The SAGE III/Meteor Mission One Year in Operation
Patrick McCormick, (PAT.MCCORMICK@hamptonu.edu) and J. Anderson, Hampton University, Hampton (VA), USA
W.P. Chu, C.R. Trepte, L.W. Thomason, J.M. Zawodny , NASA LARC, Hampton (VA), USA
The Stratospheric Aerosol and Gas Experiment (SAGE III) instrument is a fourth-generation satellite instrument designed to provide long-term measurements of ozone, aerosol, water vapour, and other gases in the atmosphere. It was launched on-board the Russian spacecraft Meteor 3M from the Russian launch site Baikonur Cosmodrome in Kazakhstan on December 10, 2001. A Ukrainian built Zenit-2 rocket was used to place the spacecraft into a sun-synchronous orbit with an inclination of 99.64o, and an ascending node crossing time of 9 am. and an orbital height of 1018 km. The SAGE III mission is part of NASA EOS program [McCormick, 1991], and is a collaborative mission between NASA and the Russian Aviation and Space Agency (RASA).
The SAGE III science team* was selected in 1990 to aid in all scientific aspects of the mission. It was composed of scientists from different U.S. government agencies (NASA, NOAA, and DOD) and researchers from various US universities (Harvard, Columbia, Georgia Tech, North Carolina State, and Wyoming), and non-U.S. institutions (University of Lille in France, Russian Institute of Atmospheric Physics, and the Russian Central Aerological Observatory). A Solar Occultation Satellite Science Team (SOSST), soon to be announced by NASA, will continue validation and perform studies using the data from SAGE III as well as other occultation data sets, e.g. SAGE II.
The Meteor-3M satellite
The Meteor-3M is a 3-axis stabilized spacecraft and is an advanced version of the Meteor series designed and built by the Electromechanics Research Institute (NIIEM) located in Istra, Russia. This series of spacecraft has been the main platform for Russian space-flight instruments serving meteorological, environmental, and natural resource research purposes. Earlier versions of the Meteor series spacecraft were developed by the All-Union Electromechanics Research Institute (VNIIEM) in Moscow, Russia. Meteors have been in operation for over 20 years and have a well-proven design with a high degree of reliability. The Meteor-3M spacecraft is equipped with advanced components such as GPS/GLONASS receiver, a refined attitude control system and an L-band transmitter with expected lifetime of over three years.
Description of the SAGE III instrument
The SAGE III instrument is designed similar to the SAGE I and SAGE II instruments with the exception of using an advanced detector package consisting of a two dimensional CCD array detector plus a near-IR photodiode. The new detector design enhances the measurement capability to provide atmospheric spectral coverage from 280 nm to 1040 nm, with a spectral resolution of about 1.2 nm, plus a channel at 1550 nm for separating aerosols and clouds, and for measuring larger aerosols.
The SAGE III instrument consists of three subsystems. The first subsystem is the scan head that consists of the scan mirror mounted on an azimuth drive that can rotate over 360o. The scan mirror scans in elevation so that it can point to the Earths limb when the instrument is in orbit. The second subsystem is the imaging optics consisting of a telescope and azimuth target acquisition detectors. The telescope is an f/4 Dall-Kirkham configuration with a one-half arc minute vertical by five arc-minute horizontal slit in the focal plane that serves as the science aperture and as the entrance slit to the grating spectrometer. The whole telescope assembly including the scan mirror can rotate together in azimuth to eliminate the problem of image rotation during azimuth rotation. The third subsystem of the SAGE III sensor is the spectrometer detector package. The spectrometer consists of a holographic grating in a Rowland configuration operating in both zeroth and first orders. The first order dispersion is imaged on the 800 x 10 element CCD, which is back-side thinned to enhance UV response. The zeroth order reflection from the grating is used with a photodiode together with a spectral bandpass filter centered at 1550 nm.
The SAGE III instrument is designed to perform the well-proven technique of Solar occultation for monitoring the different atmospheric species that scatter and/or exhibit spectral absorption characteristics in the near UV, visible, and near IR [Chu and McCormick, 1979; McCormick et al., 1979]. The instrument was also designed and built to perform Lunar occultation measurements. The capability of Lunar measurements arises from the use of the CCD detector, which can provide high sensitivity with variable signal integration time. Since SAGE III is capable of measuring the moons brightness (about 600,000 times less bright than the sun), it possesses the ability to make measurements of limb scattering on the bright side of each orbit. Limited limb scattering measurements are being made but are being treated as research products since the instrument was not optimized for such measurements.
