Earth Observation Sensor, SMILES, using state-of-the-art superconducting technology

It had been technically difficult to develop an instrument for the Earth remote sensing in submillimeter wave region where both radio and optical technologies were under developing. NICT started development of millimeter and submillimeter-wave superconducting receivers for the Earth's atmospheric observation on the frontiers of this frequency region in early 1990s. NICT demonstrated the technical usefulness in the atmospheric science of these technologies, for example, NICT succeeded in the development of the balloon-borne SMILES. SMILES was originally proposed mainly by NICT in collaboration with JAXA, and adopted as one of Research Themes for Kibo First Phase Utilization by JAXA in 1997. SMILES development was executed by NICT and JAXA, and also supported by National Astronomical Observatory of Japan, Osaka Prefecture University, and Toho University. SMILES is the fruits of the remote sensing technology on submillimeter-wave atmospheric science fostered in NICT.

SMILES will be launched aboard H-IITransfer Vehicle (HTV) Demonstration Flight via H-IIB Launch Vehicle Test Flight at 17:04 on 10 September 2009 (UT) at Tanegashima Space Center of JAXA. SMILES will do Earth environment observation for the first time at Kibo Japanese Experiment Module. By fully utilizing the infrastructure of Kibo's Exposed Facility, SMILES can realize superconducting submillimeter-wave technology, which had been difficult in conventional small satellites. SMILES will dedicate its great high sensitivity to precise Earth's atmospheric observation.

NICT has a facility of Level 3 processing which will produce global maps of the atmospheric constituents observed by SMILES and will distribute such Earth observation data. More detail, see http://smiles.nict.go.jp.

SMILES:
Superconducting Submillimeter-wave Limb-Emission Sounder (SMILES), which developed in a joint project between National Institute of Information and Communications Technology (NICT) and Japan Aerospace Exploration Agency (JAXA), is an Earth observing instrument to be aboard the Exposed Facility of Kibo Japanese Experiment Module in the international space station. SMILES has a superconducting receiver whose ability of ozone layer observation is a few or ten times higher than conventional sensors of existing satellite. (Fig.1)

Kibo Japanese Experiment Module:
Kibo Japanese Experiment Module consists of Pressurized Module and Exposed Facility. Three experiments are scheduled to start operation in the Exposed Faility in 2009. SMILES is one of the three. The other two, Monitor of All-sky X-ray Image (MAXI) and Space Environment Data Acquisition equipment (SEDA-AP), were launched with the Exposed Facilty via US space shuttle in July 2009. (Fig. 2)

Ozone layer:
Atmospheric layer between about 10 and 50 km is often called as ozone layer because of the existence of ozone which protects life on the ground from harmful ultraviolet in sunlight. Antarctic ozone hole, that was found in 1980s, is known as the result of increased strarospheric chlorine originated from anthropogenic chlorofluorocarbon. It is said that depressed ozone layer are recovering after international regulation of chlorofluorocarbon release. The future prediction of time scale of the recovering, however, has large uncertainties. Satellite observation of ozone layer is important to know the complicated chains of chemical reactions relating various kind of atmospheric molecules such as ozone and chlorine monoxide. SMILES will precisely observe more than 10 kinds of chemicals including molecules that plays an important role in ozone-related reactions, and also molecules that were only measured in very few experiments in past.

Submillimeter wave:
Submillimeter wave is radiowave in a frequency range between 300 and 3,000 GHz. It is also called Terahertz wave. (Terahertz wave is usually defined as electromagenetic wave in a frequency range between 100 and 10,000 GHz.) Most of atmospheric molecules have absorption lines at their inherent frequencies in submillimeter wave range as well as in infrared, visible, and ultraviolet. Atmospheric remote sensing using lines in near-infrared, visible, or ultraviolet usually observes absorption or scattering of sunlight, while thermal emission can be observed in submillimeter wave region so that submillimeter wave observation is available in day and night. SMILES observes submillimeter wave in 624 to 650 GHz range.

Superconducting receiver:
Superconducting receiver is the most important system in the highly sensitive SMILES. SIS mixer is a key device of the superconducting receiver. SMILES SIS mixer was developed by JAXA in collaboration with National Astronomical Observatory of Japan. The development of this device is based on the same technology used in ALMA astronomical radio telescope constructed in Chile. Although SIS mixer must be cooled down to around 4 K (lower than -268 degrees centigrade) for operation, the sensitivity is extremely high comparing with conventional semiconductor mixer. SMILES has a cryo-refrigerator to cool the SIS mixer down to around 4 K .

Balloon-borne SMILES:
NICT also developed submillimeter superconducting receivers that has comparable performance with one used in SMILES. With NICT own superconducting receiver, NICT developed Balloon-borne Superconducting Submillimeter-wave Limb-Emission Sounder, BSMILES, which was flown by balloon as observation campaigns in high altitude of more than 30 km above sea level in 2003, 2004, and 2006. In those campaigns BSMILES successfully observed stratospheric ozone, chlorine monoxide, HCl, and HO2.

H-II Transfer Vehicle, HTV:
H-II Transfer Vehicle, HTV, is Japanese unmanned cargo transporter that delivers various supplies to the international space station. HTV Demonstration Flight will be launched in 11 September 2009 (JST). SMILES is one of the largest scientific instruments carried by HTV Demonstration Flight.

Level 3 processing:
SMILES data is processed at Level 1, Level 2, and Level 3 successively. Level 1 processing is calibration of submillimeter-wave spectra which are directly obtained with SMILES instrument. Level 2 processing is retrieval of chemical constituents concentrations in the atmosphere and other atmospheric physical parameters from submillimeter-wave spectra. Level 3 processing is further processing and produces global map of chemical constituents, time variation of them, and so on. With Level 3 processing we can visually know the present state of our atmosphere and diagnose the remarkable change of atmospheric environment.

Cryo-refrigerator:
Because superconducting devices such as SIS mixer can work only at extremely cooled condition, liquid helium bath or mechanical refrigerator is indispensable. Conventional astronomical or Earth-observing satellites carried huge amount of liquid helium to cool down the devices in space. Those satellite, however, had large limitation on their lifetime because after exhaustion of the liquid helium the devices could not work any more or at least their sensitivity was very degraded. SMILES utilizes mechanical cryo-refrigerator that consists of helium compressors, thermal exchanger, and so on. Mechanical cryo-refrigerator has an ability of eternal cooling until mechanical failure so long as electricity is supplied. In past there were satellites that have mechanical refrigerators to cool devices down to around 20 K, but mechanical crefrigerator cooling down to 4 K level in space is the first attempt in SMILES.