Hirohisa Kurosaki

Development of tunable imaging spectro-polarimeter for remote sensing

Abstract The imaging spectrometer with a tunable filter, which can select the observing wavelength arbitrarily, is useful for the remote sensing of the earth. The LCTF (Liquid Crystal Tunable Filter) has a capability of selecting the observing wavelength instantaneously with high spectral resolution. Taking advantage of this filter we have developed an LCTF spectro-polarimetryr system with a polarimetry function, which can be realized by rotating the LCTF itself. Thus this LCTF spectro-polarimetryr system can acquire valuable image information of the earths environments by continuously changing both wavelength and polarization angle in response to the observation requirements. The details of the configuration and performance of the LCTF spectro-polarimeter are presented in this paper.

Application of an imaging spectropolarimeter to agro-environmental sciences

Hyperspectral analysis of solar rays reflected from the Earth’s surface is expected to play an important role in future Earth observation. Two imaging liquid crystal tunable filter (LCTF) spectropolarimeters for the visible and near-infrared wavelength bands have been developed by NAL over the past several years for such analysis. In order to realize the practical application of these optical sensors, efforts are currently under way to develop them into sensor packages for airborne observation systems. This paper first presents the concept and architecture of an optical observation system using an LCTF spectropolarimeter which is sensitive to radiation in the 650-1100 nm near-infrared wavelength band, along with its construction. The results of a farm observation conducted using a visible wavelength LCTF imaging spectropolarimeter are then presented by the spectral images of the observed areas as an example of a preliminary application to agro-environmental sciences. The results of a second farm observation conducted using a near-infrared LCTF imaging spectropolarimeter are presented by spectral images of an observed crop specimen, and radiances of solar rays reflected from the specimen are also shown. Finally, the applicability of the LCTF spectropolarimeter to agriculture observation is summarized based on the results of these agricultural observations.

Earth observation system incorporating an LCTF spectropolarimeter

There is an emerging demand for remote sensing technologies that can determine the surface characteristics of objects from the properties of reflected light. In particular, hyperspectral analysis of solar rays reflected from the Earths surface is expected to play an increasingly important role in Earth environment observation. The National Aerospace Laboratory (NAL) has developed a new type of imaging spectropolarimeter for such analysis that uses a liquid crystal tunable filter (LCTF), and efforts are now under way to develop it into a practical aircraft or spacecraft on-board sensor system for Earth environment sensing. This paper first presents the concept and architecture of an Earth observation system using an LCTF optical sensor which can sense radiation in the 400-720 nm wavelength band. The results of laboratory experiments to evaluate the performance characteristics of the observation system, e.g. hyperspectral resolution, optional selection of the plane of polarization, etc. are then presented, and the results of preliminary image acquisition experiments that demonstrate the feasibility of acquiring of spectral images is also shown. Finally, the applicability of the LCTF spectropolarimeter to Earth observation is summarized based on the results of the laboratory and field evaluation experiments.

Optical Tracking and Spectroscopic Measurement of Hayabusa Capsule Reentry Fireball

The asteroid explorer HAYABUSA finally returned to the earth on June 13rd 2010 and the sample return capsule experienced a super-orbital atmospheric reentry. To recover the sample return capsule and to conduct optical measurements, the Japan Aerospace Exploration Agency organized a ground observation team and conducted optical tracking of the sample return capsule, spectroscopy of the fireball as well as the fireball trail, and measurement of infrasounds and shock waves generated by the fireball. In this article, an overview of the ground observation is presented, and the preliminary results derived from observations are reported.

Field observation of surface conditions using LCTF spectropolarimeter

Polarimetric analysis of solar rays reflected from the Earths surface is expected to play an important role in future Earth environment observation. Research on an imaging spectropolarimeter using a liquid crystal tunable filter (LCTF), which is able to measure the polarimetric properties at selected wavelengths of solar rays reflected from land or water surfaces, has been conducted over the past five years at NAL for such analysis. Efforts are now under way to put this sensor to practical use, for airborne and ultimately space-based Earth environment remote sensing. This paper first presents the principle and construction of an LCTF spectropolarimeter which senses radiation in the 400-720 nm wavelength band. Next, an outline of an onboard observation system that incorporates an LCTF spectropolarimeter and its performance characteristics obtained by laboratory tests are presented. Third, the apparatus and procedures for the field experiment using such observation system are described, and the area for the field experiments is shown. Spectral characteristics of solar rays reflected from the observed spots are then shown by relative radiance as the analyzed results of experimental data and spectral images at various wavelengths and polarization angles are also shown as further analyzed results. It is made clear from the experimental results that solar rays reflected from targets with differing characteristics have different spectropolarimetric properties. Moreover, the result of the flight experiment conducted preliminarily to confirm the operational functions of the observation system in a flight environment is shown. Finally it is concluded that the way has been paved for determining the surface conditions from the properties of the images acquired by the LCTF spectropolarimeter.

