system. The coefficients of the quadratic equation were defined from the results of ground tests.
The cold components, detector focal plane and the shield attached for eliminating the background radiation from the warm optical system, were placed in the vacuum vessel, and were thermally combined with the cryogenic cooler displacer. The field of view (FOV) was defined by the size of the detectors as 8 km x 8 km at the ground. Figure 22 shows the outline of the detector component.
Figure 22: Outline of the detector component (image credit: CRIEPI, JAROS)
Figure 23 shows the moving mirror alignment and suspension control system diagram. The control system maintained the parallelism of the two mirrors to within 1.9 x 10-4 degrees (0 = peak) throughout the 10 cm length of the moving mirror path. An additional 3 seconds were required for the mirror to return. The alignment control system had double control loops. The basic control loop was controlled by the gap sensors to sustain the initial alignment bias obtained during the startup alignment modes. The second control loop supports the dynamic alignment mechanism that maintains the defined parallelism throughout each observation. The control system was also designed to accept the predefined alignment bias which could be modified by the ground command. This alignment bias defined the offset angle of the moving mirror.
Figure 23: Moving mirror alignment and control system diagram (image credit: CRIEPI, JAROS)
Figure 24: Overview of the IMG FTS optical system (image credit: JAXA)
Figure 25: The operation pattern of the observation/calibration cycle of the IMG in the observation (image credit: CRIEPI, JAROS)
Figure 26: Illustration of one observation cycle and corresponding footprint (image credit: CRIEPI, JAROS)
Operational experience of IMG: The IMG operated continuously day and night, but, was under the basic schedule of 4 days operation and 10 days standby. Before the start of the System Initial Checkout, the launch locks for movable parts of the IMG, such as the traveling 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), 