The Greenhouse gases Observing SATellite (GOSAT) monitors carbon dioxide (CO2) and methane (CH4) globally from space. The Thermal and Near infrared Sensor for Carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) installed on GOSAT measures spectra absorbed by atmospheric minor components including greenhouse gases in infrared wavelength regions. This paper describes the characterization and validation of the CO2 and CH4 profiles retrieved from the thermal infrared (TIR) spectra observed by GOSAT. The retrieved CO2 and CH4 profiles were compared with the corresponding aircraft data provided by the National Oceanic and Atmospheric Administration (NOAA)/Earth System Research Laboratory (ESRL)/Global Monitoring Division (GMD)/Carbon Cycle Greenhouse Gases(CCGG) group. This group has conducted an aircraft program since 1992 to collect air samples mainly in North America. Each insitu aircraft profile was compared with those retrieved from TIR spectra without considering the effect of its averaging kernel. The root mean square (RMS) and bias errors of the retrieved CO2 and CH4 profiles were evaluated seasonally and with respect to atmospheric pressure. This comparison with aircraft data provides significant information for further improvement of the TIR retrieval algorithm.
The thermal infrared band of the main sensor of the greenhouse gas observing satellite (GOSAT), the TANSO-FTS,
must be calibrated with accuracy higher than 0.3 K in the brightness temperature Tbb for retrieving CO2 concentration
with accuracy of 1% in the upper atmosphere. However, that accuracy has not been achieved because of some error
sources. One is the systematic bias in the radiance spectrum resulting from effects of radiation emitted from internal
optics and multiple scattering of target signals. Another is the polarization effect of the pointing mirror. Both effects can
be merged into two parameters, gain and offset, in the two point calibration procedure. They can be tuned by comparing
the spectrum with well-calibrated spectra such as those from the AIRS sensor. Based on the corrected radiance spectra,
global CO2 concentrations were processed. However, they show peculiar latitudinal distribution implying the existence
of temporally variant parameters that can affect the calibration. This bias can be reduced by referring to housekeeping
data of the satellite in the calibration procedure. The stratospheric ozone distribution is also analyzed. The sensor
demonstrated the difference in the ozone hole feature between spring 2009 and 2010 over the South Pole.
The greenhouse gas observing satellite (GOSAT) was launched on 23 January 2009. Its main sensor, the "thermal and
near infrared sensor for carbon observation Fourier transform spectrometer (TANSO-FTS)", is functioning normally. It
can measure a wide spectrum including three CO2 absorption bands at 1.6 μm and 2.0 μm (Short Wavelength InfraRed,
SWIR band), and 15 μm (Thermal InfraRed, TIR band). The former two bands are used to estimate columnar
concentrations of CO2. The latter is used to retrieve the vertical profile of CO2 concentration in the upper troposphere.
Simulation studies show that high radiometric calibration accuracy of 0.3 K in brightness temperature Tbb is necessary to
retrieve a CO2 concentration profile with accuracy of 1% in the upper atmosphere. The sensor's fundamental
performance was evaluated during the initial checkout period, which continued for six months. Results show that most of
the radiometric performance is achieved as designed. However, results also show that some systematic biases exist in the
TIR spectrum because of the opacity of the dichroic mirrors of SWIR bands obstructing the field of view of the TIR
band. These biases can be mostly removed by explicitly considering radiation--that emitted from inside of the optics and
multiple scattering of target signals--in the calibration procedure. Using a three-day global composite of the clear sky
spectrum, CO2 concentrations in the upper atmosphere were retrieved preliminarily. Results show a convincing
hemispheric concentration gradient, which agrees well with the climatologic distribution of CO2.
Radiometric calibration accuracy of 0.3 K in Tbb is necessary to retrieve CO2 concentration profile with accuracy of 1 %
in the upper atmosphere. In case of the thermal infrared (TIR) band (band 4) of GOSAT-TANSO-FTS, interferometric
phase correction procedure is very important because the total transmittance of the optics at the band is about 70 %
because of opacity of dichroic mirrors of band 1-3 placed obstructing the field of view of band 4, and the mirrors reflect
the radiation emitted from inside of the optics. Based on the results from the thermal vacuum tests (TVTs) of the sensor,
it is found that interferometric signal is almost zero when the sensor view a target of which temperature is about 280-
300K because the radiation emitted from inside of the spectrometer controlled at about 296 K has completely opposite
phase to that of the target. It is also found that the interferometric final phase of the calibrated signal varies when the
total signal is almost zero because of weak signals that have phases differ from both of those of targets and calibrators. A
candidate phase correction procedure is proposed based on that adopted for a previous space FTS sensor, IMG/ADEOS.
Non-linearity correction for the detector and polarization efficiency correction are also desccussed.
