sensors Article HPT: A High Spatial Resolution Multispectral Sensor for Microsatellite Remote Sensing Junichi Kurihara 1,* ID , Yukihiro Takahashi 1, Yuji Sakamoto 2, Toshinori Kuwahara 2 and Kazuya Yoshida 2 ID 1 Faculty of Science, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo 060-0810, Japan; [email protected] 2 Department of Aerospace Engineering, Tohoku University, Aramaki Aza Aoba 6-6-11, Sendai 980-8579, Japan; [email protected] (Y.S.); [email protected] (T.K.); [email protected] (K.Y.) * Correspondence: [email protected]; Tel.: +81-11-706-9244 Received: 19 January 2018; Accepted: 15 February 2018; Published: 18 February 2018 Abstract: Although nano/microsatellites have great potential as remote sensing platforms, the spatial and spectral resolutions of an optical payload instrument are limited. In this study, a high spatial resolution multispectral sensor, the High-Precision Telescope (HPT), was developed for the RISING-2 microsatellite. The HPT has four image sensors: three in the visible region of the spectrum used for the composition of true color images, and a fourth in the near-infrared region, which employs liquid crystal tunable filter (LCTF) technology for wavelength scanning. Band-to-band image registration methods have also been developed for the HPT and implemented in the image processing procedure. The processed images were compared with other satellite images, and proven to be useful in various remote sensing applications. Thus, LCTF technology can be considered an innovative tool that is suitable for future multi/hyperspectral remote sensing by nano/microsatellites. Keywords: multispectral sensor; tunable filter; microsatellite; Earth observation; image analysis 1. Introduction Artificial satellites can be categorized according to their masses, e.g., picosatellites (<1 kg), nanosatellites (1–10 kg), microsatellites (10–100 kg), minisatellites (100–1000 kg), and large satellites (>1000 kg). With the rapid development of electronic and communication technology, the adage “smaller, faster, and cheaper” summarizes the trend of satellite development activities, which has resulted in a significant increase in the number of nano/microsatellites launched over recent years, especially for Earth observation and remote sensing purposes [1]. Nano/microsatellites have great potential as remote sensing platforms because of the cost-effective implementation of satellite constellations and formations, which increase their overall temporal resolution and ground coverage [2]. However, nano/microsatellites have many limitations related to their small size, which lead to restrictions on their payload instruments in terms of design, power consumption, and data rate. For optical remote sensing, the spatial and spectral resolutions of an optical payload instrument are limited primarily by its aperture size, which is constrained by the dimensions of the satellite. For any optical system, irrespective of whether it is a spaceborne or ground-based instrument, its spatial resolution is limited theoretically by the diffraction of light, and its spectral resolution is limited practically by the signal-to-noise ratio (SNR). Both the diffraction and the SNR can be upgraded with larger apertures; thus, nano/microsatellites have fundamental disadvantages compared with larger satellites in terms of optical remote sensing [3]. Figure1 shows a diagram of the distribution of high (<10 m) and moderate (10–100 m) spatial resolution multispectral Earth observation satellites/sensors launched in the previous few decades. Sensors 2018, 18, 619; doi:10.3390/s18020619 www.mdpi.com/journal/sensors Sensors 2018, 18, x FOR PEER REVIEW 2 of 11 Sensors 2018, 18, 619 2 of 11 Figure 1 shows a diagram of the distribution of high (<10 m) and moderate (10–100 m) spatial resolution multispectral Earth observation satellites/sensors launched in the previous few decades. The mass of a satellite and the decade of its launch are indicated by the area and the color of the circle, The mass of a satellite and the decade of its launch are indicated by the area and the color of the circle, respectively. As can be seen, satellite sensors with three or four spectral bands show a trend toward respectively. As can be seen, satellite sensors with three or four spectral bands show a trend toward higherhigher ground ground resolutions resolutions and and smaller smaller satellitesatellite sizes.sizes. Another evident evident trend trend isis the the increase increase in inthe the numbernumber of of spectral spectral bands bands for for a a series series of of largelarge satellites,satellites, such as as the the Land Landsatsat and and WorldView WorldView flagship flagship series.series. Aside Aside from from the the multispectral multispectral Earth Earth observation observation satellites,satellites, onlyonly two satellites, satellites, Earth Earth Observing- Observing-1 (EO-1)1 (EO-1) and and Project Project for for On-Board On-Board Autonomy-1 Autonomy-1 (PROBA-1), (PROBA-1), areare experimentallyexperimentally equipped equipped with with high high spatialspatial resolution resolution hyperspectral hyperspectral sensors, sensors, although although additionaladditional operational hyperspectral hyperspectral missions missions are are plannedplanned for for launch launch before before 2020 2020 [4 [4].]