Fig. 1. Top view schematic of the MATR optical table.
--Mark W. Smith (mwsmith@acd.ucar.edu)
--Stephen R. Shertz, and John C. Gille, Atmospheric Chemistry Division,
National Center for Atmospheric Research,
Boulder, CO
The MOPITT Airborne Test Radiometer (MATR) is a 3-channel gas filter correlation radiometer that supports the Measurements Of Pollutants In The Troposphere (MOPITT) satellite program (Drummond 1992; Drummond et al. 1996). MATR was developed at the National Center for Atmospheric Research (NCAR) by Mark Smith and Steve Shertz, working under John Gille, lead U.S. Co-investigator for MOPITT.
MATR collects data to test and help improve the MOPITT retrieval algorithms (Pan et al. 1995; Pan et al. 1996; NCAR MOPITT Team 1996) and to help validate MOPITT data (Wang et al. 1996). MATR uses the same physical techniques as MOPITT (i.e., gas filter correlation radiometry with pressure modulation and length modulation) to remotely sense atmospheric CO profiles and CH4 columns from an aircraft. General descriptions of gas filter correlation radiometry have been published previously, as well as specific analyses of using a pressure-modulated cell (PMC), and a length-modulated cell (LMC) (Houghton et al. 1984; Taylor 1983; Tolton and Drummond 1997) .
MATR has been developed in stages, starting with a breadboard version that could be operated only in a laboratory. Next, a single-channel MATR Mk I instrument was assembled and test flown in June and September 1996. A new three-channel MATR Mk II instrument was designed and built, based on these test-flight results. The MATR Mk II instrument is described here.
MATR has been kept relatively simple and uses a mix of off-the-shelf technology and items manufactured in-house at NCAR. MATR does not duplicate MOPITT technology and is not intended to reproduce exact MOPITT Level-0 (digital counts) data. MATR is not a prototype for MOPITT, and it has not been used to refine the design of MOPITT. The purpose of MATR is to test and improve our physical and mathematical understanding of how to convert atmospheric radiance values that have been acquired using an LMC and a PMC (MOPITT Level-1 type data) to atmospheric gas amounts (MOPITT Level-2 type data), and also to validate the MOPITT Level-2 data.
While MOPITT has 6 spectral channels for CO, MATR has only 3. However, because the MOPITT channels are partially redundant, the MATR channels provide almost as much vertical profile information as the MOPITT channels. The MATR channels were chosen to span the three most important sounding components (column, mid-troposphere, and upper-troposphere) that go into the MOPITT data retrieval. Channel 1 has a spectral bandpass centered on 2.334 mm, and uses an LMC filled with CO to provide information about CO throughout an atmospheric column. Channel 2 has a spectral bandpass centered on 4.617 mm. It uses the same LMC as channel 1, but provides information about CO weighted in the mid-troposphere.
Fig. 1. Top view schematic of the MATR optical table.
Channel 3 is also centered on 4.617 mm, but it uses a PMC to provide information about CO weighted in the upper troposphere. With a typical flight altitude of 12 km, these 3 channels provide 4-km vertical resolution for the CO profile measurements. By changing a bandpass filter and the gas in the LMC, MATR can alternatively operate with a single channel centered on 2.269 mm to measure CH4 column amounts.
Fig. 1 shows the layout of the MATR optical table. The overall size is 41-cm wide by 67-cm long by 39-cm high, including thermal and electrical controls, but not including the in-flight calibration assembly. A rotating input mirror, located in the calibration assembly, selects one of four sources of input radiance: scene, cold blackbody, hot blackbody, or tungsten lamp source. The mirror does not scan during data collection, it simply selects one of the four inputs. MATR operates with a nominally nadir view of the scene, except when the aircraft is pitched up or down or is banking.
The instrument full angle field of view (FOV) is approximately 0.1 radians. The ground instantaneous FOV is therefore about one tenth of the flight altitude above ground level. However, signal averaging causes the FOV to be smeared out along the direction the aircraft is traveling. At typical flight altitudes and speeds, data are averaged over a strip that is 1.2 km wide by 10- to 30-km long. A nadir-pointing video camera records scene imagery with a field of view about 2.5 times wider than that of MATR.
MATR is designed for use aboard a pressurized-cabin jet aircraft. An unavoidable consequence of operating a radiometer from inside a pressurized-cabin aircraft is that scene radiance must pass through a pressure-sealing window.
