What is IMG ?
(Interferometric Monitor for Greenhouse gases)

Objectives of IMG

Unfortunately, our knowledge of most climatic parameters is limited so good climate models exist only for very limited areas of the earth. For example, atmospheric temperature data, a good index for measuring global warming, are very limited over the ocean. The distribution and sources of greenhouse gases are another major unknowns.

Satellite-derived data are essential to obtain global knowledges on these parameters. This requirement led to the development of the Interferometric Monitor for Greenhouse gases (IMG) by the Japan Resources Observation System Organization (JAROS) on consignment from the Ministry of Economy, Trade and Industry (METI). IMG is loaded on the Advanced Earth Observing Satellite (ADEOS) launched by the National Space Development Agency of Japan (NASDA) on Aug. 17, 1996.

Expected Results :

1. High accuracy measurement of the earth's radiation budget
2. High accuracy surface temperature and atmospheric temperature profiles
3. Measurement of atmospheric constituents:
   1. Density profiles of CO₂ and H₂O
   2. Total ozone
   3. Mixing ratios of CH₄, N₂O and CO in the troposphere

IMG Sensor System

IMG will obtain detailed spectra of thermal infrared radiation from the earth's surface and atmosphere. These spectra will be useful in evaluating the radiation budget of the earth, the temperature profile of the atmosphere, the temperature of the earth's surface, and physical properties of clouds. Thermal infrared spectra also include absorption and emission signatures of many atmospheric gases. IMG's detailed spectra will be used to infer atmospheric concentrations of water vapor and other greenhouse gases.
A global increase has been noted in tropospheric concentrations of trace gases, such as carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFCs). This increase has been caused by human activities, but we have limited knowledge of the magnitude or distribution of anthropogenic sources for these gases. Two sources whose strength must be investigated are deforestation and biomass burning. IMG will map the global and regional distributions of emission sources by measuring variations in the concentrations of trace gases.
Moreover, natural source and sink strength of trace gases may vary largely with different terrestrial and oceanic ecosystems.

IMG Main Characteristics :

Interferometer :

IMG is a Michelson-type Fourier Transform Spectrometer (FTS) with two mirrors and a beam splitter. The incident radiation received from the earth is divided by the beam splitter into two paths. One mirror is moved so that the two paths produce an interference pattern when they are recombined. The signal measured by the detector, the interferogram, can be Inverse Fourier transformed to obtain the incident spectrum. The diameter of the entrance aperture for the optics is 10 cm. The scanning mirror is suspended on magnetic bearings and scans a 10 cm long path in 10 seconds.

FOV and IMC :

The field-of-view (FOV) of each detector in IMG is 0.6 deg square. The corresponding footprint size is approximately 8 km square.
The satellite goes forward about 86 km while the scanning mirror scans and retraces its steps. The Image Motion Compensator (IMC) controls the IMC mirror so that the FOV footprint on the earth remains constant throughout each 10 sec scan. This reduces target smear caused by spacecraft motion and the rotation of the earth. The IMC mirror can also be rotated +-90 degrees from nadir to view either the on-board blackbody or deep space for calibration periodically.

Detector and Sampling Reference :

IMG has three spectral bands: band 1 (3.3 - 4.3 um), band 2 (4.0 - 5.0 um) and band 3 (5.0 - 14.0 um). InSb photovoltaic detectors are used for band 1 and band 2; a MCT (HgCdTe) photoconductive detector is used for band 3. The three detectors are mounted in a single focal plane and cooled to 80 K by a split Stirling-cycle cooler with cooling capacity of about 1 W. A He-Ne laser at 0.633 um is used to determine sampling points for digitization. The wavelength stability of the He-Ne laser will be better than 3X10^-7.

Alignment and Calibration :

An interferometer is very sensitive to optical alignment so IMG has an on-board alignment mechanism which adjusts the magnetically-suspended moving mirror to maintain alignment. Deep space and an on-board blackbody serve as the cold and warm calibration sources for IMG. Calibration data are collected every 6 measurement cycles.