Aura Mission (EOS/Chem-1)
Aura (formerly EOS/Chem-1) is the chemistry mission of NASA with the overall objective to study the chemistry and dynamics of Earth's atmosphere from the ground through the mesosphere. The goal is to monitor the complex interactions of atmospheric constituents from both natural sources, such as biological activity and volcanoes, and man-made sources, such as biomass burning, are contributing to global change and effect the creation and depletion of ozone. The Aura mission will provide global surveys of several atmospheric constituents which can be classified into anthropogenic sources (CFC types), radicals (e.g., ClO, NO, OH), reservoirs (e.g., HNO, HCl), and tracers (e.g., N2O, CO2, H2O). Temperature, geopotential heights, and aerosol fields will also be mapped. 1) 2) 3) 4) 5) 6)
In many ways, Aura is a follow-on to the very successful UARS (Upper Atmosphere Research Satellite) mission of NASA. UARS made stratospheric constituent measurements from 1991-2005. Unlike UARS, however, Aura is designed to focus on the lower stratosphere and the troposphere.
Figure 1: Artist's rendition of the Aura spacecraft )image credit: NGST)
The Aura spacecraft, like Aqua, is based on TRW's modular, standardized AB1200 bus design with common subsystems. [Note: As of Dec. 2002, TRW was purchased by NGST (Northrop Grumman Space Technology) of Redondo Beach, CA]. The S/C dimensions are: 2.68 m x 2.34 m x 6.85 m (stowed) and 4.71 m x 17.03 m x 6.85 m (deployed). Aura is three-axis stabilized, with a total mass of 2,967 kg at launch, S/C mass of 1,767 kg, payload mass =1,200 kg. The S/C design life is six years. 7)
Figure 2: Illustration of the Aura spacecraft (image credit: NGST)
The spacecraft structure is a lightweight 'eggcrate' compartment construction made of graphite epoxy composite over honeycomb core, providing a strong but light base for the science instruments (referred to as T330 EOS common spacecraft design). A deployable flat-panel solar array with over 20,000 silicon solar cells provides 4.6 kW of power.
Spacecraft attitude is maintained by stellar-inertial, and momentum wheel-based attitude controls with magnetic momentum unloading, through interaction with the magnetic field of the Earth that provide accurate pointing for the instruments. Typical pointing knowledge of the line of sight of the instruments to the Earth is on the order of one arcminute (about 0.02º).
A propulsion system of four small-thrust hydrazine monopropellant thrusters gives the spacecraft a capability to adjust its orbit periodically to compensate for the effects of atmospheric drag, so that the orbit can be precisely controlled to maintain altitude and the assigned ground track.
Figure 3: Alternate view of the Aura spacecraft (image credit: NASA)
Launch: A launch of Aura on a Delta-2 7920 vehicle from VAFB, CA, took place on July 15, 2004.
Orbit: Sun-synchronous circular orbit, altitude = 705 km, inclination = 98.7º, with a local equator crossing time of 13.45 (1:45 PM) on the ascending node. Repeat cycle of 16 days.
RF communications: Onboard storage capacity of 100 Gbit of payload data. The payload data are downlinked in X-band. The spacecraft can also broadcast scientific data directly to ground stations over which it is passing. The ground stations also have an S-band uplink capability for spacecraft and science instrument operations. The S-band communication subsystem also can communicate through NASA's TDRSS synchronous satellites in order to periodically track the spacecraft, calculate the orbit precisely, and issue commands to adjust the orbit to maintain it within defined limits.
The Aura spacecraft is part of the so-called "A-train" (Aqua in the lead and Aura at the tail, the nominal separation between Aqua and Aura is about 15 minutes) or "afternoon constellation" (formation flight starting sometime after the Aura launch). The objective is to coordinate observations and to provide a coincident set of data on aerosol and cloud properties, radiative fluxes and atmospheric state essential for accurate quantification of aerosol and cloud radiative effects. The orbits of Aqua and CALIPSO are tied to WRS (Worldwide Reference System) and have error boxes associated with their orbits. The overall mission requirements are written such that CALIPSO is required to be no greater than 2 minutes behind Aqua. The OCO mission of NASA is a late entry into the A-train sequence. The satellites are required to control their along-track motions and remain within designated "control boxes." Member satellites will exchange orbital position information to maintain their orbital separations. 8) 9) 10) 11) 12)
The A-train is part of GOS (Global Observing System), an international network of platforms and instruments, to support environmental studies of global concern. A draft implementation plan for GOS, also referred to as GEOSS ((Global Earth Observation System of Systems), was approved at the fourth Earth Observation summit in Tokyo in April 28, 2004.
Figure 4: Illustration of Aura spacecraft in the A-train (image credit: NASA)
The PARASOL spacecraft of CNES (launch on Dec. 18, 2004) is part of the A-train as of February 2005. The OCO mission (launch in 2009) will be the newest member of the A-train. Once completed, the A-train will be led by OCO, followed by Aqua, then CloudSat, CALIPSO, PARASOL, and, in the rear, Aura. 13)
• Instrument status after more than 12 years on orbit in December 2016: Three of Aura's four original instruments continue to function nominally; however, TES (Tropospheric Emissions Spectrometer) is about to reach the end of its expected life. Aura data are being widely used by the science and applications community, and in many instances Aura data are being applied in conjunction with data from other instruments that fly onboard satellites that make up the A-Train (Afternoon Constellation). 14)
- OMI (Ozone Monitoring Instrument) of KNMI has been extremely stable, making it highly suitable for ozone and solar irradiance trend analysis. OMI data have been used to study global and regional air-quality trends. 15)
- OMI and TES continue to yield significant science results that are being applied by operational environmental protection agencies for air quality assessments, regulations, and forecasts, both in the U.S. (EPA) and Europe. Although Aura does not measure carbon directly, it is making substantial contributions to understanding climate change by measurements of other climate forcing factors, such as, water vapor, solar irradiance, and aerosols. OMI and MLS (Microwave Limb Sounder) continue their crucial observations in the stratosphere that are needed for monitoring compliance of the Montreal Protocol.
