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Model-free image-based wavefront correction techniques, such as the stochastic parallel gradient descent (SPGD) algorithm, will be useful in achieving diffraction-limited optical performance in near-future optical remote sensing systems. One difficulty facing the image-based method is that the correction performance depends on the evaluation metric and the evaluated scene. We propose several evaluation functions and investigate the relationship between the optimization speed and the scene textures for each metric in the SPGD algorithm. Based on the simulation results, the study experimentally compared wavefront correction performance using four cost functions and two extended aerial images. Consequently, we found that the deviation-based cost function allowed efficient wavefront correction for versatile extended scenes. In addition, observing extended scenes with distinct structures can facilitate correction speed. Furthermore, we numerically validated this approach in a segmented-aperture imaging system for large telescopes. We believe that the presented approach allows us to realize spaceborne remote sensing with unprecedented high angular resolution.

Earth remote sensing from geostationary orbit (GEO) realizes high time resolution that is essential for disaster monitoring; however, the spatial resolution is commonly worse than observation from low Earth orbit (LEO). In order to achieve high-resolution and high-frequency GEO remote sensing, we have proposed a “Formation Flying Synthetic Aperture Telescope (FFSAT)” with multiple micro-satellites. The FFSAT can improve the spatial resolution by using the technique of a synthetic aperture, and therefore the relative positions and attitudes between the optical units of each satellite must be controlled with an accuracy better than 1/10 of the observation wavelength. In order to verify feasibility of such highly accurate control, μ-class control experiments were conducted by using COTS components, and numerical models of the components were constructed. Results of the experiments were integrated into a software simulator, and the μ-class formation flying control of the entire FFSAT system was numerically evaluated. In this simulation, highly accurate control was achieved with dual-stage actuators, which consist of piezo actuators and thrusters. The simulation results show that the formation can be controlled in μ-class accuracy under some assumptions.

Wavefront compensation techniques that do not require wavefront sensors are demanded in the on-orbit telescopes on Earth observation satellites. This is especially true for segmented or sparse aperture telescopes that could realize unprecedented high angular resolution. A promising wavefront sensorless approach is the stochastic parallel gradient descent (SPGD) algorithm. The wavefront correction by the SPGD optimization relies only on the intensity data in the acquired image. However, many previous observation targets are point light sources, not the extended ground scenes generally acquired by Earth observation satellites. This paper derives an efficient wavefront control method for imaging systems for the fast SPGD optimization. Wavefront compensation has been demonstrated by experiment on extended objects in single aperture optics, in which a microelectrome- chanical system deformable mirror controls the wavefront. Subsequent numerical simulations are reported for multi-aperture imaging systems. The paper also discuses a method to reduce the computational cost of SPGD optimization.

Small satellites have been used for remote sensing with a spatial resolution of several meters from LEO in recent years. However, it is difficult to increase temporal resolution for LEO remote sensing due to the short orbital period. Therefore, GEO remote sensing which enables observation of high temporal resolution from GEO or its nearby orbit is getting important. In order to obtain enough spatial resolution in GEO remote sensing, an optical system having a diameter of several meters is required because of the diffraction limit. It takes huge cost to realize such a large diameter primary mirror due to manufacturability and required accuracy. To address this problem, we propose a synthetic aperture telescope by small satellites formation flying. The synthetic aperture telescope is composed of several mirror satellites constituting a primary mirror of the telescope and an imaging satellite having a focal plane assembly. By optically synthesizing the light collected by each mirror satellite with the imaging satellite, a virtual large aperture telescope is constructed. In this paper, we assume the observation at near infrared to short wavelength infrared and show the specifications of the system. The apperture telescope and the image processing method used to extract high spatial frequency information from the observed images are also described.

A demand for responsive, high-resolution Earth observations is emerging for mitigating the human suffering and damage that follow large-scale disasters. One of the most promising advances is a sophisticated optical imager with a large, 3.6-m satellite-mounted telescope in geostationary orbit. The imager of the proposed space telescope has a segmented mirror and offers a ground sampling distance of better than 10 m and a latency of shorter than 30 minutes. For the imager to realize diffraction-limited performance, deformable mirrors are planned to be installed at the exit pupil of the telescope system. One candidate for the deformable mirrors in segmented telescope is based on a micro-electromechanical system (MEMS) that offers a small actuator pitch, fine step resolution, and excellent hysteretic motion response. This paper presents the wavefront correction of aberrations with a high and low spatial frequency using MEMS deformable mirror on an optical testbed. The expected image quality is also evaluated through numerical simulation.

