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Solar Backscatter Ultraviolet Instruments (BUV) on Satellites in L-1 and GEO Orbits Lawrence Flynn1, Kelly Chance2, Jhoon Kim3, Ernest Hilsenrath4, Jay Herman4, Berit Ahlers5, and Ruediger Lang6 1NOAA NESDIS, 2Smithsonian Astrophysical Observatory, 3Yonsei University, 4Joint Center for Earth Systems Technology UMBC, 5ESA/ESTEC, and 6EUMETSAT Introduction This poster describes four planned missions which will add Lagrange Point 1 (L-1) and geostationary (GEO) assets to the global BUV satellite instrument complement and examines their relation to projects of the newly formed GSICS Research Working Group Ultraviolet Subgroup (GRWG UVSG). Measurements from the new instruments / platforms will expand our capability to monitor the diurnal variations of atmospheric constituents including ozone, UV absorbing aerosols, SO2, NO2 and other trace gases. The three instruments on GEO platforms will provide opportunities to apply Low-Earth Orbit (LEO)/GEO comparison techniques (currently in use by GSICS Research Working Groups for infrared and visible sensors) as measurements from these new instruments become available. This will lead to improved calibration products for existing polar-orbiting BUV instruments. Even before the GEO instruments become available there will be a new BUV/Visible instrument, the Earth Polychromatic Imaging Camera (EPIC), operating from L-1 opening new areas for LEO/L-1 and GEO/ L-1 under-flight comparisons. Looking back in time, the BUV experience already includes SS/LEO [Space Shuttle SBUV / NOAA Polar-orbit Operational Environmental Satellite (POES) SBUV-2] under-flight comparisons. The first section of the poster describes the missions, measurements, products and applications for the four new instruments. The second part looks at how these new measurements will be used with GSICS. The recently formed GSICS Research Working Group UV Subgroup (GRWG UVSG) has started projects which will grow to include these new instruments. Some details on the projects are in the latest issue of the GSICS quarterly available at http://www.star.nesdis.noaa.gov/smcd/GCC/newsletters.php . Additional information on how some of the projects will include these new measurements is also given in the second part of this poster. Geostationary Missions By 2020, the geostationary orbits are expected to be filled with three UV-visible spectrometers, the NASA Tropospheric Emissions: Monitoring of Pollution (TEMPO) (P.I.: Kelly Chance, HarvardSmithonian Center for Astrophysics) over North America, the ESA Sentinel-4 Ultraviolet Visible Near infrared (UVN) spectrometer over Europe (Mission Scientist: Ben Veihelmann, ESTEC), and the KARI GEMS over Asia (P.I.: Jhoon Kim, Yonsei University), with the TROPOspheric Monitoring Instrument (TROPOMI) and Ozone Mapping and Profiler Suite (OMPS) flying underneath in Low-Earth, sun-synchronous, polar Orbits (LEO). Instrument attributes are provided in the table to the right, and their expected coverage is shown below. Spatial coverage Full Disk Obs. time Onboard calibration Volume (m3) Mass (Kg) Power (W) Data rate (Mbps) Ref. TEMPO 290 – 490 / 540 - 740 0.6 (3 samples) Sentinel-4 UVN 305-500 / 750-775 0.5 / 0.12 2.1 × 4.7 8 × 8 @ 45 N 19°N – 57.5°N 73°W – 130°W 1 hour Solar 1.0 × 1.1 × 1.0 100 30°N – 65°N 30°W – 150°W 1 hour Solar, cal light source 1 × 1 × 1.5 200 100 180 30 Kelly Chance 30 Ben Veihelmann GEMS Instrument Description DSCOVR EPIC Instrument The GEMS(Geostationary Environment Monitoring Spectrometer) is going to be launched into orbit at the end of 2018. It will be positioned over Asia. The instrument is basically a step-and-stare scanning UVvisible imaging spectrometer, with scanning Schmidt telescope and Offner spectrometer. A UV-enhanced 2-D CCD array takes images with one axis spectral, and the other N-S spatial, with E-W scanning over time. On-orbit calibrations are planned by making daily solar measurements and weekly LED light