• October 27, 2016: The size and depth of the ozone hole over Antarctica was not remarkable in 2016. As expected, ozone levels have stabilized, but full recovery is still decades away. What is remarkable is that the same international agreement that successfully put the ozone layer on the road to recovery is now being used to address climate change. 16)
- The stratospheric ozone layer protects life on Earth by absorbing ultraviolet light, which damages DNA in plants and animals (including humans) and leads to health issues like skin cancer. Prior to 1979, scientists had never observed ozone concentrations below 220 Dobson Units. But in the early 1980s, through a combination of ground-based and satellite measurements, scientists began to realize that Earth's natural sunscreen was thinning dramatically over the South Pole. This large, thin spot in the ozone layer each southern spring came to be known as the ozone hole.
- The image of Figure 5 shows the Antarctic ozone hole on October 1, 2016, as observed by the OMI (Ozone Monitoring Instrument) on NASA's Aura satellite. On that day, the ozone layer reached its average annual minimum concentration, which measured 114 Dobson Units. For comparison, the ozone layer in 2015 reached a minimum of 101 Dobson Units. During the 1960s, long before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 260 to 320 Dobson Units.
- The area of the ozone hole in 2016 peaked on September 28, 2016, at about 23 million km2. "This year we saw an ozone hole that was just below average size," said Paul Newman, ozone expert and chief scientist for Earth Science at NASA's Goddard Space Flight Center. "What we're seeing is consistent with our expectation and our understanding of ozone depletion chemistry and stratospheric weather."
- The image of Figure 6 was acquired on October 2 by the OMPS (Ozone Mapping Profiler Suite) instrumentation during a single orbit of the Suomi-NPP satellite. It reveals the density of ozone at various altitudes, with dark orange areas having more ozone and light orange areas having less. Notice that the word hole isn't literal; ozone is still present over Antarctica, but it is thinner and less dense in some areas.
- In 2014, an assessment by 282 scientists from 36 countries found that the ozone layer is on track for recovery within the next few decades. Ozone-depleting chemicals such as chlorofluorocarbons (CFCs) — which were once used for refrigerants, aerosol spray cans, insulation foam, and fire suppression — were phased out years ago. The existing CFCs in the stratosphere will take many years to decay, but if nations continue to follow the guidelines of the Montreal Protocol, global ozone levels should recover to 1980 levels by 2050 and the ozone hole over Antarctica should recover by 2070.
- The replacement of CFCs with hydrofluorocarbons (HFCs) during the past decade has saved the ozone layer but created a new problem for climate change. HFCs are potent greenhouse gases, and their use — particularly in refrigeration and air conditioning — has been quickly increasing around the world. The HFC problem was recently on the agenda at a United Nations meeting in Kigali, Rwanda. On October 15, 2016, a new amendment greatly expanded the Montreal Protocol by targeting HFCs, the so-called "grandchildren" of the Montreal Protocol.
- "The Montreal Protocol is written so that we can control ozone-depleting substances and their replacements," said Newman, who participated in the meeting in Kigali. "This agreement is a huge step forward because it is essentially the first real climate mitigation treaty that has bite to it. It has strict obligations for bringing down HFCs, and is forcing scientists and engineers to look for alternatives."
Figure 5: Image of the Antarctic Ozone Hole acquired with OMI on Aura on October 1, 2016 (image credit: NASA Earth Observatory, Aura OMI science team)
Figure 6: An edge-on (limb) view of Earth's ozone layer, acquired with OMPS on the Suomi-NPP on October 2, 2016 (image credit: NASA Earth Observatory, image by Jesse Allen, using Suomi-NPP OMPS data)
• June 1, 2016: Using a new satellite-based method, scientists at NASA, Environment and Climate Change Canada, and two universities have located 39 unreported and major human-made sources of toxic sulfur dioxide emissions. Data from NASA's Aura spacecraft were analyzed by scientists to produce improved estimates of sulfur dioxide sources and concentrations worldwide between 2005 and 2014. 17)
- A known health hazard and contributor to acid rain, sulfur dioxide (SO2) is one of six air pollutants regulated by the U.S. Environmental Protection Agency. Current, sulfur dioxide monitoring activities include the use of emission inventories that are derived from ground-based measurements and factors, such as fuel usage. The inventories are used to evaluate regulatory policies for air quality improvements and to anticipate future emission scenarios that may occur with economic and population growth.
- But, to develop comprehensive and accurate inventories, industries, government agencies and scientists first must know the location of pollution sources.
- "We now have an independent measurement of these emission sources that does not rely on what was known or thought known," said Chris McLinden, an atmospheric scientist with Environment and Climate Change Canada in Toronto and lead author of the study published this week in Nature Geosciences. "When you look at a satellite picture of sulfur dioxide, you end up with it appearing as hotspots – bull's-eyes, in effect — which makes the estimates of emissions easier." 18)
- The 39 unreported emission sources, found in the analysis of satellite data from 2005 to 2014, are clusters of coal-burning power plants, smelters, oil and gas operations found notably in the Middle East, but also in Mexico and parts of Russia. In addition, reported emissions from known sources in these regions were — in some cases — two to three times lower than satellite-based estimates.
