n Resources Observation System Organization), this is a nadir-looking Fourier transform infrared spectrometer (FTIR). Objectives: Observation of detailed spectra of IR radiation of the Earth's surface and the atmosphere, mapping greenhouse gases on a global scale (CO2, CH4, N2O, H2O and the total ozone mixing ratios of the troposphere).
MITI funded the instrument development. It selected a company developing the sensor, TOSHIBA. JAROS had supervised the sensor development. Subsequently, MITI selected the Central Research Institute of Electric Power Industry (CRIEPI) as a consignor for the IMG software system development. 38) 39)
Continuous measurements of the infrared spectrum in the range from 3.3 - 14.0 µm with a very fine spectral resolution of 0.1 cm-1 (apodized) absolute accuracy of measurement=<1 K, stability of measurement<0.1K, IFOV=10 mrad (~64 km2 footprint); vertical resolution about 2-6 km depending on species; interferogram scan time ≤ 10 s. Operation requirements: full time operation for 3 days out of 13 days. A mechanical cryogenic coolant system is used to regulate the temperature of the quantum detectors. An image motion compensation mirror is used to compensate for the satellite orbital motion. Measurements are made in 20 km swaths at 8 km resolution.
Table 6: Some characteristics of the IMG instrument
Figure 20: The observation concept of IMG (image credit: NASDA)
IMG is a Michelson-type Fourier Transform Spectrometer (FTS) with two mirrors and a beam splitter. The incident radiation is divided by the beam splitter into two paths. One mirror is moved so that the two paths produce an interference pattern when recombined. The signal measured by the detector, the interferogram, can be Fourier-transformed to obtain the incident spectrum. The diameter of the entrance aperture is 10 cm. The moving mirror is suspended on magnetic bearings and scans a 10 cm long path in 10 seconds. 40) 41)
Figure 21 shows a schematic of the IMG optical system. The heart of the IMG is the magnetically suspended moving mirror system. The design team studied a bearing system for the linearly moving mirror which had an expected lifetime of three years, and selected to develop a magnetic suspension/drive system in order to avoid problems caused by lubrication of the mechanism in vacuum.
Since the IMG adopted a pair of moving and fixed flat mirrors for the interferometer, the magnetic suspension system not only had to suspend the 10 cm diameter mirror attached to the armature, but also enable alignment control in the x- and y-directions of the armature moving in the z-direction to maintain accurate parallelism. The magnetic circuit of the moving mirror armature was composed of an inner yoke, a drive coil fixed to the optics bench, inner permanent magnets at the end of the armature, and the outer armature itself. Eight other suspension electromagnets with eight eddy current type detection gap sensors were used to suspend and control the armature alignment.
Figure 21: Schematic view of IMG optical system (image credit: CRIEPI, JAROS)
To satisfy the wavelength accuracy requirement of less than 3 x 10-7 in the ratio at 25 ±5ºC for three years, the newly designed HeNe laser made of a glass block with an ultralow thermal expansion coefficient (less than ± 5 x 10-8/K) was used. The laser beam was expanded and injected into the interferometer, and light interference was detected with four Si detectors positioned circumferentially in order to detect the position and the inclination of the moving mirror.
The IMG used three bands with two PV-type InSb detectors for the shorter wavelength range and one PC-type HgCdTe (MCT) detector for the longer wavelength range to cover the required wavelengths between 3.3 µm and 14 µm. The MCT detector is well known to have a nonlinear relation between the input power and the output signal. The ground test showed that nonlinearity arose when the sensor target temperature was higher than 280 K. This effect was compensated for by a quadratic equation at the processing stage of the interferograms in the ground 