - 200µm. Data from the CERES units is ideally suited to some of the following applications: • Determining the effects of solar radiation as an input to global atmospheric models. • Extended range weather predictions. • Increasing our understanding of long-term climatic change. MISR – nine cameras, four wavelengths With the majority of remote-sensing satellite instruments either aimed directly towards the Earth or towards a fixed point in the atmosphere, much information regarding the effects of energy scattering within the atmosphere has been overlooked. The Multi-Angle Image Spectroradiometer (MISR) instrument was de­signed to address this need through the use of nine cameras. One is aimed straight down and there are fore and aft pairs angled at 26.1°, 45.6°, 60.0° and 70.5° from the Earth’s surface. As Terra orbits, the Earth’s surface is progressively mapped by each of the nine cameras in four wavelengths: blue (446nm), green (558nm), red (672nm) and infrared (866nm). MISR is able to acquire a global picture around once every nine days. Operating only during the day due to its visible light requirements, the 149kg instrument draws an average of 72W power and produces an output stream 32  Silicon Chip averaging 3.3 megabits/second. Built by Jet Propulsion Laboratories for NASA, the MISR has already provided a good insight into monthly, seasonal and long term trends of the following: • The amount and composition of atmospheric aerosol particles, both man-made and natural. • The amount, type and heights of cloud cover around the world. • The composition of land surface cover, including vegetation density, health and structure. MODIS – measures infrared The Moderate-Resolution Imaging Spectroradiometer, built by Raytheon (previously Hughes), Santa Barbara, is designed to provide a broad range of observations of the Earth’s atmosphere, land and oceans, in both the visible and infrared regions, with the ability to construct a global snapshot over a two-day period. MODIS features a viewing width of 2330km over 36 separate spectral bands, ranging from 0.4 to 14.4µm wavelength, with spatial resolution ranging from 250 10000 metres. Operating 24 hours a day, MODIS collects data from all spectral bands during daylight hours. In darkness, scanning is solely done in the thermal infrared bands, reducing the output data stream from 10.8Mbps (day) to 2.5Mbps (night). The 274kg MODIS draws an average of 162W of power. Typical applications of MODIS include: www.siliconchip.com.au • • Surface temperature monitoring of both land and sea. Global detection of fires (including fires in underground coal seams). • Ocean colour, to aid detection of contaminants (sediment, photo-plankton, etc). • Cloud characteristics. MOPITT – measures pollution The rather lengthily named “Measurement of Pollution in the Troposphere” (MOPPIT) instrument is designed to increase under­standing of the Earth’s lower atmosphere and its interaction with the land and seas. In order to achieve this, MOPPIT focuses on the distribution, transport, sources and sinks of carbon monoxide and methane in the Earth’s lower atmosphere. MOPPIT consists of a scanning radiometer using gas spec­troscopy to measure reflected radiance in the three absorption bands of carbon monoxide and methane. By measuring the power levels of reflected spectra at 2.3µm (methane) and 2.4 and 4.7µm (carbon monoxide), MOPPIT will be able to determine the concen­trations of these gases within the troposphere. The 184kg MOPPIT was supplied to the Terra team by the Canadian Space Agency. To date, the unit has been generating global maps of carbon monoxide and methane distribution. ASTER – for high resolution images A joint venture between NASA and Japan’s Ministry of Inter­national Trade and Industry, the Advanced Spaceborne Thermal Emission and Reflection Radio­ meter (ASTER) has provided high resolution images of the Earth during its operation. Operating three distinct telescope systems, ASTER includes the following spectral subsystems: visible near infrared (VNIR), shortwave-infrared (SWIR) and thermal infrared (TIR). Each of these subsys­tems employs its own instrumentation and was NASA’s Terra spacecraft is now providing daily views of fires around the world. With the high resolution and sensitivity of the Moderate-Resolution Imaging Spectroradiometer (MODIS) and the instrument’s regular global coverage, Terra is providing an improved fire-detection capability over previous space-based sensors. MODIS is also capable of much higher-resolution imaging of fires, as seen in this image of fires in northern Australia taken on October 2, 2000. www.siliconchip.com.au This globe shows data collected from multiple sensors and integrated into one image. Notice the three-dimensional cloud measurements; these are collected by ASTER and MISR aboard NASA’s Terra spacecraft, while MODIS measures total cloud cover on a daily basis. The El Nino temperature anomaly is visible as red in the Pacific Ocean while the red dots on land show the locations of forest fires. Terra’s ASTER, MISR, MODIS, and MOPITT instruments are all uniquely-designed to observe fires and help measure the smoke and gases they release. Together with CERES, Terra’s instruments help scientist’s understand the Earth as a whole, integrated system. constructed by a different corporation. The ASTER as a whole provides spectral separation to its three units via bandpass and dichroic filters. The VNIR subsystem was constructed by NEC and consists of two telescopes, one pointing downwards and one pointing behind (along the orbit path) to produce stereo