- Altogether, the unreported and underreported sources account for about 12 percent of all human-made emissions of sulfur dioxide – a discrepancy that can have a large impact on regional air quality, said McLinden.
- The research team also located 75 natural sources of sulfur dioxide — non-erupting volcanoes slowly leaking the toxic gas throughout the year. While not necessarily unknown, many volcanoes are in remote locations and not monitored, so this satellite-based data set is the first to provide regular annual information on these passive volcanic emissions.
- "Quantifying the sulfur dioxide bull's-eyes is a two-step process that would not have been possible without two innovations in working with the satellite data," said co-author Nickolay Krotkov, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
- First was an improvement in the computer processing that transforms raw satellite observations from the Dutch-Finnish Ozone Monitoring Instrument aboard NASA's Aura spacecraft into precise estimates of sulfur dioxide concentrations. Krotkov and his team now are able to more accurately detect smaller sulfur dioxide concentrations, including those emitted by human-made sources such as oil-related activities and medium-size power plants.
- Being able to detect smaller concentrations led to the second innovation. McLinden and his colleagues used a new computer program to more precisely detect sulfur dioxide that had been dispersed and diluted by winds. They then used accurate estimates of wind strength and direction derived from a satellite data-driven model to trace the pollutant back to the location of the source, and also to estimate how much sulfur dioxide was emitted from the smoke stack.
- "The unique advantage of satellite data is spatial coverage," said Bryan Duncan, an atmospheric scientist at Goddard. "This paper is the perfect demonstration of how new and improved satellite datasets, coupled with new and improved data analysis techniques, allow us to identify even smaller pollutant sources and to quantify these emissions over the globe." - The University of Maryland, College Park, and Dalhousie University in Halifax, Nova Scotia, contributed to this study.
• June 2015: The Aura satellite was launched in July 2004 as part of the A-Train. The three operating instruments on-board Aura MLS (Microwave Limb Sounder), OMI (Ozone Monitoring Instrument), and TES (Tropospheric Emissions Spectrometer) provide profiles and column measurements of atmospheric composition in the troposphere, stratosphere, and mesosphere. OMI is contributed from the Netherlands Space Office and the Finnish Meteorological Institute. The suite of observations from MLS, OMI and TES is very rich, with nearly 30 individual chemical species relevant for stratospheric chemistry (O3, HCl, HOCl, ClO, OClO, BrO, NO2, N2O, HNO3, etc.), tropospheric pollutants (O3, NO2, CO, PAN, NH3, SO2, aerosols), and climate-related quantities (CO2, H2O, CH4, clouds, aerosol optical properties). The Aura spacecraft is healthy and is expected to operate until at least 2022, likely beyond. There is great value in continuing the mission to: 19)
1) extend the unique 10-year record of stratospheric composition, variability, and trends as well as the chemical and dynamical processes affecting ozone recovery and polar ozone chemistry
2) continue to map-out rapidly changing anthropogenic emissions of NO2, SO2, and aerosol products influencing air quality
3) continue to develop greater vertical sensitivity by combining radiances from separate sensors
4) use Aura data to further evaluate global chemistry-climate, climate, and air quality models
5) extend observations of short-term solar variability overlapping with SORCE and providing a bridge to future measurements (GOME-2 TROPOMI)
6) continue the development of new synergetic products combining multiple Aura instruments and instruments from the A-Train
7) provide continuity and comparison to current and future satellite missions (Suomi-NPP, SAGE-III, TROPOMI)
8) deliver operational products: volcanic monitoring, aviation safety, operational ozone assimilation at NOAA for weather and UV index forecasting, OMI Aerosol Index and NO2 products for air quality forecasting. As such, the Panel concludes that Aura mission be continued as currently baselined.
The Aura spacecraft flight systems are operating on primary hardware with redundant systems intact and are expected to continue to perform very well through the proposed mission extension period. Aura Mission Operations have been very successful (Ref. 19).
• April 29, 2015: Late on April 22, 2015, the Calbuco volcano in southern Chile awoke from four decades of slumber with an explosive eruption. Ash and pumice particles were lofted high into the atmosphere, and the debris has been darkening skies and burying parts of Chile, Argentina, and South America for nearly a week. Along with 210 million cubic meters of ash and rock, the volcano has been spewing sulfur dioxide (SO2) and other gases. 20)
- Near the land surface, sulfur dioxide is a acrid-smelling gas that can cause respiratory problems in humans and animals. Higher in the atmosphere, it can have an effect on climate. When SO2 reacts with water vapor, it creates sulfate aerosols that can linger for months or years. Those small particles can have a cooling effect by reflecting incoming sunlight.
- The images of Figure 7 show the average concentration of sulfur dioxide over South America and surrounding waters between April 23–26, 2015. The maps were made with data from OMI (Ozone Monitoring Instrument) on NASA's Aura satellite. Like ozone, atmospheric sulfur dioxide is sometimes measured in Dobson Units. If you could compress all the sulfur dioxide in a column of atmosphere into a single layer at the Earth's surface at 0º Celsius, one Dobson Unit would be 0.01 mm thick and would contain 0.0285 grams of sulfur dioxide/m2.