For small satellite remote sensing missions, a large aperture telescope which has more than 400 mm of diameter is required to realize less than 1m GSD observations. However, it is difficult or expensive to realize the large aperture telescope using monolithic primary mirror with high surface accuracy. Generally, not only high accuracy of optical surface but also high accuracy of optical alignment is required for large aperture telescope. For a segmented mirror telescope, aligning optical elements in high accuracy is more difficult and more important. For conventional systems, optical alignment is adjusted before launch to achieve desired imaging performance. However, it is difficult to adjust the alignment for large sized optics in high accuracy. Furthermore, thermal environment in orbit and vibration in launch vehicle cause the misalignments of the optics. We are developing an adaptive optics system using a MEMS deformable mirror (DM) for Earth observing remote sensing sensor. Image based adaptive optics system compensate the misalignments and wavefront aberration of optical elements using DM by feedback of observed images. Because, it is difficult to use a reference point source unless the satellite controls its attitude toward a star for Earth observing systems. Furthermore, total amount of incident light can enter an image sensor. We propose the control algorithm of DM and mirror segments for a segmented mirror telescope by using of observed images. Numerical simulation results represent that misalignment and wavefront aberration of the segmented mirror telescope are corrected and image quality is improved.

For small satellite remote sensing missions, a large aperture telescope more than 400mm is required to realize less than 1m GSD observations. However, it is difficult or expensive to realize the large aperture telescope using a monolithic primary mirror with high surface accuracy. A segmented mirror telescope should be studied especially for small satellite missions. Generally, not only high accuracy of optical surface but also high accuracy of optical alignment is required for large aperture telescopes. For segmented mirror telescopes, the alignment is more difficult and more important. For conventional systems, the optical alignment is adjusted before launch to achieve desired imaging performance. However, it is difficult to adjust the alignment for large sized optics in high accuracy. Furthermore, thermal environment in orbit and vibration in a launch vehicle cause the misalignments of the optics. We are developing an adaptive optics system using a MEMS deformable mirror for an earth observing remote sensing sensor. An image based adaptive optics system compensates the misalignments and wavefront aberrations of optical elements using the deformable mirror by feedback of observed images. We propose the control algorithm of the deformable mirror for a segmented mirror telescope by using of observed image. The numerical simulation results and experimental results show that misalignment and wavefront aberration of the segmented mirror telescope are corrected and image quality is improved.

University of Tokyo and Kobe University conducted a sounding rocket experiment to deploy large “Furoshiki” net in space, which is a promising candidate for the future large antenna or solar power satellites. The experimental system consisting of mother and three daughter satellites as well as a folded net was separated from S-310 sounding rocket of JAXA/ISAS at the altitude of 110km, where the daughter satellites, after separated from mother satellite with 1.2 m/s velocity, deployed a large net of 14m-sized triangle. The deployment was quire successful without any tangling and the dynamics during deployment has been captured by cameras, INS and radar distance measurement system. Retro-directive antenna micro wave transmission experiment using 4 antennae on the bottom of satellites were conducted also successfully.

We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial resolution and high signal to noise ratio has to be realized by a lightweight sensor system due to the launcher’s requirements. Moreover, simple hardware architecture has to be used to achieve high reliability, low cost, and short development period. Image based AOS realize these requirements without wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its attitude toward a star. We propose the multi-mode phase diversity method using deformable mirror.

We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial resolution has to be achieved by a lightweight sensor system due to the launcher’s requirements. Moreover, simple hardware architecture have to be selected to achieve high reliability. Image based AOS realize these requirements without wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its attitude toward a star. We propose the control algorithm of the deformable mirror on the basis of the extended scene instead of the point source.

We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial resolution has to be achieved by a lightweight sensor system to be used for small satellites. Moreover, simple hardware architecture has to be selected to achieve high reliability and low development cost. Image based AOS realize these requirements without wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its attitude toward a star. We propose the control algorithm of the deformable mirror on the basis of two methods; model-based wavefront estimation method and direct optimization of acquired images. We described simulation results of the proposed methods.

We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial resolution has to be achieved by a lightweight sensor system due to the launcher’s requirements. Moreover, simple hardware architecture has to be selected to achieve high reliability. Image based AOS realize these requirements without wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its attitude toward a star or it has a reference point source in itself. We propose the control algorithm of the deformable mirror on the basis of the extended scene instead of the point source. In our AOS, a cost function is defined using acquired images on the basis of the contrast in spatial or Fourier domain. The cost function is optimized varying the input signal of each actuator of the deformable mirror. In our system, the deformable mirror has 140 actuators. We use basis functions to reduce the number of the input parameters to realize real-time control. We constructed the AOS for laboratory test, and proved that the modulated wavefront by DM almost consists with the ideal one by directly measured using a Shack- Hartmann wavefront sensor as a reference.