images with a very high (15 metres) resolution. These images can later be used to generate 3-dimensional perspectives of the land being analysed. The wave­lengths captured by this instrument are especially useful in monitoring the health of crops and vegetation. The SWIR subsystem, produced by Mitsubishi Electric, oper­ates in six shortwave infrared channels with a 30-metre resolu­ tion. Employing a pointing mirror, the SWIR system can focus on nearby areas of interest, allowing it to study the same area with each orbital pass for several passes. The SWIR system is espe­cially useful for identifying the geological structure of the land being studied. The TIR subsystem, supplied by Fujitsu Ltd, scans five thermal infrared channels with a resolution of 90 metres. As with the SWIR system, a mirror setup is used to allow areas of inter­est to receive higher coverage. Once again, this apparatus is especially useful for geological sensing of ground structure. ASTER weighs in at 450kg and is the largest of the Terra’s five instruments. Together, the three subsystems combine to give the ASTER the following capabilities: • Surface temperatures and emissivities. March 2002  33 This computer-generated image shows the EOSAM1 Spacecraft, with the MISR instrument on board, orbiting Earth. Direction of flight is toward the lower left. The actual locations imaged by the nine cameras, each with four colour bands, along the Earth’s surface are illustrated here with translucent surfaces. • Digital elevation (topological) maps from stereo images con­taining geological information. • Surface composition and vegetation maps. • Mapping of polar ice movements and formations. • Mapping of volcanic activity, both geological and thermal. Power and telemetry systems Terra, like many satellites, is powered by an array of solar cells. Together with a nickel-hydrogen battery bank, the power system provides an average of 2530 watts of power to the craft. All data recorded by Terra is transmitted back to Earth via the Ku-band (15.25 17.25GHz) at a rate of 150 Megabits/second. Command and configuration information is upload­ed to Terra via the S-band (1.7 - 2.3GHz). For each Earth orbit, two 12-minute periods of radio contact are used for these data transfers. Terra global studies As already mentioned, each of the instruments aboard Terra produces separate data sets. It is the combining of the results of several of these instruments by teams of scientists from varied disciplines that is the exciting part of this story. Currently, there are many studies being undertaken using data generated from Terra, with the fire 34  Silicon Chip fighting tools demonstrated during the NSW bushfires being just one of these. Volcanoes, fire and flood Volcanoes and fires around the world not only pose a huge threat to the safety of people and property but also generate a large amount of atmospheric aerosols. Using the MODIS instrument, Terra can scan globally and instantly produce alarms from fires or volcanic eruptions from anywhere in the world. One of the EOS teams currently produces constant lists of fires and lava flows globally, which are quite useful for risk management and evacua­tions. Floods are also a major danger to communities around the world. Based on data sourced from Terra, scientists now have quite accurate models to help predict an area’s susceptibility to flooding. When severe flooding occurs, researchers are able to measure the land area affected, contributing basin area, peak discharge, suspended sediment concentration and meteorological factors, so that the risk can be minimised in the future. Vegetation Terra is able to provide valuable data on the health and distribution of ground vegetation around the world. www.siliconchip.com.au With immense areas of forests being constantly turned into farmland, the Earth’s climate is definitely affected. Scientists are able to use data from the ASTER, MODIS and MISR instruments in studying the impacts of climate change of global vegetation, in addition to providing large scale analysis of agricultural methods. Researchers from The University of California, use the MODIS to monitor the burning of forest land in the Amazon and Africa. These burning activities release carbon dioxide, carbon monoxide and methane, along with other aerosols into the atmosphere. The MODIS can be used to measure the exact quantities and compo­sitions of these gases, as well as vegetation regrowth and change later on down the track. Cities have great impact on the local environment. Dense city planning results in higher summer temperatures, increased health risks, energy consumption and pollution levels. The in­struments onboard Terra are able to assist urban planners in minimising these effects. In fact, since Terra’s launch, studies have shown that vegetation cover and surface albedo (reflectivi­ty) of urban areas have major affects on air temperatures and ozone pollution levels. This is an artist’s rendition of the MISR instrument in cutaway view. The back ends of the nine MISR cameras appear as yellow cylinders. In this orientation, MISR would look down toward Earth. General climatology studies Over the last century there has been an immense impact from human development on the Earth’s landscape. Despite carbon diox­ide levels having increased by almost one third since recording began, global temperatures have only increased by around 0.5°C over the last century – rather less than predicted by traditional climate models. This has revealed limitations in the ways traditional models took in the effects of atmospheric aerosols, changes