- "Satellite sulfur dioxide data are critical for understanding the impacts of volcanic eruptions on climate," said Simon Carn, a part of the OMI team and professor at the Michigan Technological University. "Climate modelers need estimates of SO2 mass and altitude to run their models and accurately predict the atmospheric and climate impacts of volcanic eruptions. The SO2 plume images also provide unique insights into the atmospheric transport and dispersion of trace gases in the atmosphere, and on upper atmospheric winds."
- So far, Calbuco has released an estimated 0.3 to 0.4 teragrams (0.3 to 0.4 million tons) of SO2 into the atmosphere. The gas was injected into the stratosphere (as high as 21 km), where it will last much longer and travel much farther than if released closer to the surface. The SO2 will gradually convert to sulfate aerosol particles. However, it is not clear yet if there will be a cooling effect from this event.
Figure 7: The OLI measurements were acquired on four days in April 2015. On the maps, data appear in stripes or swaths, revealing the areas observed (colored) or not observed (clear) by the Aura spacecraft on a given day. Note how the plume moves north and east with the winds. By April 28, the plume of SO2 had reached the Indian Ocean (image credit: NASA Earth Observatory, Jesse Allen)
Figure 8: The natural color image, acquired on April 25, 2015 by the ALI (Advanced Land Imager) instrument on NASA's EO-1 (Earth Observing-1) satellite, shows Calbuco's plume rising above the cloud deck over Chile (image credit: NASA, EO-1 team)
• In early 2015, Aura is operating "nominally" in its extended mission phase. Although HIRDLS is no longer operational and the TES instrument shows signs of wear that have limited its operations, OMI and MLS continue to operate well. OMI's highly successful advanced technology has been and will continue to be employed by new NASA satellite instruments, such as OMPS (Ozone Mapper Profiling Suite) on the Suomi-NPP (Suomi National Polar-orbiting Partnership) and on the TEMPO (Tropospheric Emissions: Monitoring of Pollution) missions. Overall, the Aura mission continues to operate satisfactorily and there is enough fuel reserve for Aura to operate safely in the A-Train until 2023. 21)
- To date, the mission has met or surpassed nearly all mission success criteria. Aura data are being used by the U.S. GCRP (Global Change Research Program) and for three international assessments, including the AR5 (Fifth Assessment Report) of the IPCC (Intergovernmental Panel on Climate Change), the WMO (World Meteorological Organization) and UNEP/SAOD (United Nations' Environmental Program/Scientific Assessments of Ozone Depletion), and the TF HTAP (Task Force on Hemispheric Transport of Air Pollution).
- Aura's data have proven to be valuable for air quality applications such as identifying the trends that result from regulation of emissions on decadal time scales, and shorter time scale applications are being assessed. The original Aura science questions have surely been addressed or answered and serendipitous discoveries have been realized.
• On July 15, 2014, the Aura Spacecraft of NASA was 10 years on orbit. Aura has provided vital data about the cause, concentrations and impact of major air pollutants. With its four instruments measuring various gas concentrations, Aura gives a comprehensive view of one of the most important parts of Earth - the atmosphere. 22)
Figure 9: Nitrogen dioxide pollution, averaged yearly from 2005-2011, has decreased across the United States (image credit: NASA Goddard's Scientific Visualization Studio, T. Schindler)
Legend to Figure 9: The OMI (Ozone Monitoring Instrument) on the Aura satellite began monitoring levels of nitrogen dioxide worldwide shortly after its launch. OMI data show that nitrogen dioxide levels in the United States have decreased at a rate of 4% per year from 2005 to 2010 — a time period when stricter government policies on power plant and vehicle emissions came into effect. As a result, ground-level ozone concentrations also decreased. OMI data also showed a 2.5% decrease of nitrogen dioxide per year during the same time period in Europe, which had enacted similar legislation.
While air quality in the United States has improved, the issue still persists nationwide. Since bright sunlight is needed to produce unhealthy levels of ozone, ozone pollution is largely a summertime issue. As recent as 2012, about 142 million people in America— 47 % of the population— lived in counties with pollution levels above the National Ambient Air Quality Standards, according to the EPA (Environmental Protection Agency). The highest levels of ozone tend to occur on hot, sunny, windless days.
Air pollution also remains an issue worldwide. The WHO (World Health Organization) reported that air pollution still caused one in eight deaths worldwide in 2012. Outside of the United States and Europe, OMI showed an increase in nitrogen dioxide levels. Data from 2005 to 2010 showed China's nitrogen dioxide levels increased at about 6 % and South East Asia increased levels at 2% per year. Globally, nitrogen dioxide levels increased a little over half a percent per year during that time period (Ref. 22).
• Figure 10, released on June 8, 2014 in NASA's Earth Observatory program, was assembled from observations made by the OMI (Ozone Monitoring Instrument) on the Aura satellite. The map shows the concentration of stratospheric ozone over the Arctic—63º to 90º North—on April 1, 2014. Ozone is typically measured in Dobson Units, the number of molecules required to create a layer of pure ozone 0.01 mm thick at a temperature of 0º Celsius and an air pressure of 1 atmosphere (the pressure at the surface of the Earth). Reaching 470 Dobson Units, April 1 marked the highest average concentration of ozone over the region so far this year. The average amount of ozone in Earth's atmosphere is 300 Dobson Units, equivalent to a layer 3 mm in thickness. 23)
Figure 10: OMI map of Arctic ozone in spring acquired on April 1, 2014 (image credit: NASA Earth Observatory)
• The Aura spacecraft and three of its four instruments are operating nominally in 2014.