. FigureFigure 1. 1.Diagram Diagram of of the the high highand and moderatemoderate spatialspatial resolution multispectral multispectral Earth Earth observation observation satellites/sensorssatellites/sensors launched launched in in the the past.past. HyperspectralHyperspectral sensors, sensors, which which are are also also known known asas imaging spectrometers, spectrometers, are are capable capable of of obtaining obtaining images in hundreds of spectral bands, and they can provide abundant information in relation to a images in hundreds of spectral bands, and they can provide abundant information in relation to a wide wide variety of Earth observation and remote sensing applications. Although the challenges in variety of Earth observation and remote sensing applications. Although the challenges in adapting adapting high spatial resolution hyperspectral sensors to small satellites have been studied high spatial resolution hyperspectral sensors to small satellites have been studied theoretically in theoretically in terms of the resolution, quality, and volume of hyperspectral data, the practical terms of the resolution, quality, and volume of hyperspectral data, the practical feasibility appears feasibility appears low for satellites <100 kg, even when applying state-of-the-art technology [5,6]. lowThe for hyperspectral satellites <100 sensor kg, even considered when applying in the state-of-the-artabove studies technologywas a push-broom/line-scanning [5,6]. The hyperspectral sensorspectrometer, considered which in the had above been studies conventionally was a push-broom/line-scanning mounted on large satellites spectrometer, or on manned which aircraft. had This been conventionallytype of hyperspectral mounted sensor on large has satellites a two-dimensional or on manned (2D) aircraft. detector This array, type where of hyperspectral one axis records sensor hasspatial a two-dimensional information and (2D) the detector other axis array, records where spec onetral axis information. records spatial The informationspatial axis andis arranged the other axisperpendicular records spectral to the information. satellite track; The thus, spatial by scanni axis isng arranged the Earth’s perpendicular surface along to the the path satellite of movement track; thus, byof scanning the satellite, the Earth’s a three-dimensiona surface alongl (3D) the hyperspectral path of movement data cube of the is obtained. satellite, aConsequently, three-dimensional both the (3D) hyperspectralresolution and data the cube quality is obtained. of the acquired Consequently, data ar bothe constrained the resolution considerably and the qualityby the ground of the acquired track datavelocity are constrained of the satellite considerably and the aperture by the size ground of the track sensor. velocity These of constraints the satellite make and it thedifficult aperture to realize size of theeffective sensor. Thesedeployment constraints of high make spatial it difficult resolution to realizehyperspectral effective sensors deployment on nano/microsatellites. of high spatial resolution hyperspectralAnother sensors type of onmulti/hyperspectral nano/microsatellites. sensor is a wavelength-scanning imager using a filter wheel orAnother a tunable type filter. of multi/hyperspectralThis type of sensor also sensor employs is a wavelength-scanning a 2D detector array, imagerbut it captures using a a filter spatial wheel 2D or a tunableimage filtered filter. This at a typecertain of sensorspectral also band employs that is switched a 2D detector by mechanically array, but it rotating captures the a spatial filter wheel 2D image or filteredby electrically at a certain tuning spectral the band filtered that wavelength. is switched byWhile mechanically a finite response rotating time the is filter required wheel to or switch by electrically from tuning the filtered wavelength. While a finite response time is required to switch from one spectral band to another for wavelength scanning, the resolution and the quality of the data do not depend directly on the Sensors 2018, 18, 619 3 of 11 satellite speed. A filter wheel mounted with 15 spectral