In the hyperspectral era, the demand for data registration in spectral region is an important issue in addition to spatial region. Detection of smile and keystone phenomena that are caused by aberrations in spectrometer is related to registration activity, which is crucial for data fusion research. Hyperspectral Imager Suite (HISUI) is a next-generation Japanese optical sensor that is composed of a hyperspectral imager and a multispectral imager, which will be launched on Advanced Land Observation Satellite 3 (ALOS-3). Three similarity measures, normalized cross correlation (NCC), phase correlation (PC) and mutual information (MI), for spatial-spectral registration of hyperspectral data are discussed for Level-1 data processing of HISUI.

Remote sensing missions have been conventionally performed by using satellite-onboard optical sensors with extraordinarily high reliability, on huge satellites. On the other hand, small satellites for remote-sensing missions have recently been developed intensely and operated all over the world. This paper gives a Japanese concept of the development of nano-satellites(10kg to 50kg) based on "Hodoyoshi" (Japanese word for "reasonable") reliability engineering aiming at cost-effective design of optical sensors, buses and satellites. The concept is named as "Hodoyoshi" concept. We focus on the philosophy and the key features of the concept. These are conveniently applicable to the development of optical sensors on nano-satellites. As major advantages, the optical sensors based on the "Hodoyoshi" concept are "flexible" in terms of selectability of wavelength bands, adaptability to the required ground sample distance, and optimal performance under a wide range of environmental temperatures. The first and second features mentioned above can be realized by dividing the functions of the optical sensor into modularized functional groups reasonably. The third feature becomes possible by adopting the athermal and apochromatic optics design. By utilizing these features, the development of the optical sensors become possible without exact information on the launcher or the orbit. Furthermore, this philosophy leads to truly quick delivery of nano-satellites for remote-sensing missions. On the basis of the concept, we are now developing nano-satellite technologies and five nano-satellites to realize the concept in a four-year-long governmentally funded project. In this paper, the specification of the optical sensor on the first satellite is also reported.

The purpose of this paper is to develop sub-pixel registration method for adaptive optics system using phase diversity wavefront sensing with a spatial light modulator (SLM). The SLM which is used for wavefront compensation applies multiple time-series known wavefronts as a priori information to the optical system. By using the SLM for the phase diversity generator, it is possible to select the optimal number and shape of phase diversities for various kinds of natural modes of wavefront aberrations which are represented by the Zernike polynomials. In this case, a misregistration of several diversity images has to be compensated before using phase diversity algorithm. We extracted phase diversity method to estimate not only wavefront aberration but also parallel shift between images simultaneously. The suggested method was validated by numerical simulations, and the high estimation accuracy of the distorted wavefront was demonstrated, and nearly diffraction limited images were acquired by wavefront compensation by preventing noise due to misregistrations.

The purpose of this paper is to develop adaptive optics system which estimates degrading factors using observed images and automatically compensates the distorted wavefronts using a spatial light modulator (SLM). The system estimates optical wavefront aberrations by phase diversity method. The SLM which is used for wavefront compensation applies multiple time-series known wavefronts as a priori information to the optical system. By using the SLM for the phase diversity generator, it is possible to select the optimal number and shape of phase diversities for various kinds of natural modes of wavefront aberrations which are represented by the Zernike polynomials. In laboratory test, wavefront aberrations were generated by optical misalignments, and they were estimated as the coefficients of Zernike polynomials for an extended scene. The suggested method was validated by numerical simulations and laboratory tests. The high estimation accuracy of the distorted wavefront was demonstrated, and nearly diffraction limited images were acquired by wavefront compensation.

Hyperspectral imaging sensors suffer from spectral and spatial misregistrations. These artifacts prevent the accurate acquisition of the spectra and thus reduce classification accuracy. The main objective of this work is to detect and correct spectral and spatial misregistrations of hyperspectral images. The Hyperion visible near-infrared (VNIR) subsystem is used as an example. An image registration method using normalized cross-correlation for characteristic lines in spectrum image demonstrates its effectiveness for detection of the spectral and spatial misregistrations. Cubic spline interpolation using estimated properties makes it possible to modify the spectral signatures. The accuracy of the proposed postlaunch estimation of the Hyperion properties has been proven to be comparable to that of the prelaunch measurements, which enables the precise onboard calibration of hyperspectral sensors.