Sulfur dioxide (SO2), emitted from both man-made and volcanic activities, significantly impact air quality and climate. Advanced sensors including OMI (Ozone Monitoring Instrument) flying on NASA's Aura spacecraft have been employed to measure SO2 pollution. This however, remains a challenging problem owing to relatively weak signals from most anthropogenic sources and various interferences such as ozone absorption and stray light.
The project has developed a fundamentally different approach for retrieving SO2 from satellites. Unlike existing methods that attempt to model different interferences, we directly extract characteristic features from satellite radiance data to account for them, using a principal component analysis technique. This proves to be a computationally efficient way to use hundreds of wavelengths available from OMI, and greatly decreases modeling errors.
The new approach has the following features:
- 50% noise reduction compared with the operational OMI algorithm.
- Reduction of unrealistic features in the operational product.
- Computation efficiency (10 times faster than comparable methods relying on online radiative transfer calculation).
- Applicability to many instruments such as GOME-2 and the Suomi National Polar Partnership (NPP) Ozone Mapping and Profiler Suite (OMPS).
This new algorithm will significantly improve the SO2 data quality from the OMI mission. Once applied to other sensors, it will enable the production of consistent long-term global SO2 records essential for climate and air quality studies.
Figure 11: Monthly mean SO2 maps over the Eastern U.S. for August 2006 generated using (a) the new algorithm and (b) the operational algorithm (image credit: NASA/GSFC)
Legend to Figure 11: The circles represent large SO2 sources (e.g., coal-fired power plants) that emit more than 70,000 tons of SO2 annually. The colors represent the amount of SO2 in the atmospheric column above the surface in Dobson Unit (DU). 1 DU means 2.69 x 1016 SO2 molecules above a surface of 1 cm2 . Because the SO2 signals from anthropogenic sources are relatively weak, small errors in the estimated interferences (e.g., ozone absorption) may lead to substantial biases in the retrieved SO2. Negative values can arise when, for example, the contribution from ozone absorption in the SO2 spectral window is only slightly overestimated. As shown above, the negative retrieval biases become much smaller in the new algorithm.
Relevance for future science and NASA missions: The new algorithm has been proposed to reprocess data from the Aura OMI mission. It can be applied to Suomi-NPP and future JPSS OMPS instruments to ensure SO2 data continuity from the EOS era. It can also help extend satellite SO2 data records if applied to other current and future NASA and European missions such as TEMPO, GEO-CAPE, TROPOMI and GOME-2. TEMPO is the first selected NASA EVI (Earth Venture Instrument) and will be launched on a geostationary platform near the end of the decade (Ref. 24).
• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 26)
- The satellite is in excellent health. The Aura MLS, OMI and TES instruments are showing signs of aging, but are still producing science data of excellent quality, and there is an excellent chance of extending measurements beyond the current proposal cycle. The data are highly utilized in the research and operational communities.
- The reasons for extending the Aura mission include: (1) to allow current scientific and applied benefits to continue; (2) to increase the value of the Aura data for climate studies through increasing the length of the Aura data sets; (3) to allow continued collection of data that are unique since the loss of the European Envisat satellite; and (4) to continue to generate synergistic products by combining different Aura and measurements from other A-Train satellite missions.
• The Aura spacecraft and two of its four instruments (MLS and OMI) are operating nominally in 2013. The TES (Tropospheric Emission Spectrometer) instrument continues to make special observations in targeted regions, but no longer makes global measurements. A moving part of TES is suffering from lubricant degradation. 27)
• The Aura spacecraft and three of its four instruments are operating nominally in 2011.
• The Aura spacecraft and three of its four instruments are operating nominally in 2010. Aura entered its extended mission phase in October 2010 (extension until the end of 2012).
• The Aura spacecraft and three of its four instruments are operating nominally in 2009 (MLS, OMI and TES Operations are nominal while HIRDLS is collecting limited science data). 28)
• The spacecraft has been declared "operational" by NASA as of Oct. 14, 2004 (ending the commissioning phase). 29)
• Shortly after launch, the HIRDLS science team discovered, that a piece of plastic was blocking 80% of the optical instrument aperture. Engineers concluded that the plastic was torn from the inside of the instrument during the explosive outgassing on spacecraft ascent at launch. This plastic remains caught on the scan mirror despite efforts to free it. In spite of these setbacks, the HIRDLS team has shown that it can use the remaining 20% of the aperture to produce their promised data products at high vertical resolution. Unfortunately, the instrument no longer has its azimuthal scanning capability. 30) 31) 32)
Sensor complement: (HIRDLS, MLS, OMI, TES)
The Aura instrument package provides complementary observations from the UV to the microwave region of the EMS (Electromagnetic Spectrum) with unprecedented sensitivity and depth of coverage to the study of the Earth's atmospheric chemistry from its surface to the stratosphere. MLS is on the front of the spacecraft (the forward velocity direction) while HIRDLS, TES, and OMI are mounted on the nadir side. 33)
Figure 12: Auro atmospheric profile measurements (image credit: NASA)
Legend to Figure 12: OMI also measures UVB flux, cloud top/cover, and column abundances of O3, NO2, BrO, aerosol and volcanic SO2. TES also measures several additional 'spectral products' such as ClONO2, CF2Cl2, CFCl3, NO2, and volcanic SO2.