source linearity checks. For the solar calibration, there are two transmissive diffusers, a daily working one and a reference diffuser used twice a year to check the degradation of the working one. Dark current measurements are planned twice a day, before and after the daytime imaging. In order to avoid dark current issues and random telegraph signal (RTS), the CCD is cooled to temperatures well below 0°C. Spectral stability is required to be better than 0.02 nm over 24 hours, stray light less than 2%, polarization sensitivity less than 2% at the instrument level, and the instrument system level MTF better than 0.3. Preflight calibration will be carried out by using the NIST standards. Sentinel-4 (hourly) TEMPO (hourly) Much of the material on given here on EPIC is taken from publically available materials contained in the posters and presentations of the session ‘Earth Observations From the L-1 (Lagrangian Point No. 1)’ at the AGU 2011 fall meeting, San Francisco. The Deep Space Climate Observatory (DSCOVR a joint NASA/NOAA project) will have a continuous view of the entire sunlit face of the Earth. (http://www.nesdis.noaa.gov/DSCOVR/), DSCOVR will make unique space measurements from the first sun-Earth Lagrange point (L-1). The L-1 point is on the direct line between Earth and the sun located 1.5 million kilometers sunward from Earth, and is a neutral gravity point between Earth and the sun. The spacecraft will be orbiting this point in a six-month orbit with a spacecraft-Earth-sun angle varying between 4 and 15 degrees. The NASA Earth Polychromatic Imaging Camera (EPIC) instrument provides spectral images of the entire sunlit face of Earth, as viewed from an orbit around L-1. EPIC is able to view the entire sunlit Earth from sunrise to sunset. EPIC's observations will provide a unique angular perspective, and will be used in science applications to measure ozone and aerosol amounts, cloud height, vegetation properties and ultraviolet reflectivity of Earth. The data from EPIC will be used by NASA for a number of Earth science developments including dust and volcanic ash maps of the entire Earth. EPIC makes images of the sunlit face of the Earth in ten narrowband spectral channels. As part of EPIC data processing, a full disk true color Earth image will be produced about every two hours. This information will be publicly available through NASA Langley Research Center in Hampton, Virginia, approximately 24 hours after the images are acquired. The Earth Polychromatic Imaging Camera (EPIC shown to the right) is a Cassegrain Telescope imaging the disc of the Earth on a 2048 x 2048 pixel CCD array temperature stabilized at -40°C. The irradiances pass through two filter wheels with six positions each (open hole plus five spectral channels) providing ten narrow band channels in the UV and visible. Nominal exposure time is 40 msec for each channel providing signal to noise ratios for 250:1 at 80% well filling. Its temporal resolution will depend on the final schedule of the data-downlink for all instruments on DSCOVR. There is minimal overlap between EPIC’s and other satellites’ scattering angles. Therefore EPIC’s observations from L-1 will provide a unique angular perspective and can be combined with other measurements to obtain particle shapes, phase selection, optical depth, 3-D effects and stereo heights. Spectral ranges (nm) Spectral resolution (nm) Spatial Resolution (NS km × EW km) GEMS 300 – 500 0.6 (3 samples) 7 ×8 @ Seoul, 3.5 ×8 for aerosol 5°S – 45°N 75°E – 145°E 30 min Solar, cal light source 1.1 × 1.2 × 0.9 140 200 (on orbit) /100 (transfer) 40 Jhoon Kim GEMS (hourly) TEMPO Science Traceability Matrix Science Questions Science Objective Science Measurement Requirement Observables Q1. What are the temporal and spatial variations of emissions of gases and aerosols important for AQ and climate? Q2. How do physical, chemical, and dynamical processes determine tropospheric composition and AQ over scales ranging from urban to