For Solar occultation measurements, the operation of the SAGE III instrument in orbit is similar to the operation of the previous SAGE instruments. Before a Solar occultation event, the telescope and scan head is first slewed to the azimuth position where the sun will appear. As soon as the sun appears in the instruments field-of-view, the scan mirror begins to scan in elevation to acquire the Solar image. The 0.5 arc-minute science aperture in the vertical direction provides approximately a one-half kilometer vertical resolution in the atmosphere. Measurements are obtained by repeatedly scanning up and down over the Solar disk at the Earths limb over a height region from the ground to about 300 km altitude as the sun rises or sets from the satellite perspective. Radiometric data are sampled at a rate of 64 samples per second. Due to the limitation on the data downlink from the spacecraft, the instrument can only sample 85 spectral channels of data from the CCD instead of the total available 800 pixels. These 85 spectral channels are selected to provide optimum information for the retrieval of ozone in the stratosphere, mesosphere, and down into the troposphere, plus information on aerosol, NO2, water vapour, and the oxygen A-band used for the retrieval of temperature and pressure.
Lunar occultation measurements are being performed by the SAGE III instrument when the brightness of the moon is 40 % or greater of a full moon. Lunar measurements are sampled at 10 samples per second due to the needed long integration time for the weaker signal. The spectral coverage for the Lunar measurement can, therefore, be increased to 340 spectral channels over the CCD. The limb scattering measurements are considered to be a research mode for SAGE III and are just now beginning to be conducted. Early limb scattering results are of high quality and showing success, especially for ozone profile measurements.
Retrieval algorithm and data processing
The algorithm used to process the SAGE III measurements is similar to the SAGE II algorithm [Chu, et al., 1989]. A complete description of the SAGE III retrieval algorithm is available in the SAGE III Algorithm Theoretical Base Document: Solar and Lunar Algorithm, which is available from the NASA Earth Observing System Project Science Office Web Site (http://eospso.gsfc.nasa.gov).
The algorithm consists of two main modules. The first module performs calibration of the measured radiance over the 280 to 1040, and 1550 nm spectral channels, and converts the measurements into slant-path transmission profiles of the atmosphere. This procedure involves both geometric calibration and radiometric calibration. The geometric calibration is to precisely locate each measured data point through a detailed spacecraft and Solar ephemeris calculation, including atmospheric refraction. The position information consists of slant-path tangent height, the latitude and longitude of the ground location for the tangent point, and the angular position of the viewing direction on the Solar disk. The radiance calibration is done simply by rationing the measurements within the atmosphere to the exoatmospheric Solar limb profiles. The second module performs the retrieval from the transmission data into species profile data. The multi-wavelength slant-path transmission profiles are first separated into transmission profiles for individual species, such as ozone, nitrogen dioxide, water vapour and oxygen across their absorption bands, and aerosol attenuation at select wavelengths. The slant-path transmission profile for aerosol, ozone, and nitrogen dioxide is then inverted into vertical concentration profiles using an onion-peeling procedure. For water vapour and oxygen, a nonlinear least-squares retrieval method is used to retrieve water vapour concentration and temperature profiles.
Data production software for routine processing of the SAGE III measurements has been implemented at the NASA Langley Research Center, SAGE III Science Computing Facility (SCF). Both level 1 and level 2 data from the SAGE III measurements are currently being produced, archived and available from the NASA Atmospheric Sciences Data Center at Langley. The level 1 data products consist of the multi-wavelength atmospheric slant-path transmission data from the Solar occultation measurements. The level 2 data products from the Solar measurements consist of O3 profiles from cloud-top to 85 km, aerosol extinction profiles at nine wavelengths from cloud-top to about 40 km, pressure and temperature profiles from cloud-top to 85 km, H2O profiles from cloud-top to 50 km, and NO2 profiles from 10 to 50 km. For Lunar measurements, the level 2 data products consist of O3 profiles from 10 km to 50 km, NO2 profiles from 15 to 45 km, NO3 profiles from 20 to 55 km, and OClO profiles, under perturbed atmospheric conditions, from 15 to 25 km.
Post launch status and early results
After the launch of SAGE III on December 10, 2001, the instrument was powered up and put on standby for out-gassing. Two major spacecraft problems occurred during this period. The primary spacecraft transmitter failed on January 1, 2002. Fortunately, a back-up L-band transmitter worked well when turned-on. The second was the failure of the GPS/GLONASS receiver. Since accurate spacecraft ephemeris data are necessary for the operation of the instrument and processing of the data, another means had to be found to provide the needed ephemeris data. Fortunately, a newly designed retro-reflector built by the Russians for ground-based laser tracking is aboard the spacecraft. The SAGE III team was given permission by RASA to allow the International U.S. Laser Ranging System (ILRS) to track the Meteor 3M spacecraft. With the availability of the laser tracking data, accurate spacecraft ephemeris information is easily being calculated.