Figure 13: Instrument field-of-view accommodation (image credit: NASA)
HIRDLS (High-Resolution Dynamics Limb Sounder):
HIRDLS is a joint instrument between the University of Colorado at Boulder and Oxford University, Oxford, UK. PIs: J. Gille of the University of Colorado and J. Barnett, Oxford University; prime contractors are Lockheed Martin and Astrium Ltd., UK. The instrument is a mid-infrared limb-scanning spectroradiometer designed to sound the upper troposphere, stratosphere, and mesosphere emissions within the spectral range of 6 - 18 µm (21 channels). The instrument measures infrared thermal emissions from the atmosphere which are used to determine vertical profiles as functions of pressure of the temperature and concentrations of several trace species in the 8-100 km height range. The HIRDLS design is of LRIR (Nimbus-6), LIMS and SAMS (Nimbus-7), ISAMS and CLAES (UARS) heritage.
HIRDLS observes global distributions of temperature and trace gas concentrations of O3, H2O, CH4, N2O, HNO3, NO2, N2O5, CFC11, CFC12 ClONO2, and aerosols in the upper troposphere, stratosphere, and mesosphere plus water vapor, aerosol, and cloud tops. The swath width is 2000-3000 km (typically six profiles across swath). Complete Earth coverage (including polar night) can be obtained in 12 hours. High horizontal resolution is obtained with a commandable azimuth scan which, in conjunction with a rapid elevation scan, provides profiles up to 3,000 km apart in an across-track swath. Spatial resolution: standard profile spacing is 500 x 500 km horizontally (equivalent to 5º longitude x 5º latitude) x 1 km vertically; averaging volume for each data sample 1 km vertical x 10 km across x 300 km along line-of-sight. 34) 35)
Figure 14: Illustration of the HIRDLS instrument (image credit: Oxford University)
HIRDLS performs limb scans in the vertical at multiple azimuth angles, measuring infrared emissions in 21 channels (temperature distribution) ranging from 6.12 - 17.76 µm. Four channels measure the emission of CO2. Taking advantage of the known mixing ratio of CO2, the transmittance is calculated, and the equation of radiative transfer is inverted to determine the vertical distribution of the Planck black body function, from which the temperature is derived as a function of pressure. Winds and potential vorticity are determined from spatial variations of the height of geopotential surfaces. These are determined at upper levels by integrating the temperature profiles vertically from a known reference base. The HIRDLS instrument will improve knowledge in data-sparse regions by measuring the height variations of the reference surface with the aid of a gyro package. This level (near the base of the stratosphere) can also be integrated downward using nadir temperature soundings to improve tropospheric analyses. 36)
FOV (scan range): elevation from 22.1º to 27.3º below horizontal, azimuth: -21º (sun side) to +43º (anti-sun side). The instrument has 21 photoconductive HgCdTe detectors cooled to 65 K; each detector has a separate bandpass interference filter. Thermal control by paired Stirling cycle coolers, heaters, sun baffle, radiator panel; the thermal operating range is 20-30º C.
Table 1: Overview of some HIRDLS parameters
Status 2005: The instrument is performing correctly except for a problem with radiometric views out from the main aperture. A series of tests using the (i) the in-orbit instrument, (iii) the Engineering Model, (iii) purpose-built ground rigs, has led to the conclusion that the optical beam is obstructed between the scan mirror and the entrance aperture by what is believed to be a piece of Kapton film that became detached during the ascent to orbit. This film was intended to prevent movement of contamination, but itself moved from behind the scan mirror to in front. The lines along which that film tore can only be deduced from the in-orbit behavior.
Extensive tests have been performed on the HIRDLS instrument to understand the form of the optical blockage and how it occurred. A clear picture has emerged of the geometry, and this adds to the confidence in the approach to extracting atmospheric profiles which appears to be giving good results. All other aspects of the instrument are performing as well as or better than expected and there is every reason to expect that a long series of valuable atmospheric data will be obtained. 37) 38) 39) 40)
Figure 15: Illustration of the HIRDLS instrument and its components (image credit: UCAR, Ref. 36)
Figure 16: Internal view of the HIRDLS instrument (image credit: UCAR)
MLS (Microwave Limb Sounder):
The MLS instrument is of UARS MLS heritage; PI: J. W. Waters, NASA/JPL. The instrument measures thermal emissions from the atmospheric limb in submillimeter and millimeter wavelength spectral bands and is intended for studies of the following processes and parameters: 41) 42) 43) 44) 45) 46)
• Chemistry of the lower stratosphere and upper troposphere. Measurement of lower stratospheric temperature and concentrations of: H2O, O3, ClO, BrO, HCl, OH, HO2, HNO3, and HCN, and N2O. Measurement of upper tropospheric H2O and O3 (radiative forcing on climate change).
• Chemistry of the middle and upper stratosphere. Monitoring of ozone chemistry by measuring radicals, reservoirs, and source gases in chemical cycles which destroy ozone
• The effects of volcanoes on global change. MLS measures SO2 and other gases in volcanic plumes.
Measurements are performed continuously, at all times of day and night, altitude range from the upper troposphere to the lower thermosphere. The vertical scan is chosen to emphasize the lower stratosphere and upper troposphere. Complete latitude coverage is obtained each orbit. Pressure (from O2 lines) and height (from a gyroscope measuring small changes in the FOV direction) are measured to provide accurate vertical information for the composition measurements. 47) 48) 49)
The MLS instrument consists of three modules (Figure 18):
9) GHz module: This module contains the GHz antenna system, calibration targets, switching mirror, optical multiplexer, and 118, 190, 240, and 640 GHz radiometers
10) THz module: contains the THz scan and switching mirror, calibration target, telescope, and 2.5 THz radiometers at both polarizations. Measurement of the OH emissions near 2.5 THz (119 µm).
11) Spectrometer module: that contains spectrometers, command and data handling systems, and power distribution systems.