continental, diurnally to seasonally? Q3. How do episodic events affect atmospheric composition and AQ? Expected EPIC Data Products Q4. How does AQ drive climate forcing and climate change affect AQ on a continental scale? Ozone: Total Column Aerosol Properties: aerosol Index, optical thickness & height Cloud & Surface Properties: surface albedo, cloud fraction & height Vegetation Properties: vegetation index & leaf area index RGB: colored image of the Earth’s sunlit face Q5. How can observations from space improve AQ forecasts and assessments for societal benefit? Q6. How does transboundary transport affect AQ? A. High temporal resolution measurements to capture changes in pollutant gas distributions. [Q1, Q2, Q3, Q4, Q5, Q6] B. High spatial resolution measurements that sense urban scale pollutant gases across GNA and surrounding areas. [Q1, Q2, Q3, Q5, Q6] C. Measurement of major elements in tropospheric O3 chemistry cycle, including multispectral measurements to improve sensing of lower-tropospheric O3, with precision to clearly distinguish pollutants from background levels. [Q1, Q2, Q4, Q5, Q6] D. Observe aerosol optical properties with high temporal and spatial resolution for quantifying and tracking evolution of aerosol loading. [Q1, Q2, Q3, Q4, Q5, Q6] E. Determine the instantaneous radiative forcings associated with O3 and aerosols on the continental scale. [Q3, Q4, Q6] F. Integrate observations from TEMPO and other platforms into models to improve representation of processes in the models and construct an enhanced observing system. [Q1, Q2, Q3, Q5, Q6] G. Quantify the flow of pollutants across boundaries (physical & political); Join a global observing system. [Q2, Q3, Q4, Q5, Q6] Spatially imaged & spectrally resolved, solar backscattered earth radiance, spanning spectral windows suitable for retrievals of O3, NO2, H2CO, SO2 and C2H2O2. [ A, B, C, E, F, G ] Measurements at spatial scales comparable to regional atmospheric chemistry models. [ A, B, C, D, F, G ] Multispectral data in suitable O3 absorption bands to provide vertical distribution information. [ A, B, C, E, F, G ] Spectral radiance measurements with suitable quality (SNR) to provide multiple measurements over daylight hours (solar zenith angle < 70°) at precisions to distinguish pollutants from background levels. [ A to G ] Spatially imaged, wavelength dependence of atmospheric reflectance spectrum for solar zenith angles <70°.[ B, D, E, F, G ] Physical Parameters Instrument Function Requirements Parameter Required Predicted Baseline#* Trace gas column densities (1015 cm-2) hourly @ 8.9 km x 5.2 km Species Precision Band Signal to Noise O3: 0-2 km 10 ppbv O3:Vis (540-650 nm) ≥1413 1765 O3: FT 10 ppbv O3: UV (290-345 nm) ≥1032 1247 O3: SOC 5% O3: Total 3% NO2 1.00 423-451 nm ≥781 2604 H2CO 17.3 327-354 nm ≥742 2266 SO2 17.3 305-330 nm ≥1100 1328 C2H2O2 0.70 433-465 nm ≥1972 2670 Property AOD AAOD AI CF COCP Baseline#* Aerosol/Cloud properties hourly @ 8.9 km x 5.2 km Precision Band Signal to Noise 0.10 0.06 354, 388 nm ≥1414 2158 0.2 0.05 346-354 nm ≥1200 2222 100 mb Spectral Imaging Requirements Relevant absorption bands for trace gases & windows for aerosols Spectral Range (nm) Spectral Resolution (nm) Spectral Sampling (nm) 290-490, 540-740 ≤0.6 < 0.22 Solar irradiance and Earth backscattered radiance spectrally resolved over spectral range Wavelength-dependent Albedo Calibration Uncert. (%) Wavelength-independent Albedo Calibration Uncert. (%) Spectral Uncertainty (nm) Polarization Factor (%) 290-490, 540-740 0.6 0.2 Radiometric Requirements ≤1 0.8 ≤2 2.0 < 0.02 <5 UV, <20 Vis < 0.02 ≤4 UV, <20 Vis Spatial Imaging Requirements Observations at relevant urban to synoptic scales and multiple times during daytime Revisit Time (hr) FOR Geolocation Uncertainty (km) IFOV*: N/S × E/W (km) E/W Oversampling (%) MTF of IFOV*: N/S × E/W ≤1 CONUS <4.0 ≤2.2 × ≤5.2 7.5 ± 2.5 ≥0.16 × ≥0.30 1 GNA 2.8 2.2 × 5.2 7.5 0.16 × 0.36 Investigation Requirements UVN Calibration