Kei Shiomi

Vicarious Calibration of the GOSAT Sensors Using the Railroad Valley Desert Playa

Japans Greenhouse Gases Observing Satellite (GOSAT) was successfully launched into a sun-synchronous orbit on January 23, 2009 to monitor global distributions of carbon dioxide ( CO2) and methane (CH4). GOSAT carries two instruments. The Thermal And Near-infrared Sensor for carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) measures reflected radiances in the 0.76 μm oxygen band and in the weak and strong CO2 bands at 1.6 and 2.0 μm. The TANSO Cloud and Aerosol Imager (TANSO-CAI) uses four spectral bands at 0.380, 0.674, 0.870, and 1.60 μm to identify clear soundings and to provide cloud and aerosol optical properties. Vicarious calibration was performed at Railroad Valley, Nevada, in the summer of 2009. The site was chosen for its flat surface and high spectral reflectance. In situ measurements of geophysical parameters, such as surface reflectance, aerosol optical thickness, and profiles of temperature, pressure, and humidity, were acquired at the overpass times. Because the instantaneous field of view of TANSO-FTS is large (10.5 km at nadir), the spatially limited reflectance measurements at the field sites were extrapolated to the entire footprint using independent satellite data. During the campaign, six days of measurements were acquired from two different orbit paths. Spectral radiances at the top of the atmosphere were calculated using vector radiative transfer models coupled with ground in situ data. The agreement of the modeled radiance spectra with those measured by the TANSO-FTS is within 7%. Significant degradations in responsivity since launch have been detected in the short-wavelength bands of both TANSO-FTS and TANSO-CAI.

Long-Term Vicarious Calibration of GOSAT Short-Wave Sensors: Techniques for Error Reduction and New Estimates of Radiometric Degradation Factors

This work describes the radiometric calibration of the short-wave infrared (SWIR) bands of two instruments aboard the Greenhouse gases Observing SATellite (GOSAT), the Thermal And Near infrared Sensor for carbon Observations Fourier Transform Spectrometer (TANSO-FTS) and the Cloud and Aerosol Imager (TANSO-CAI). Four vicarious calibration campaigns (VCCs) have been performed annually since June 2009 at Railroad Valley, NV, USA, to estimate changes in the radiometric response of both sensors. While the 2009 campaign ( VCC2009) indicated significant initial degradation in the sensors compared to the prelaunch values, the results presented here show that the stability of the sensors has improved with time. The largest changes were seen in the 0.76 μm oxygen A-band for TANSO-FTS and in the 0.380 and 0.674 μm bands for TANSO-CAI. This paper describes techniques used to optimize the vicarious calibration of the GOSAT SWIR sensors. We discuss error reductions, relative to previous work, achieved by using higher quality and more comprehensive in situ measurements and proper selection of reference remote sensing products from the Moderate Resolution Imaging Spectroradiometer used in radiative transfer calculations to model top-of-the-atmosphere radiances. In addition, we present new estimates of TANSO-FTS radiometric degradation factors derived by combining the new vicarious calibration results with the time-dependent model provided by Yoshida (2012), which is based on analysis of on-board solar diffuser data. We conclude that this combined model provides a robust correction for TANSO-FTS Level 1B spectra. A detailed error budget for TANSO-FTS vicarious calibration is also provided.

Retrievals of Total and Tropospheric Ozone From GOSAT Thermal Infrared Spectral Radiances

Total and tropospheric ozone columns have been retrieved from thermal infrared spectral radiances recorded with the Thermal And Near infrared Sensor for carbon Observation-Fourier Transform Spectrometer (TANSO-FTS), which is onboard the Greenhouse gases Observing SATellite launched on January 23, 2009. We present ozone retrievals that were performed over about two years of observations (during the period from April 2009 to December 2010) over four climatically distinct regions (the Sapporo, Tsukuba, Naha, and Syowa sites). Annual variations of the total and tropospheric ozone columns over the four sites were derived. We compare TANSO-FTS total ozone columns with ground-based data from the Dobson spectrophotometer, and the seasonal trends and patterns of the retrieved total ozone are consistent with those of Dobson measurements. The TANSO-FTS total ozone columns are in good agreement with the Dobson data, with a correlation coefficient of about 0.98. On average, TANSO-FTS total ozone retrievals exhibit a positive bias of 8.8 DU (3.0%) with a root-mean-square difference of 10.9 DU (4.1%) compared to the Dobson measurements. Comparisons of the TANSO-FTS tropospheric ozone columns to ozonesondes available from the four sites have been performed. The TANSO-FTS tropospheric ozone columns compare well with the ozonesonde measurements, with correlation coefficients of 0.96 and 0.92 for the surface-tropopause and surface-6 km partial columns, respectively. Average differences of 0.7 ± 4.2 DU (2.5% ± 12.8%) and -0.7 ±2.2 DU (-3.0% ±12.2%) are found for the surface-tropopause and surface-6 km partial columns, respectively.