Measurement approach: passive limb sounder; thermal emission spectra collected by offset Cassegrain scanning antenna system; Limb scan = 0 - 120 km; spatial resolution = 3 x 300 km horizontal x 1.2 km vertical. MLS contains heterodyne radiometers in five spectral bands.
Table 2: MLS instrument parameters
Figure 17: Schematic view of the MLS instrument (image credit: NASA)
FOV: boresight 60-70º relative to nadir; IFOV = ±2.5º (half-cone, along-track); spatial resolution: measurements are performed along the suborbital track; the resolution varies for different bands, at 640 GHz the spatial resolution is: 1.5 km vertical x 3 km cross-track x 300 km along-track at the limb tangent point; a typical resolution is: 3 km vertical x 5 km cross-track x 500 km along-track. Spectral bands at millimeter and submillimeter wavelengths. Instrument mass = 490 kg, power = 550 W; duty cycle = 100%; data rate = 100 kbit/s; thermal control via radiators and louvres to space as well as heaters; thermal operating range is 10-35ºC.
Table 3: MLS instrument frequency bands
Figure 18: Line drawing of the MLS instrument (image credit: NASA)
Figure 19: Signal flow block diagram of the MLS instrument (image credit: NASA)
Figure 20: The MLS GHz module antenna concept, showing Cassegrain configuration, edge tapers, and surface tolerances of the reflectors (image credit: NASA)
Figure 21: The MLS THz module optical scheme (image credit: NASA)
OMI (Ozone Monitoring Instrument):
The OMI instrument is a contribution of NIVR (Netherlands Institute for Air and Space Development) of Delft in collaboration with FMI (Finnish Meteorological Institute), Helsinki, Finland, to the EOS Aura mission. The PI is Pieternel F. Levelt of KNMI; co-PIs are: Gilbert W. Leppelmeier of FMI and Ernest Hilsenrath of NASA/GSFC. OMI was manufactured by Dutch Space/TNO-TPD in The Netherlands, in cooperation with Finnish subcontractors VTT and Patria Finavitec. 50) 51) 52) 53) 54) 55) 56) 57) 58) 59)
OMI is a nadir-viewing UV/VIS imaging spectrograph which measures the solar radiation backscattered by the Earth's atmosphere and surface over the entire wavelength range from 270 to 500 nm, with a spectral resolution of about 0.5 nm. The design is of GOME heritage, flown on ERS-2, as well as of SCIAMACHY and GOMOS heritage, flown on Envisat. The overall objective is to monitor ozone and other trace gases (continuation of the TOMS measurement series) and to monitor tropospheric pollutants worldwide. The OMI measurements are highly synergistic with the HIRDLS and MLS instruments on the Aura platform. The OMI observations provide the following capabilities and features:
• Mapping of ozone columns at 13 km x 24 km and profiles at 36 km x 48 km (continuation of TOMS and GOME ozone column data records and the ozone profile records of SBUV and GOME)
• Measurement of key air quality components: NO2, SO2, BrO, OClO, and aerosol (continuation of GOME measurements)
• Distinguish between aerosol types, such as smoke, dust, and sulfates
• Measurement of cloud pressure and coverage
• Mapping of the global distribution and trends in UV-B radiation
• A combination of processing algorithms is employed including TOMS version 7, DOAS (Differential Optical Absorption Spectroscopy), Hyperspectral BUV retrievals and forward modeling to extract the various OMI data products
• Near real-time production of ozone and other trace gases.
Figure 22: The OMI instrument (image credit: KNMI)
OMI is the first of a new generation of UV-Visible spaceborne spectrometers that use two-dimensional detectors (CCD arrays). These detectors enable OMI to daily observe the entire Earth with small ground pixel size (13x24 km2 at nadir), which makes this instrument suitable for tropospheric composition research and detection of air pollution at urban scales. OMI is a wide-angle, non-scanning and nadir-viewing instrument measuring the solar backscattered irradiance in a swath of 2600 km. The telescope has a FOV of 114º. The instrument is designed as a compact UV/VIS imaging spectrograph, using a two-dimensional CCD array for simultaneous spatial and spectral registration (hyperspectral imaging in frame-transfer mode). The instrument has two channels measuring in the spectral range of 270-500 nm.
The Earth is viewed in 1500 bands in the along-track direction providing daily global coverage. OMI employs a polarization scrambler to depolarize the incoming radiance. The radiation is then focussed by the secondary telescope mirror. A dichroic element separates the radiation into a UV and a VIS channel. The UV channel is split again into two subchannels UV1 (270-314 nm) and UV2 (306-380 nm). In the UV1 subchannel, the spatial sampling distance per pixel is a factor two larger than in the UV2 subchannel. The idea is to increase the ratio between the useful signal and the dark current signal, hence, to increase SNR in UV1. The resulting IFOV values of a pixel in the cross-track direction are 6 km for UV1 and 3 km for UV2 and VIS. The corresponding spatial resolution is twice as good as the sampling distances. Groups of 4 or 8 CCD detector pixels are binned in the cross-track direction. The basic detector exposure time is 0.4 s, corresponding to an along-track distance of 2.7 km. In OMI, five subsequent CCD images are electronically co-added, resulting in a FOV of 13 km in the along-track direction. In addition, one column (wavelength) of each CCD data is downlinked without co-adding (monitoring of clouds, ground albedo). The pixel binning and image co-adding techniques are used to increase SNR and to reduce the data rate.