Sources Mission lifetime: 1-yr (Threshold), 20-mon (Baseline), 10-yr (Goal) Orbit Longitude °W: 90-110 (Preferred), 75-137 (Acceptable) GEO Bus Pointing: Control <0.1° Knowledge <0.04° On-orbit Calibration, Validation, Verification FOR encompasses CONUS and adjacent areas Provide near-realtime products to user communities within 2.5-hr to enable assimilation into chemical models (NOAA & EPA) and use by smart-phone applications Distribute and archive TEMPO science data products These figures on either side show how the absorption patterns for trace gases are revealed in the Earth BUV radiances. The figure on the left compares spectral albedos (normalized radiance / irradiance ratios) for four scenes. The figure on the right isolates the optical depth spectra for select atmospheric constituents. Some signals/retrieval estimates can be obtained by using simple pairs of weakly and strongly absorbing channels while other require very high signal to noise measurements over spectral intervals. Effective Reflectivity and Aerosol Indices Recognized by the Committee on Earth Observing Satellite (CEOS) Atmospheric Composition Constellation (ACC), the geostationary constellation of UV-visible spectrometers will enlighten us on the global distribution of ozone, aerosol, and their precursors. To integrate the datasets for global measurements, harmonized data quality is very important. Thus the inter-calibration among the three different UV-visible satellite instruments is very important, in addition to the quality of the data processing. Therefore, the standardization of data products and pre-calibration/ post-calibration / validation methods are being discussed. The GSICS Research Working Group UV Sub Group has a project on “Best Practices” for BUV calibration. The GSICS Quarterly has an article by Ruediger Lang, “In-flight Characterization of the Solar Diffuser of GOME-2 on Metop-A” with some lessons learned. Images provide by C. Seftor, SSAI. To L1 Schematic for L-1 & and LEO matched viewing the conditions at Equinox. Sun Matches shift north or south seasonally “following” the sun. Local Match for viewing geometry Solar Noon LEO Orbital Equator Track or LEO Cross Track FOV Great Circle aligned with Cross-track FOV Sunlit side of the Earth The schematic above shows a Simultaneous View Path (SVP) match up between DSCOVR EPIC at 0º offset with the Earth/Sun line and Suomi-NPP OMPS. Similar matches will be present for any BUV instrument on a GEO platform with one in a LEO orbit as the LEO orbital tracks pass near the GEO sub-satellite point. An even more extensive set of matches between L-1 and GEO observations will occur as the GEO sub-solar point moves through local solar noon for latitudes in the sun/L-1 line. The GEMS instrument with its coverage extending across the Equator will offer the best match ups with EPIC and LEO instruments. The TEMPO and UVN match up geometries will often require allowances for differences in scattering or viewing path angles depending on the season cycle of solar angles, although TEMPO at 19ºN is only 3º off nadir. Matched viewing conditions will depend on the DSCOVR-Earth-Sun angles, the seasonally varying tilt of the Earth relative to the sun, and each GEO instrument’s Field-of-Regard (FOR). The best match ups for the northern hemisphere will occur in the late fall and early spring. Comparisons of Solar Spectra The GRWG UV Subgroup has a project on comparisons of Solar Spectra from satellite-based instruments. The two initial actions are to collect and compare high spectral resolution solar data sets for the UV, and to collect solar measurements from BUV sensors with sufficient information on their spectral bandpasses and wavelength scales to allow comparisons. The recent GSICS Quarterly article, “Use of Solar Reference Spectra for Satellite Instruments” by Matthew DeLand, provided a survey and summary of several choices for reference spectra. We plan to add the new instruments’ solar spectra to the data