The Greenhouse gases Observing Satellite (GOSAT) is a Japanese satellite that is intended to observe CO2
concentrations from space and to contribute to advancement of research related to CO2 source/sink estimation. The
GOSAT main sensor is a Fourier Transform Spectrometer (FTS) named "TANSO-FTS", which covers a wide terrestrial
radiation spectrum including CO2 absorption bands at 1.6 μm (Short Wavelength InfraRed, SWIR), and 15 μm (Thermal
InfraRed, TIR). The former band is used to estimate columnar concentration of CO2; the latter is used to retrieve the
vertical profile of CO2 in the upper atmosphere above the ca. 700 hPa pressure level. We adopt the maximum a posteriori
method (MAP) to retrieve the vertical profile of CO2 concentrations using the meteorological analysis data for
temperature profiles. Key techniques for retrieving CO2 concentrations are 1) reduction of temperature estimation error
through channel selection, 2) optimization of the a priori CO2 profile based on the output from a CO2 transport model,
and 3) usage of SWIR data as an additional constraint in retrieval of vertical profiles of CO2. Simulation studies using
the output from a CO2 transport model show that, although thermal infrared spectrum has poor sensitivity to the CO2
concentration change in the lower atmosphere, particularly in the boundary layer, we expect that CO2 concentration
profiles in the lower atmosphere can be reproduced statistically by combining CO2 columnar data derived from SWIR as
an additional constraint in retrieving a CO2 concentration profile from TIR data.
The Greenhouse Gases Observing Satellite (GOSAT) will be launched in 2008 for global observations of greenhouse
gases such as CO2 and methane. This study examines the feasibility of retrieving CO2 concentrations from the infrared
spectra of the GOSAT/Thermal and near infrared Sensor for Carbon Observation (TANSO)-FTS. Retrieval simulations
in which the maximum a posteriori (MAP) method was applied to pseudo-spectra at 700-800 cm-1 from TANSO-FTS
("CO2 15-?m band") showed that retrieved CO2 profiles agreed with true CO2 profiles to within the total errors
throughout the troposphere above 700-800 hPa when atmospheric conditions such as temperature used in the
computation of the spectra were known. In contrast, discrepancies between retrieved CO2 and true CO2 concentrations
increased if temperatures used in the retrieval included random errors; a random scatter of ±0.5 K caused a discrepancy
that was 12 times larger at ~750 hPa. However, appropriate channel selection based on CO2 and temperature information
could reduce the effect of temperature uncertainty on CO2 retrievals in this spectral region: the discrepancy between
retrieved and true concentrations at ~750 hPa in the case with channel selection was about one-third of the discrepancy
without any channel selection.
The Greenhouse gases Observing Satellite (GOSAT) is a Japanese satellite that is intended to observe CO2 concentration
from space and to contribute to advancement of research of the source/sink estimation of CO2. The GOSAT main sensor
is a Fourier Transform Spectrometer (FTS) named "TANSO-FTS", which covers a wide terrestrial radiation spectrum
including CO2 absorption bands at 1.6 μm, 2.0 μm, and 15 μm. The former two bands are used to estimate columnar
concentration of CO2; the latter is used to retrieve the vertical profile of CO2 in the upper atmosphere above about 700
hPa pressure level. In addition, another installed on the satellite is an imaging sensor that will be used to detect clouds
and aerosols: Cloud and Aerosol Imager (CAT). The Center for Climate System Research (CCSR) has contracted with
the Japan Aerospace Exploration Agency (JAXA) to develop an algorithm to retrieve CO2 concentration profiles from
data measured by the thermal infrared (TIR) band of the TANSO-FTS sensor. We adopt the maximum a posteriori
method (MAP) to retrieve the vertical profile of atmospheric parameters from thermal infrared spectra. Key techniques
for retrieving CO2 concentrations are 1) reduction of temperature estimation error through channel selection, 2)
optimization of the initial guess for CO2 profile based on the output from a chemical transport model (CTM), and 3)
usage of data from the 1.6 μm band of TANSO-FTS as an additional constraint in retrieval of vertical profiles of CO2.
Although thermal infrared spectrum data have poor vertical resolving power for CO2 concentration in the lower
atmosphere, particularly in the boundary layer, we expect that CO2 amount in the lower atmosphere can be deduced by
substituting the upper level concentration from the columnar concentration estimated from the 1 .6 μm band data.
The successor of the Improved Limb Atmospheric Spectrometer (ILAS), ILAS-II, aboard the Advanced Earth Observing Satellite-II (ADEOS-II) measured atmospheric absorption spectra at a wavelength region from 753 nm to 784 nm, including the molecular oxygen (O2) A-band centered at 762 nm, with a FWHM spectral resolution of 0.06 nm. Temperature and pressure profiles between ~10 km and 80 km were retrieved from the solar occultation measurements of the O2A-band spectra during the operational period of ADEOS-II in 2003. Based on the actual measured data during the smallest atmospheric variability, the repeatability of the measurement, which is a measure of the measurement precision, for temperature and pressure was estimated to be 1-2 K and 0.5-2%, respectively. Comparisons between ILAS-II and the U.K. Met. Office (UKMO) stratospheric analyses or the NASA's UARS/HALOE and TIMED/SABER temperature data are performed. Regardless of the good precision, it is found that the ILAS-II temperatures are systematically lower in the stratosphere and significantly higher in the lower mesosphere.
The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) successfully observed atmospheric profiles over the Arctic and Antarctic from November 1996 through June 1997. It revealed the frequent occurrence of Polar Stratospheric Clouds (PSCs) over the Arctic between January and mid-March 1997. The ILAS provides a unique data set, including aerosol extinction at 780 nm, nitric acid, water vapor, and nitrous oxide, simultaneously. This paper demonstrates the validity of the ILAS aerosol data and presents an approach to estimate the chemical composition of PSCs. Comparisons are made with data from the Stratospheric Aerosol and Gas Experiment (SAGE) II.
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