Figure 23: Conceptual design of the OMI instrument (image credit: KNMI)
Table 4: OMI instrument parameters
The CCD detector arrays are of the back-illuminated and frame-transfer type, each with 576 (rows) x 780 (columns) pixels in the image section and the same amount in the storage or readout section. The frame transfer layout allows simultaneous exposure and readout of the previous exposure. This in turn permits fair pixel readout rates (130 kHz) and good data integrity. There are two zoom modes, besides the global observation mode, for regional studies with a spatial resolution of 13 km x 13 km. In one zoom mode, the swath width is reduced to 725 km; in the other zoom mode, the spectrum is reduced to 306 - 432 nm. Cloud coverage information is retrieved with a high spatial resolution, independent of the operational mode. 60)
Instrument calibration: Daily measurements of the sun are taken with a set of reflective quartz volume diffusers (QVD) for absolute radiometric calibration. Relative radiometric calibration is performed using a WLS (White Light Source) and two LEDs per spectral (sub-) channel. The two LEDs fairly uniformly illuminate the CCD's. Spectral calibration is being performed using Fraunhofer features in the sun and nadir spectra. This is supported by a dedicated spectral correction algorithm in the level 0-1 b software. Dark signal calibration is performed at the dark side of the orbit using long-exposure time dark measurements. Straylight is always monitored at dedicated rows at the side of the images; there are also covered CCD pixels measuring dark current and, at the top and bottom of the image, exposure smear. 61) 62) 63) 64)
Table 5: Characteristics of the main observation modes of OMI
Figure 24: The optical bench of OMI (image credit: KNMI)
Figure 25: Schematic layout of the OMI optical bench (image credit: SRON, KNMI)
Table 6: Performance parameters of OMI
Figure 26: Schematic of measurement principle of the OMI instrument (image credit: Dutch Space)
Figure 27: Photo of the OMI instrument (image credit: SRON, Ref. 64)
TES (Tropospheric Emission Spectrometer):
The TES instrument is of ATMOS (ATLAS), and AES (Airborne Emission Spectrometer) heritage (PI: Reinhard Beer, NASA/JPL). TES has been developed for NASA by JPL. TES is a high-resolution infrared imaging Connes-type FTS (Fourier Transform Spectrometer), with spectral coverage from 3.2 - 15.4 µm (spectral resolution of 0.03 cm-1). TES has the capability to make both limb and nadir observations. Limb mode: height resolution = 2.3 km, height coverage = 0 - 34 km. In the nadir modes, TES has a spatial resolution of 0.53 km x 5.3 km with a swath of 5.3 km x 8.5 km. TES is a pointable instrument; it can access any target within 45º of the local vertical, or produce regional transects up to 885 km in length without any gaps in coverage. TES employs both, the natural thermal emission of the surface and atmosphere, and reflected sunlight, thereby providing day and night coverage anywhere on the globe. 65) 66) 67)
Figure 28: Schematic layout of the TES optics (image credit: NASA/JPL)
Observations from TES will further understanding of long-term variations in the quantity, distribution, and mixing of minor gases in the troposphere, including sources, sinks, troposphere-stratosphere exchange, and the resulting effects on climate and the biosphere. TES will provide 3-D global maps of tropospheric ozone (primary objective) and its photochemical precursors (chemical species involved in ozone formation and destruction). Other objectives: 68) 69)
- Simultaneous measurements of NOy, CO, O3, and H2O, determination of the global distribution of OH.
- Measurements of SO2 and NOy as precursors to the strong acids H2SO4 and HNO3
- Measurements of gradients of many tropospheric species
- Determination of long-term trends in radiatively active minor constituents in the lower atmosphere.
The following key features are part of the TES instrument design:
• Back-to-back cube corner reflectors to provide the change in optical path difference
• Use of KBr (potassium bromide) material for the beam splitter-recombiner and the compensator
• Only one of two input ports for actual atmospheric measurements. The other input views an internal, grooved, clod reference target
• A diode-pumped solid-state Nd:YAG laser for interferogram sampling control
• Cassegrain telescopes for condensing and collimating wherever possible to minimize the number of transmissive elements in the system.
• A passive space-viewing radiator to maintain the interferometer and optics at 180 K.
• A two-axis gimbaled pointing mirror operating at ambient temperature to permit observation of the full field of regard (a 45º cone about nadir plus the trailing limb).
• Two independent focal plane assemblies maintained at 65 K with active pulse-tube coolers. 70)
The TES instrument operates in a step-and stare configuration when in downlooking mode. At the limb the instrument points to a constant tangent height. Thus, the footprint is smeared along the line-of-sight by about 110 km during the 16 s limb scan (this is comparable to the effective size of the footprint itself). Hence, atmospheric inhomogeneity in the atmosphere becomes an issue; it must be dealt with in data processing (usually through a simplified form of tomography).
The routine operating procedure for TES is to make continual sets of nadir and limb observations (plus calibrations) on a one-day on, one-day off cycle. During the off-days, extensive calibrations and spectral product observations are made.
Table 7: TES performance characteristics
Figure 29: Observation geometry of the TES instrument (image credit: NASA/JPL)
TES status: After launch, TES went through a lengthy outgassing procedure to minimize the ice buildup on the detectors. After seven month of operation, the translator mechanism (which moves the reflecting surfaces of the spectrometer) began to show signs of bearing wear. The TES instrument team commanded the instrument to skip the limb sounding modes in May 2005. TES is now operating only in the nadir mode. This will increase the bearing life of the translator and the life of the instrument.
Figure 30: Schematic view of TES on the Aura spacecraft (image credit: NASA/JPL)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).