Toshiyuki Nishibori

Submillimeter Limb-emission Sounder JEM/SMILES aboard the Space Station

A submillimeter limb-emission sounder, that is to be aboard the Japanese Experiment Module (JEM, dubbed as KIBO) at the International Space Station, has been designed. This payload, Superconducting Submillimeter-wave Limb-emission Sounder (SMILES), is aimed at global mappings of stratospheric trace gases by means of the most sensitive submillimeter receiver ever operated in space. Such sensitivity is ascribed to a Superconductor-Insulator- Superconductor (SIS) mixer, which is operated at 4.5 K in a dedicated cryostat combined with a mechanical cooler. SMILES will observe ozone-depletion-related molecules such as ClO, Hcl, HO2, HNO3, BrO and O3 in the frequency bands at 624.32-626.32 GHz and 649.12-650.32 GHz. A scanning antenna will cover tangent altitudes from 10 to 60 km in every 53 seconds, while tracing the latitudes form 38 S to 65 N along its orbit. This global coverage makes SMILES a useful tool of observing the low- and mid- latitudinal areas as well as the Arctic peripheral region. The molecular emissions will be detected by two units of acousto-optic spectrometers (AOS), each of which has coverage of 1.2 GHz with a resolution of 1.8 MHz. This high-resolution spectroscopy will allow us to detect weak emission lines attributing to less-abundant species.

Measurement of the Offset-Cassegrain Antenna of JEM/SMILES Using a Near-Field Phase-Retrieval Method in the 640-GHz Band

This communication describes the results of the measurements made for the flight model of the offset Cassegrain antenna of superconducting submillimeter-wave limb-emission sounder (SMILES) aboard the International Space Station. We have employed a near-field phase retrieval method in which the aperture phase distribution is estimated only from the amplitude distribution measurements over two near-field planes. The far-field patterns estimated from the estimated near-field patterns were compared with theoretical calculations based on physical optics in which the surface errors measured for the main and sub reflectors were taken into account. As a result of the comparison, the far-field patterns estimated from the phase retrieval method were found to be in very good agreement with the physical-optics calculations to the sidelobe levels as low as -55 dB. We have also found that patterns of machined flaws on the surface of the main reflector were clearly identified in the retrieved near-field phase pattern. This demonstrates that the phase retrieval is an effective method to evaluate aperture antennas in the submillimeter-wave region, where accurate phase measurement is rather difficult.

Receiver Performance of the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station

Superconducting devices were used to make atmospheric limb observations from space for the first time. The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) is a superconductor-insulator-superconductor (SIS) receiver in frequency bands of 625 and 650 GHz. SMILES was deployed on the International Space Station. SMILES observed atmospheric limb spectra for six months from October 2009 to April 2010. The sensitivity of the receiver is the most important performance parameter for microwave atmospheric limb observation, in which the receiver measures, sometimes very weak, thermal line emissions. The SMILES SIS receivers demonstrated limb observations with a sensitivity more than one order of magnitude better than that of conventional limb sensors. The sensitivity of the SMILES receivers in space was 315 K-322 K in a definition of single-sideband system noise temperature at the antenna; this met the instrument requirement with a large margin. The SMILES-receiver stability also met the requirement; the stability time of the receiver was 8 s (and 500 s for spectroscopic data) in a frequency resolution of about 1.1 MHz. Although the stability time is shorter than the calibration period (53 s) in operational observation, the variance increment by the drift noise is found to be insignificant. The temperature resolution for the continuum signal is estimated to be better than 0.27 K. There was no evidence that the stability of the SIS receiver was influenced by the temperature fluctuation of the 4-K cooling system, which consists of a two-stage Stirling cooler and a Joule-Thomson cycle cooler. The suppression of baseline ripples is another important performance parameter of the receiver for spectral measurement like the SMILES receivers. As a result of our design of low-standing-wave optics, we found no baseline ripple in the observed spectra of SMILES in practical level.

In-Orbit Measurement of the AOS (Acousto-Optical Spectrometer) Response Using Frequency Comb Signals

The in-orbit response characteristic of the AOS (Acousto-Optical Spectrometer) was estimated from spectral profiles taken by using a frequency comb generator, which was originally used for frequency axis calibration. Frequency comb signals taken under various environmental conditions in orbit were consolidated to deduce the response profile of the spectrometer. The fluctuation bandwidth calculated from the deduced spectral response compared well with that derived from the noise characteristic of the spectrometer.

