eling mirror, were released on September 5, 1996. The Initial Checkout on the IMG unit was performed between September 20 and October 10, 1996. After the functional tests, the first observation was performed on October 10, 1996.
Routine IMG operations began on November 26, 1996. At the beginning of the operation, the moving mirror alignment system occasionally stuck at an anomalous alignment position, resulting in unacceptable degradation of the interferogram. After troubleshooting, it was found that when the moving mirror vibrated, the dynamic alignment system slipped. It was determined that such vibration was induced by the control system’s insufficient gain which was defined in the ground test in which zero gravity was simulated.
The slippage of the dynamic alignment system, which introduced the degradation of data performance (reproducibility, NESR), occurred randomly. Occasionally, the slippage was followed by sticking and caused fatal alignment deviation. It was anticipated that such anomalies could be reduced by adjusting the moving mirror alignment control bias, because the control bias was related to the slippage margin. Fortunately, this control bias could be preset through commands given from the ground. Trials to identify the optimum bias continued during the routine operations until the end of June 1997, when satellite failure occurred.
Using the IMG, the project was able to measure the Earth’s radiation budget, the surface temperature and atmospheric temperature profiles, and the atmospheric constituents. The operation of the IMG established the validity of the high-resolution, high-throughput interferometer in space. Global maps of the atmospheric constituents were obtained, and the results were very impressive. The atmospheric thermal radiation spectra obtained by the IMG show a good correspondence with the anticipated spectra calculated using the atmospheric optical model (Ref. 38).
ILAS (Improved Limb Atmospheric Spectrometer):
ILAS is a sensor developed by the Environment Agency of Japan (EA), heritage of LAS on Ohzora (EXOS-C). Objectives: Measurement of atmospheric trace gases and the pressure and temperature profiles by the solar occultation technique (vertical profiles of O3, NO2, aerosols, H2O, CFC11, CH4, N2O). ILAS has two grating spectrometers, one is used for the IR-band trace gases, the other measures aerosols, air temperature and pressure in the VIS-band. The ILAS occultation measurements are performed only at high latitudes in both hemispheres (due to orbit characteristics). ILAS keeps looking at the sun during sunrise and sunset, it measures the sequence of spectral intensity changes in the solar light which pass through the various tangent heights of the atmosphere. Since atmospheric trace gases have their own characteristic absorption spectra, the concentration of gases can be derived from these absorption measurements. 42), 43), 44) 45) 46) 47)
Table 7: ILAS instrument parameters
Figure 27: Illustration of the ILAS observation concept (image credit: NASDA)
Figure 28: Schematic overview of the ILAS instrument (image credit: NASDA)
RIS (Retroreflector in Space):
RIS is a JEA (Japan Environmental Agency) funded sensor. RIS is a large aperture single element corner-cube reflector for Earth-satellite-Earth laser long-path absorption measurements of atmospheric trace gases. The RIS experiment was the first implementation of the laser long-path absorption method between the ground and a satellite. 48) 49)
Objective: Used for laser long-path absorption measurements of atmospheric trace species and to support two color laser ranging experiments. Measurements of ozone (O3), CFC12, HNO3, methane (CH4), CO, and other gases by the laser beam absorption technique. Wavelength range: 0.4 - 14 µm.
In the RIS experiments a laser beam is transmitted from a SLR ground station, reflected by RIS, and received back at the ground station. The absorption of the intervening atmosphere is measured in the round-trip optical path (measurement of absorption spectra by the Doppler shift of the reflected beam). The co