With the two major problems solved, and with a sufficient time allowed for out-gassing, the instrument was turned on February 27, 2002, and Solar occultation measurements were made. By early March 2002, the SAGE III instrument was acquiring all the available Solar measurements. Similarly, routine Lunar measurements began on March 4, 2002. The first attempt at acquiring limb scattering data with the SAGE III/Meteor instrument was performed on June 30, 2002. Currently, limb scattering measurements have only been taken on an occasional basis and the data are considered to be for research studies only.
The first public release of the SAGE III dataset is available through the website http://eosweb.larc.nasa.gov/PRODOCS/sage3/table_sage3.html. The SAGE III aerosol, ozone, and nitrogen dioxide dataset can also be accessed through the SAGE III website at http://www-sage3.larc.nasa.gov/data/login_form.php. The second public release of the data should be available in the summer of 2003. Figures 1-4 illustrate the type of data products that are being produced by the SAGE III instrument.
Work is underway to validate the existing products and to refine the algorithms to produce new products. One major emphasis of the SOSST will be to validate the publicly released SAGE III data with comparisons to other instruments and model outputs.
Figure 1. The mean difference and standard deviation of composite comparisons between coincident SAGE III and SAGE II ozone profiles during the time period of May to December 2002 are shown. Coincidence is defined as measurements made within 4º latitude, 12º longitude, and 12 hours. For these coincidences, the mean time difference is 1.3 hours, mean latitude difference is 2o, and the mean longitude difference is 5.9o. The number of coincidences at a given altitude is given on the right hand side of the right panel. The mean is defined as SAGE III-SAGE II / the average of SAGE III + SAGE II. The yellow shading in each panel indicates the range of tropopause heights for the comparisons and the horizontal dashed green line is the mean tropopause level. Note that the two instruments agree within 3% from 15 to 40 km.
Figure 2. The SAGE III sunrise time-history of ozone density profiles from May 2002 through February 2003 in the southern hemisphere is shown. The measurement latitudes and tropopause heights (thin black line) at the measurement location are also shown.
Figure 3. The SAGE III sunset time-history of aerosol extinction profiles at 1020 nm wavelength from May 2003 through February 2003 in the northern hemisphere is shown. The latitude and tropopause height for each measurement are also shown.
Figure 4. Time-series of SAGE II (red circles) and SAGE III (blue circles) monthly zonal averages centered near 56ºS is plotted for a number of altitudes. The black line is the model fit of SAGE II ozone data between October 1984 and June 2000, and extended to August 2002. It is clear that the SAGE III data follow the natural seasonal cycle (climatology) and fill in monthly gaps of the SAGE II time series. The SAGE II model climatology consists of a mean, linear trend, seasonal, quasi-biennial oscillation, and Solar cycle terms.
After some initial problems, SAGE III is working well and producing high quality data. After an initial validation, the O3, NO2, and aerosol data are being routinely archived and are publicly available at NASAs Atmospheric Sciences Data Center at the Langley Research Center. The SAGE III Ozone Loss and Validation Experiment (SOLVE-2), sponsored by NASAs Office of Earth Sciences, was held from December 2002 through February 2003, as an intensive field campaign staged out of Kiruna, Sweden. It involved coordinated balloon launches, ground-based measurements, and aircraft deployments. It was coordinated with not only SAGE III overflights but with a number of international satellite experiments and thereby provided validation data for ILAS-II on ADEOS-II, and SCIAMACHY, GOMOS, and MIPAS on ENVISAT. These data and other validation data will become available over the near future and used to further validate and improve the SAGE III data products. It is expected that the remainder of the SAGE III Solar and all of the Lunar data will become publicly available during the summer of 2003.
Chu, W.P. and M.P. McCormick, Inversion of stratospheric aerosol and gaseous constituents from Spacecraft Solar extinction data in the 0.38-1.0 micrometer wavelength region. Appl. Opt., 18, 1404-1413, 1979.
Chu, W.P., et al., SAGE II inversion algorithm. J. Geophys. Res., 94, 8339-8351, 1989.
McCormick, M. P., SAGE III capabilities and global change, AIAA-91-0051, 29th Aerospace Sciences Meeting, Jan. 7-10, 1991, Reno, Nevada.
McCormick, M. P., et al., Satellite studies of the stratospheric aerosol. Bull. Am. Meteorol. Soc., 60, 1038-1046, 1979.
*The SAGE III Science Team (1990-2002)
M. Patrick McCormick, C. Brogniez, A.A. Chernikov, W.P. Chu, D.M. Cunnold, J. DeLuisi, P.A. Durkee, N.P. Elansky, B.M. Herman, P.V. Hobbs, G.S. Kent, J. Lenoble, A.J. Miller, V.A. Mohnen, V. Ramaswamy, D.H. Rind, P.B. Russell, V.K. Saxena, E.P. Shettle, L.W. Thomason, C.R. Trepte, G. Vali, S.C. Wofsy, and J.M. Zawodny.