in cloud cover and the Earth’s oceans in af­fecting climate. Aerosols are tiny particles suspended in the atmosphere. While some occur naturally, human activities have increased atmospheric aerosols by around 10%. Naturally occurring aerosols include material from volcanoes, dust storms, forest fires, biological materials and sea spray. Common human aerosols include materials from the burning of fossil fuels, factories and motor vehicles. While there is still much to be learnt, it is known that aerosols have both direct and indirect effects on the Earth’s climate. The direct effect is the cooling of the Earth by re­ flecting incident sunlight back into space, with the magnitude of this dependent on the type and size of the particles in question. It is known that aerosol cooling from human-generated particles offsets some of the effects of carbon dioxide produced global warming. The indirect effect of these particles is to change the properties of clouds, themselves formed by water droplets adher­ing to aerosol particles. In regions of low particle densities, clouds tend to be composed of large droplets, with regions of high aerosol concentrations having clouds composed of very small droplets. Large droplets do not scatter light well and allow more light to pass through, while clouds of small droplets (caused by lots of aerosols) scatter light and restrict light passed towards the Earth. Also, these brighter, more reflective clouds composed of smaller droplets are less likely to generate rain than www.siliconchip.com.au This is the “business” end of the MISR instrument, which includes the cameras and calibration equipment. The photograph was taken in October 1996, as MISR was being assembled. Subsequently, the parts that supply power, communications and temperature control were added. The entire package was then encased in a protective housing, which was covered with highly reflecting thermal blankets. larger droplet types. Using the MISR instrument, scientists have been able to improve their understanding of the concentrations of aerosols in the atmosphere and their effects on cloud formation, sunlight reflectivity, ground and ocean temperatures and rainfall. With oceans comprising around 70% of the Earth’s surface, this mass of water stores large amounts of heat energy. While originally thought to be a large inert “heatsink”, later studies have revealed the ocean to be a major March 2002  35 The Red Sea golf resort in Sharm El Sheik, Egypt, where President Clinton met with Israeli Prime Minister Ehud Barak and Palestinian Authority President Yasser Arafat, stands out against the desert landscape in this image acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on August 25, 2000. This image of the southern tip of the Sinai Peninsula shows an area about 30 x 40km in the visible and near infrared wavelength region. The vegetation appears in red while the blue areas in the water at the top and bottom of the image are coral reefs. The airport is visible just to the north of the golf resort. ASTER is the only high resolution imaging sensor on Terra and its primary goal is to obtain high-resolution image data in 14 channels over the entire land surface, as well as black and white stereo images. With a “revisit time” of between 4 and 16 days, ASTER will provide the capability for repeat coverage of changing areas on Earth’s surface. The broad spectral coverage and high spectral resolution of ASTER will provide scientists in numerous disciplines with critical information for surface mapping and monitoring temporal change. Example applications are: monitoring glacial advances and retreats, monitoring potentially active volcanoes, identifying crop stress, determining cloud morphology and physical properties, wetlands evaluation, thermal pollution monitoring, coral reef degradation, surface temperature mapping of soils and geology, and measuring surface heat balance. contributor to climatic phenomenon. With heat escaping the ocean and creating atmospheric temperature gradients in the surrounding air, winds are generated, creating horizontal currents. Water temperature measurements With temperatures and salt levels affecting vertical cur­ rents, complex water circulations around the world arise. This results in warm surface waters moving poleward where heat escapes more readily to outer space, while cold, deep currents are estab­lished in the ocean depths. Through this system of ocean circula­tion, the oceans and atmosphere work together to distribute heat and regulate climate. This circulation transports enormous amounts of heat, resulting in more moderate climates on land areas that are near the ocean. Using Terra’s instruments, scientists have been able to measure ocean surface temperatures to within 0.5°C. This information, coupled with wind measurements made on Earth, have been invaluable in understanding the effects of the world’s oceans on global climate. With better understanding of most of the variables which affect climate, researchers have been improving a set of tools for the prediction and assessment of the effects of large scale seasonal climate fluctuations, including the El Nino Southern Oscillation index. The Future An artist’s impression of Terra on its way to orbit, follow­ ing its launch from Vandenberg Air Force Base, California, on December 18, 1999. 36  Silicon Chip Since its launch in December 1999, Terra has provided in­valuable data concerning the Earth’s natural processes. If all future missions are as successful as Terra, a lot will SC be learnt about our won­derful planet. www.siliconchip.com.au