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.