We propose an adaptive optics system for a lightweight remote sensing sensor. The phase diversity (PD) technique, in which known wavefronts (Phase Diversity) are applied to the optics and the inherent aberrations are estimated using the acquired images without a priori information, is a key to realizing the system. For the reduction of computing cost and the enhancement of the estimation accuracy of aberration, a spatial light modulator (SLM) is adopted not only for wavefront compensator but also for PD generator. The SLM produces arbitrary "aberration modes" that are each represented by a Zernike polynomial. Therefore, optimal phase diversities are applied to the optical system and particular modes are effectively obtained, which makes it possible to overcome the conventional PD generated by defocusing that describes only quadratic form and lacks information of a particular mode. In order to solve the complex inverse problem of phase diversity with low computing cost, a general regression neural network (GRNN) is used. Moreover, principal component analysis compresses the input data for GRNN by extracting information from collected images in Fourier space, and reduces computation cost considerably. The performance is validated by numerical simulation, and the result of experiment using SLM is described.

Fourier transform spectrometer (FTS) has fast optics, and it can realize high resolution within the range from visible light to thermal infrared radiation. FTS intrinsically has the problem that it takes long time to obtain spectrum, because it needs mechanical scanning. But we developed spaceborne FTS system which has the ability of high speed scanning and data handling. By high speed scanning, FTS makes it possible to have high altitude resolution in occultation, and imaging in nadir observation.

Large aperture optical system is required for high resolution and high signal to nose ratio remote sensing observations. In this case, adaptive optics is used to compensate the wavefront aberration generated by the misalignment or the thermal deformation of the optical elements. We use a liquid crystal on silicon spatial light modulator (LCOS-SLM) for the optical wavefront control, and image-based wavefront sensing which realize simple hardware architecture. For image-based sensing, a priori information is required in addition to the acquired images. We use phase diversity (PD) wavefront sensing method which applies a priori information called PD to the optics. By using PDs and acquired images, we can estimate arbitrary wavefront aberration. In this case, the sensitivity of the acquired image to the aberration mode depends on the applied PD. We use LCOS-SLM to apply the optimal set of PDs. We constructed adaptive optics system testbed using LCOS-SLM and USB camera. In this system, we used a Shack-Hartmann wavefront sensor (SHWS) to compare the estimated wavefront aberration with the actual wavefront measured by the SHWS. The laboratory test results show that the proposed system improves the optical performance of the remote sensing sensors.

Hyperspectral imaging sensors suffer from spectral and spatial misregistrations due to optical-system aberrations and misalignments. These artifacts distort spectral signatures that are specific to target objects and thus reduce classification accuracy. The main objective of this work is to detect and correct spectral and spatial misregistrations of hyperspectral images. The Hyperion visible near-infrared subsystem is used as an example. An image registration method based on phase correlation demonstrates the accurate detection of the spectral and spatial misregistrations. Cubic spline interpolation using estimated properties makes it possible to modify the spectral signatures. The accuracy of the proposed postlaunch estimation of the Hyperion characteristics is comparable to that of the prelaunch measurements, which enables the accurate onboard calibration of hyperspectral sensors.

We propose an adaptive optics system for a lightweight remote sensing sensor. The phase diversity (PD) technique, in which known wavefronts (Phase Diversity) are applied to the optics and the inherent aberrations are estimated using the acquired images without a priori information, is a key to realizing the system. For the reduction of computing cost and the enhancement of the estimation accuracy of aberration, a spatial light modulator (SLM) is adopted not only for wavefront compensator but also for PD generator. The SLM produces arbitrary “aberration modes” that are each represented by a Zernike polynomial. Therefore, optimal phase diversities are applied to the optical system and particular modes are effectively obtained, which makes it possible to overcome the conventional PD generated by defocusing that describes only quadratic form and lacks information of a particular mode. In order to solve the complex inverse problem of phase diversity with low computing cost, a general regression neural network (GRNN) is used. Moreover, principal component analysis compresses the input data for GRNN by extracting information from collected images in Fourier space, and reduces computation cost considerably. The performance is validated by numerical simulation, and the result of experiment using SLM is described.

We propose an adaptive optics system for a lightweight remote sensing sensor. The phase diversity (PD) technique, in which known wavefronts (phase diversity) are applied to the optics and the inherent aberrations are estimated using the acquired images without a priori information, is a key to realizing the system. For the reduction of computing cost and the enhancement of the estimation accuracy of aberration, a spatial light modulator (SLM) is adopted not only for the wavefront compensator but also for the PD generator. The SLM produces arbitrary "aberration modes" that are each represented by a Zernike polynomial. Therefore, optimal phase diversities are applied to the optical system and particular modes are effectively obtained, which makes it possible to overcome the conventional PD generated by defocusing that describes only the quadratic form and lacks information of a particular mode. To solve the complex inverse problem of phase diversity with low computing cost, a general regression neural network (GRNN) is used. Moreover, principal component analysis compresses the input data for GRNN by extracting information from collected images in Fourier space, and reduces computation cost considerably without degrading estimation accuracy. The mathematical model is implemented and its performance is validated by numerical simulation.