Measurement and Evaluation of Submillimeter-Wave Antenna Quasioptical Feed System by a Phase-Retrieval Method in the 640-GHz Band

A phase-retrieval method is applied to the quasioptical feed system of the offset Cassegrain antenna of the Superconducting Submillimeter-Wave Limb-Emission Sounder (JEM/SMILES) to be aboard the International Space Station for evaluating the beam alignment by estimating the phase pattern from the beam amplitude pattern measurements. As the result, the application of the phase retrieval method is demonstrated to be effective for measuring and evaluating the quasioptical antenna feed system. It is also demonstrated that the far-field radiation pattern of the antenna main reflector can be estimated from the phase-retrieved beam pattern of the feed system.

Gain Nonlinearity Calibration of Submillimeter Radiometer for JEM/SMILES

Correcting the gain nonlinearity of an atmospheric sounder is important because nonlinearity causes a systematic error in the measured atmospheric emission. In spectroradiometers consisting of a nonlinear broadband receiver followed by a nonlinear spectrometer, the distortion of the received atmospheric spectrum due to nonlinearity should be corrected using the linearity information both on the broadband receiver and on the spectrometer. It is necessary to measure the input-output relation of two parts, the broadband and narrow-band sections, of the spectroradiometer. We propose a nonlinearity measurement method in which the input-output relations of two sections can be measured using broadband noises as input. The method is advantageous for satellite-borne sounders, which do not always measure each section separately. We successfully applied the method to the radiometer for the Japanese Experiment Module/Superconducting Submillimeter-Wave Limb-Emission Sounder.

LiteBIRD: mission overview and design tradeoffs

We present the mission design of LiteBIRD, a next generation satellite for the study of B-mode polarization and inflation from cosmic microwave background radiation (CMB) detection. The science goal of LiteBIRD is to measure the CMB polarization with the sensitivity of δr = 0:001, and this allows testing the major single-field slow-roll inflation models experimentally. The LiteBIRD instrumental design is purely driven to achieve this goal. At the earlier stage of the mission design, several key instrumental specifications, e.g. observing band, optical system, scan strategy, and orbit, need to be defined in order to process the rest of the detailed design. We have gone through the feasibility studies for these items in order to understand the tradeoffs between the requirements from the science goal and the compatibilities with a satellite bus system. We describe the overview of LiteBIRD and discuss the tradeoffs among the choices of scientific instrumental specifications and strategies. The first round of feasibility studies will be completed by the end of year 2014 to be ready for the mission definition review and the target launch date is in early 2020s.

Detection of Intact Lava Tubes at Marius Hills on the Moon by SELENE (Kaguya) Lunar Radar Sounder

Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moons composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.

On orbit performance of radio spectrometers of Superconducting Submillimeter-Wave Limb-Emission Sounder (JEM/SMILES)

On-orbit performance of the radio spectrometer of SMILES is discussed. We focused on the telemetry data of photodiode current, laser diode current, and laser diode operating temperature. The data showed degradation trend as the mission went on. This is due to a wear-out phenomenon of commercially available laser diode, which is used as the light source of the radio spectrometer. Since the laser diodes have a certain lifetime, both screening procedure and operating condition for them must be properly defined and implemented for ensuring a good performance of the spectrometer throughout designed mission life. For these purposes, we conducted several kinds of qualification tests including an accelerated life time test during design phase, and expected life time of the laser diode was derived on the basis of these test results. In this paper, the results from the qualification tests at ground and the actual performance on orbit with the telemetry and mission data will be presented.

Optical designing of LiteBIRD

LiteBIRD aims to detect the footprint of the primordial gravitational wave on the Cosmic Microwave Background (CMB) in a form of polarization pattern called B mode. In order to separate CMB from the Galactic emission, our measurements cover 35 GHz to 450 GHz. The LiteBIRD optics consists of two telescopes: a crossed Dragone type for lower frequencies, which provides a compact configuration with a wide field of view, and a refractor type for higher frequencies. The whole optical system is cooled down to around 5 K to minimize the thermal emission. We use two kinds of approaches of designing calculations as well as the experimental confirmation particularly for the lower frequency telescope.