TIR Spectral Radiance Calibration of the GOSAT Satellite Borne TANSO-FTS With the Aircraft-Based S-HIS and the Ground-Based S-AERI at the Railroad Valley Desert Playa

The thermal infrared (TIR) band of Thermal and Near-Infrared Sensor for carbon Observations Fourier Transform Spectrometer (TANSO-FTS) on the Greenhouse gases Observing SATellite (GOSAT) measures a wide range of scene temperatures using a single detector band with broad spectral coverage. This work describes the vicarious radiometric calibration over a large footprint (10.5 km) and high temperature surface using well-calibrated ground-based and airborne FTS sensors. The vicarious calibration campaign of GOSAT was conducted at Railroad Valley, NV in June 2011. During the campaign, the Scanning High-resolution Interferometer Sounder (S-HIS) mounted on the high-altitude NASA ER-2 aircraft observed upwelling radiation and the ground-based Surface-Atmospheric Emitted Radiance Interferometer (S-AERI) observed infrared thermal emission from the atmosphere and the surface at the same location and time as the GOSAT TANSO-FTS. We validated TANSO-FTS TIR radiance with S-HIS radiance using double difference method, which reduces the effect of differences in the observation geometry. In this paper, we estimated the TANSO-FTS Instantaneous Field of View average temperature and emissivity by the coincident S-AERI and S-HIS observed radiance. The double difference between TANSO-FTS and S-HIS result in a difference of 0.5 K at atmospheric window channels (800 ~ 900 cm-1) and CO2 warm brightness temperature channels (700 ~ 750 cm-1), 0.1 K at ozone channels (980 ~ 1080 cm-1), and more than 2 K at CO2 cool brightness temperature channels (650 ~ 700 cm-1). The main reason of remaining errors is attributed to a calibration error in the TANSO-FTS Level 1B product version under evaluation.

Updated level-1 processing after two-years operation of TANSO-FTS

To monitor the global column concentration of carbon dioxide (CO2) and methane (CH4) from space, the Greenhouse gases Observing SATellite (GOSAT) was launched on January 23, 2009, and has started the operational observation. Thermal and Near Infrared Sensor for Carbon Observation- Fourier Transform Spectrometer (TANSO-FTS) has been continuously measuring CO2 and CH4 distributions globally every three days, and data distribution to the public started from Feb. 16, 2010. During two years operational periods, the radiometric, geometric and spectroscopic characterizations of TANSO have been continuously conducted with updating the Level-1 processing algorithm. To make a precise spectroscopic observation, correction algorithms were newly developed, demonstrated and installed on operational processing. Two major corrections are discussed. One is correction of the scan-speed instability caused by microvibration from satellite. Through the on-orbit data analysis, degrading spectroscopic accuracy caused by periodically micro-vibrations was found, and these distortion effects were compensated with applying the re-sampling technique for interferogram. The other is non-linearity correction in the electronics. In this presentation, the detail of on-orbit characteristics and the current status of Level-1procesing for TANSO will be presented.

On-orbit performance and level 1 data processing of TANSO-FTS and CAI on GOSAT

The Greenhouse gases Observing SATellite (GOSAT) monitors carbon dioxide (CO2) and methane (CH4) globally from space. It is a joint project of Japan Aerospace Exploration Agency (JAXA), Ministry of the Environment (MOE) and National Institute for Environmental Studies (NIES). GOSAT is placed in a sun-synchronous orbit of 666km and 12:48 local time, with an inclination angle of 98 deg. It was launched on January 23, 2009 from Tanegashima Space Center. There are two instruments on GOSAT. The Thermal And Near infrared Sensor for carbon Observation Fourier- Transform Spectrometer (TANSO-FTS) detects the Short wave infrared (SWIR) reflected on the earths surface as well as the thermal infrared (TIR) radiated from the ground and the atmosphere. TANSO-FTS is capable of detecting wide spectral coverage; three narrow bands (0.76, 1.6, and 2 μm) and a wide band (5.5-14.3 μm) with 0.27 cm-1 spectral resolution. The TANSO Cloud and Aerosol Imager (TANSO-CAI) is a radiometer of ultraviolet (UV), visible, and SWIR to correct cloud and aerosol interference. For three months after the launch, the on-orbit function and performance have been checked out. Now level 1A (raw interferogram) and level 2B (spectra) are now being processed and provided regularly with calibration data.

Retrieval of minor constituents from thermal infrared spectra observed by GOSAT TANSO-FTS sensor

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.

GOSAT level 1 processing and in-orbit calibration plan

Greenhouse gases Observing SATellite (GOSAT) is a Japanese mission to observe greenhouse gases, such as CO2 and CH4, from space. The GOSAT carries a Fourier transform spectrometer and a push broom imager. The development of GOSAT satellite and sensors has almost finished after the characterization of sensor performance in laboratory. In orbit, the observation data will be evaluated by onboard calibration data and implemented by ground processing system. Level 1 algorithm and processing system are developed by JAXA. The post-launch calibration items are planned and the methods are developed before launching. We show the Level 1 processing and in-orbit calibration of GOSAT sensors.

On orbit status of TANSO-FTS on GOSAT

To map the global column dry mole fractions of carbon dioxide (CO2) and methane (CH4), the Green house gases Observing SATellite (GOSAT) was launched on January 23, 2009. The Thermal And Near infrared Sensor for carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) and Cloud and Aerosol Imager (TANSO-CAI) are onboard on GOSAT to derive the precise amount of CO2 and CH4 in atmosphere measuring the solar light intensity reflected and scattered on the earths surface and the thermal radiation. The first high spectral resolution Short Wave Infrared (SWIR) spectra by TANSO-FTS and the image by TANSO-CAI were acquired on February 7, 2009. TANSO has been continuously measuring CO2 and CH4 distributions in global every three days periods, and data distribution for public users was started from February 16, 2010. After the launch, the on-orbit characterization of performance, calibration, and health monitoring of TANSO has been continuously conducted with updating the Level-1 and -2 processing algorithm. During the over one-year operation period, a few anomalies such as instability of pointing mechanism, varying offset of pointing position, small wave-number shift and Zero Path Difference position change, were observed. The radiometric responses for FTS and CAI are also slightly changing. To minimize these effects in data using, quality flags were additionally included in product, response functions are updated and the regular operation procedure was slightly changed. In this presentation, the detail of on-orbit status of TANSO will be reported.

Radiometric calibration accuracy of GOSAT-TANSO-FTS (TIR) relating to CO2 retrieval error

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.