set for comparisons. Signal variations in the UV from changes in solar activity (both on the 11-year solar cycle of 27-day solar rotation time scales) complicate comparisons of solar spectra. Recent work by Marchenko and Deland (2014) “Sun as a Star with Aura OMI: Spectral Changes in the on-Going Cycle 24”, provide very accurate determination of the spectral patterns associated with this activity. These results, shown to the right, will help to identify changes associate with instrument from those do to measurements at different times. DSCOVR-EPIC Band Channel λ FWHM Primary Application No. (nm) (nm) 1 Ozone, SO2 317.5±0.1 1±0.2 2 Ozone 325±0.1 2±0.2 3 Ozone, Aerosols 340±0.3 3±0.6 4 Ozone, Aerosols, 388±0.3 3±0.6 5 Aerosols, RGB 443±1 3±0.6 6 Aer,Veg, RGB 551 ± 1 3±0.6 7 680 ± 0.2 2±0.4 Aer, Veg, Cloud, RGB 8 Cloud Height 687.75 ± 0.2 0.8±0.2 9 Cloud Height 764 ± 0.2 1±0.2 10 Clouds 779.5 ± 0.3 2±0.4 Suomi-NPP Band Channel λ (nm) OMPS (NM) 317 OMPS (NM) 325 OMPS (NM) 340 OMPS (NM) 360-380 VIIRS-M2 444 VIIRS-M4 551 VIIRS-M5 672 VIIRS-M5 672 VIIRS-M6 745 FWHM (nm) 1 1 1 1 19.8 14.3 20 20 14 The table above shows DSCOVR-EPIC versus Suomi-NPP OMPS and VIIRS Band Comparison. Suomi-NPP OMPS Nadir Mapper (NM) has spectral coverage every 0.42-nm from 300 to 380 nm at 1nm FWHM. Selections and aggregations of the 196 spectral measurements can be used to approximate the three EPIC UV channels (1st to 3rd channel for EPIC) very well. Extrapolation of the radiance/irradiance ratios for channels in the 360 to 380 nm range can be used to estimate the 388 nm values. VIIRS has 2 bands that include the EPIC VIS/NIR (5th -6th) channels well. The 7th to 9th VIS/NIR channels of EPIC can be calibrated with extrapolation and radiative transfer modeling of VIIRS M5 and M6 channels. Stability of all EPIC channels can be monitored with Validation Time Series (VTS) at vicarious sites. The GRWG UVSG has a project to compare effective reflectivity and aerosol index products for BUV measurements in spectral regions with little trace gas absorption, primarily from 340 nm to 405 nm. All four instruments will make measurements within this spectral range and create these products. The GSICS quarterly has an article by Omar Torres, a primer on the properties of UVAbsorbing Aerosol Incides (AAI) and their relationship to inter-channel calibration. The statement within it—“For a well calibrated sensor the AAI is close to zero in the absence of aerosols and clouds”—can be inverted to give a check on inter-channel calibration for regions with known clear sky and low aerosol loading. This UVSG project’s goals are to develop vicarious calibration methods and provide comparisons for monitoring the BUV measurements for these channels. Approaches for checking reflectivity channel calibration will follow the work in Jaross and Krueger (1993), where they looked at using ice radiances to characterize timedependent changes in reflectivity channel performance. Additional target regions have been identified and used by BUV practitioners since then. The image immediately to the left shows an Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Red-Green-Blue image over the Arabian Peninsula region for January 30, 2012. The image to its left shows a false color map of the OMPS effective reflectivity (from a single Ultraviolet channel at 380 nm) with 5×10 km2 FOVs at nadir for the same day. The color scale intervals range from 0 to 2 % in dark blue to 18 to 20 % in yellow. Products like these from LEO instruments can use the corresponding EPIC or GEO products as transfers comparison standards. Space Shuttle Under-flights of LEO Flights of the SSBUV on the Space Shuttle demonstrated the value of comparisons for match up observations with LEO instruments. Hilsenrath et al. (1995) describes corrections of prelaunch calibration for the NOAA-11 SBUV/2 by using in-orbit comparisons of ultraviolet geometric albedos measured by shuttle SBUV (SSBUV) and the NOAA 11 SBUV/2. Geometric