This window will invariably modify the scene radiance by reflection, absorption, and re-emission. MATR uses a novel in-flight radiometric calibration assembly that places the calibration sources outside of the pressure-sealing window (so that window effects are included in the in-flight radiometric calibration), but which keeps air from blowing over the calibration sources (so that the sources can be maintained at stable temperatures). Fig. 2 shows a sketch of the calibration assembly.
 | Fig. 2. Side view schematic of the MATR in-flight radiometric calibration assembly. |
A set of flights was made with MATR in February and March 1998 as part of the Pre-MOPITT Oklahoma Validation Experiment (Pre-MOVE), centered at the Department of Energy's Atmospheric Radiation Measurement Program/Cloud and Radiation Testbed (DOE/ARM CART) site near Enid, Oklahoma. Pre-MOVE was a collaborative effort that involved several groups that will all work to validate MOPITT data. Data from these flights appear to be of good quality, but the data reduction and analysis are not yet complete. The retrieved CO values will be compared to results obtained simultaneously with ground-based spectrometers and from independent airborne in situ sampling. The MATR instrument was flown aboard a Cessna Citation II operated by the DOE Remote Sensing Laboratory. Fig. 3 shows a picture of MATR installed in the Citation.
 | Fig. 3. Steve Shertz filling a detector Dewar with liquid nitrogen, with MATR installed in the DOE Cessna Citation II. Calibration assembly in foreground. |
Future flight plans for MATR include a mission over Table Mountain Observatory in California or over Kitt Peak Observatory in Arizona in February 1999. CO values retrieved with MATR will again be compared to results from ground-based spectrometers as well as in situ sampling. Plans are also being developed to operate MATR as part of the SAFARI-2000 field campaign in southern Africa.
Project support is provided by NASA under contract NAS5-30888 administered through Dr. Michael King, the EOS Senior Project Scientist at NASA's Goddard Space Flight Center. The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.
Drummond, J. R. 1992: Measurements of Pollution in the Troposphere (MOPITT), The Use of EOS for Studies of Atmospheric Physics, J. C. Gille and G. Visconti, Eds., North-Holland, Amsterdam, 77-101.
Drummond, J. R., G. P. Brasseur, G. R. Davis, J. C. Gille, J. C. McConnell, G. D. Peskett, H. G. Reichle, Jr., and N. Roulet, 1996: MOPITT Mission Description Document. Department of Physics, University of Toronto, Canada. Available HTTP: http://www.atmosp.physics. utoronto.ca; Directory: /MOPITT/home.html
Houghton J. T., F. W. Taylor, and C. D. Rodgers, 1984: Remote Sounding of Atmospheres. Cambridge: Cambridge University Press, 93-107.
NCAR MOPITT Team, 1996: MOPITT: Algorithm Theoretical Basis Document. National Center for Atmospheric Research, Boulder, CO. Available HTTP: http://eos.acd.ucar.edu Directory: /mopitt/atbds.html File: Level-2 ATBD (in various formats).
Pan, L., D. P. Edwards, J. C. Gille, M. W. Smith and J. R. Drummond, 1995: Satellite remote sensing of tropospheric CO and CH4: forward model studies of the MOPITT instrument. Appl. Opt. 34, 6976-6988.
Pan, L., J. C. Gille, C. D. Rodgers, D. P. Edwards, P. L. Bailey, L. A. Rokke, and J. Wang, 1996: Analysis and characterization of the retrieval algorithm for measuring tropospheric CO using the MOPITT instrument. Proc. SPIE, 2830, 159-168.
Taylor, F. W., 1983: Pressure Modulator Radiometry. Spectrometric Techniques, Vol. III, G. A. Vanasse, Ed., Academic Press, New York, 137-196.
Tolton, B.T., and J. R. Drummond, 1997: Characterization of the length-modulated radiometer. Appl. Opt., 36, 5409-5420.
Wang, J., J. Gille, P. Bailey, M. Smith, D. Edwards, L. Pan, L. Rokke, J. Drummond, G. Davis, and H. Reichle, 1996: MOPITT Data Validation Plan. National Center for Atmospheric Research, Boulder, CO. Available HTTP: http://eos.acd.ucar.edu Directory: /mopitt/val_plans.html File: V3 validation plan (in various formats).