Experimental checking of the spectropolarimeter for airborne remote sensing

The National Aerospace Laboratory of Japan has developed a new type of optical sensor, an imaging spectropolarimeter which uses a liquid crystal tunable filter, for airborne and satellite-based remote sensing of the Earths environment. Ground-based field experiments conducted as a preliminary to flight evaluations have demonstrated the feasibility of acquiring spectral images of objects irradiated by solar rays, and confirmed that solar rays reflected from different targets have characteristic spectropolarimetric properties. This paper first presents an outline of the developed spectropolarimeter. Next, the apparatus and procedures for the field experiments are described. The spectropolarimetric characteristics of solar rays reflected from a range of targets are then shown by relative radiance as the results of analyzed experimental data, and spectral images acquired at various wavelengths and polarization angles are shown. Plans to evaluate the sensor in a flight environment are described. Possible applications of the optical sensor are also introduced: observation of water quality deterioration in brackish lakes, applications to agro-environmental science, and applicability to a fish-finding system. Finally, it is concluded that results of the field experiments demonstrate that the way has been paved for determining surface characteristics from the optical sensor output.

Development of an imaging hyperspectral camera using the ultraviolet and visible wavelength AOTF

A spectroscopic camera has been developed which has spectral resolution of less than 1.5nm in the ultraviolet (UV) and visible wavelength bands (320-580 nm). Its main components are a specially coated UV objective lens, a UV Acousto-Optic Tunable Filter (AOTF) with a thermo-electric cooling system, and a imaging system based on a high-gain avalanche rushing amorphous photoconductor (HARP) developed by NHK Science and Technical Research Laboratories. Research is currently under way to develop the hyperspectral camera into a sensor package for airborne and ultimately space-based remote sensing applications. This paper presents the basic principle and configuration of the hyperspectral camera, and gives details of tests to measure its performance. The results of spectral resolution tests analyzing very close two spectra from a helium-discharge lamp demonstrate the cameras high spectral resolution performance. Full color and spectral images obtained by a spectrometry experiment are also presented to demonstrate the cameras hyperspectral capabilities.

Development of an onboard spectro-polarimeter for Earth observation at NAL

Various methods, techniques and sensors for Earth observation are being developed worldwide as the necessity of protecting the Earths environment increases. In particular, polarimetric analysis of solar rays reflected from the Earths surface is expected to play an important role in future Earth environment observation. A new type of spectro-polarimeter based on a liquid crystal tunable filter (LCTF) has been developed at NAL for such analysis. Efforts are now under way to put this sensor to practical use in airborne or satellite-based remote sensing of the Earths environment by developing a sensor package and onboard observation system based around it. This paper first presents the operational principle and construction of the LCTF spectro-polarimeter, which captures images in the 400 - 720 nm wavelength band. Next, an outline of an onboard observation system incorporating the spectro-polarimeter is described and its applicability to airborne remote sensing discussed. The performance of the observation system is then shown based on experimental results. Other possible applications of the sensor are presented, and finally, the results of the evaluation of the observation system, e.g. hyper-spectral resolution of less than 10 nm, are summarized in the conclusion.

Detection of LEO Objects Using CMOS Sensor

We succeeded in detecting 10 cm LEO objects at 1000 km altitude with a CMOS sensor installed to the 18cm telescope by using fast frame rate of the CMOS sensor and FPGA-based image-processing technique. The LEO survey system using numerous sets of the CMOS sensor will contribute the monitoring LEO environments which is currently done with the space surveillance network of the United States and solving the space debris problem in the future.

Research and Development on Space Debris Observation Technologies in Japan Aerospace Exploration Agency

JAXA is developing optical observation technologies for space debris. For GEO debris, the stacking method, which uses numerous CCD frames to detect objects under background noise level, is developed and shown to work well, so far. However, the method has the disadvantage that is time-consuming to detect objects whose movements are unpredictable. In order to overcome this, a new algorithm which uses binarization of CCD images and calculates sum values instead of median, is developed. Moreover, the algorithm is applied to the FPGA board system. These reduce analysis time to about one thousandth which enables us to analyze one night data till next night observation. For LEO debris, a ground-based optical observation system for monitoring LEO objects ,which uses a lot of optical sensors to cover the vast region of the sky, is being proposed. Detection abilities and orbital determination possibilities of the system were examined. About 30cm LEO objects at 1000km altitude are detectable using a 18cm telescope, a CCD camera and the developed analysis software. Simulations and a test observation showed that two longitudinally separate observation sites with arrays of optical sensors can identify same objects from a lot of data set and determine their orbits precisely. The proposed system may complement or replace the current radar observation system for monitoring LEO objects like space situation awareness in the near future.