albedo comparison data were further corrected using a radiative transfer code to account for the small difference in observing conditions between the two spacecraft. Comparison of data from three SSBUV flights, which occurred about one year apart, with concurrent SBUV/2 data provided an independent check of the time-dependent change derived from the in-flight calibration data. The SSBUV instrument was returned to the laboratory after each flight check its performance. More recently, LEO/LEO comparisons were used to set the calibration of the Total Ozone Unit (TOU) instrument on FengYun-3 (W. Wang, 2014). GEO / LEO Comparison Example The two figures to the right give information on a comparison of GOME-2 Metop-A with Seviri / MSG measurements. There are two steps to create the match ups. First there is a spatial collocation where the average of all Seviri measurements (~4km) in one GOME-2 ground pixel (40 by 80 km) is computed. Match up must be within ±15 minutes. The second step is to use the high resolution measurements from GOME-2 to approximate the Seviri channel bandpass. The figure immediately to the right shows the spectral coverage. Match up collected from 1st Jan to 22nd Nov 2013 are show in the far figure to the right. The red symbols are two week averages. The results find an ~8% gain in reflectivity for Severi Channel 1 compared to GOME-2 Time-averaged irradiance differences: (mid-y2012+y2013) vs. (mid-y2007+y2008+mid-y2009) Summary The four new BUV instruments in L-1 and GOE orbits will provide exciting new measurements and products for atmospheric composition researchers and practitioners. The GRWG UV Subgroup is already starting projects that will lead to improved measurements and products not just from these instruments but from from the whole constellations of space-based BUV instruments. References Bhartia, P.K., et al., 1995, “Applications of the Langley Plot Method to the Calibration of SBUV Instrument on Nimbus-7 Satellite,” J. Geophys. Res., 100, 2997-3004. Cebula, R.P., & DeLand, M.T., 1998, “Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II solar activity proxy indexes,” Solar Physics, 177, 117–132. Chance, K., 2014, Overview of TEMPO status. TEMPO Science Team Meeting, Hampton, VA. Committee on Earth Observation Satellites Atmospheric Composition Constellation (CEOS ACC), 2011, A Geostationary Satellite Constellation for Observing Global Air Quality: An International Path Forward, position paper. CEOS http://www.ceos.org/images/ACC/AC_Geo_Position_Paper_v4.pdf GSICS Quarterly, Special Issue on Ultraviolet (2014), 8(2) doi: 10.7289/V5N29TWP http://docs.lib.noaa.gov/noaa_documents/NESDIS/GSICS_quarterly/v8_no2_2014.pdf Use of Solar Reference Spectra for Satellite Instruments by Matthew DeLand, SSAI The Absorbing Aerosol Index by Omar Torres, NASA Ozone Measurements from FY-3A by Weihe Wang, CMA Measurement of Atmospheric Composition using UV-visible Spectrometers from Geostationary Orbits by Jhoon Kim, Department of Atmospheric Sciences, Yonsei University, Seoul, Korea Hilsenrath, E., R. P. Cebula, M. T. DeLand, K. Laamann, S. Taylor, C. Wellemeyer, and P. K. Bhartia(1995), Calibration of the NOAA 11 solar backscatter ultraviolet (SBUV/2) ozone data set from 1989 to 1993 using in-flight calibration data and SSBUV, J. Geophys. Res., 100(D1), 1351–1366, doi:10.1029/94JD02611. Jaross, G. and Krueger, A. J., 1993. Ice radiance method for BUV instrument monitoring. Proc. SPIE Int. Soc. Opt. Eng., 2047, 94–101. Kim, J., 2014, Overview and status of GEMS. TEMPO Science Team Meeting, Hampton, VA. Marchenko, S., and DeLand, M.T. , “Sun as a Star with Aura OMI: Spectral Changes in the on-Going Cycle 24”, submitted to The Astrophysical Journal, January 2014. Veihelmann, B., 2013, Sentinel-4 status. CEOS ACC-9 meeting, Darmstadt, Germany. A bibliography of papers on SSBUV underflights of the SBUV(/2) instruments can be found at disc.sci.gsfc.nasa.gov /ozone/documentation/publications/sensor/ssbuv_pubs.shtml.