ALOS-3 (Advanced Land Observing Satellite-3; optical mission)
ALOS-3 is the follow-on JAXA optical satellite mission of ALOS/Daichi (launch Jan. 24, 2006), to complement the SAR services of the ALOS-2 mission (launch planned for 2013). ALOS-3 carries an optical sensor complement to succeed PRISM (Panchromatic Remote-sensing Instrument for Stereo Mapping) and AVNIR-2 (Advanced Visible and Near-Infrared Radiometer-2) aboard ALOS/ The goal of the ALOS-3 mission is to provide operational support services in the following areas:
1) Disaster monitoring of stricken regions is one of the most important requirements of the post-ALOS program.
2) Continuous updating of data archives related to national geographical information, including topographic maps, land use, and vegetation
3) Survey of crops and coastal fishing conditions
4) Environmental monitoring, including illegal dumping of industrial wastes.
The basic requirements include high resolution, wide swath, prompt observation and information delivery after a disaster. To meet their requirements, JAXA has been conducting the conceptual design of the post-ALOS satellite system, including a constellation of optical satellites and radar satellites. 1) 2) 3) 4)
The main requirements are:
- Provision of high-resolution imagery with a GSD of < 1 m on a swath of 50 km
- Provide pan-sharpen imagery with simultaneous acquisition of panchromatic band (Pan) images and four-color band (MS) images
- Acquire stereo imagery from different view angles
- Enable the capability of image acquisition of any point in a wide area with pointing to achieve timely monitoring if a disaster occurs.
The sensor complement of ALOS-3 will feature more enhanced capabilities than those provided by the PRISM and AVNIR-2 instruments. The mission concept of ALOS-3 calls for the following services:
• ALOS-3 will supply high-resolution imagery of 0.8 m and a wide swath of 50 km to contribute to archives of satellite image maps all over the world. ALOS-3 will also produce pan-sharpened images with simultaneous acquisition of panchromatic images and four-band multi-spectral images.
• ALOS-3 will perform simultaneous observations of ground surfaces from different angles using the nadir-looking and backward-looking panchromatic imager in order to produce precise DSMs (Digital Surface Models) by stereo pair images to be used in many fields.
• ALOS-3 will enable image acquisition of any point in a wide area by satellite body-pointing to achieve timely monitoring when a disaster occurs.
Note: The ALOS (Daichi) spacecraft was retired on May 12, 2011. The JAXA recovery team had been trying to communicate with ALOS for about three weeks after it developed a power generation anomaly.
Figure 1: Artist's view of the deployed ALOS-3 spacecraft (image credit: JAXA)
A proper description of the spacecraft will be provided when available.
The event monitoring requirement for quick disaster response services calls for an agile spacecraft with a body-pointing capability of ±60º into any direction using reaction wheels. Hence, such technologies as a highly stable spacecraft control function and a precise image location system will be inherited from ALOS.
The design of the ALOS-3 bus will adopt the design of the ALOS-2 bus in almost all aspects. There are some differences between the ALOS-2 bus and the ALOS-3 bus; for example, the pointing capability and the amount of data storage are different.
RF communications: The large volume of the payload source data requires a high-capacity onboard storage system as well as a high-quality irreversible (lossy) compression technique.
ALOS-3 uses an X-band downlink and an ISL (Intersatellite Link) system to handle the several Gbit/s of mission source data. The data rate is 800 Mbit/s with a modulation scheme of 16 QAM (Quadrature Amplitude Modulation). Use of the newly developed XMOD (Multi-mode High Speed Modulator) as described in ALOS-2. The onboard storage capacity is > 200 GByte. The maximum data volume is 1440 GByte/day.
Launch: A launch of ALOS-3 is planned for ≥2016 on a H-2A vehicle from TNSC, Japan.
Orbit: Sun-synchronous orbit, altitude = 618 km, inclination = 97.9º, LTDN (Local Time on Descending Node) at 10:30 hours ±15 minutes. The nominal repeat cycle is 60 days.
Sensor complement: (PRISM-2, HISUI)
The sensor complement is comprised of the optical instruments PRISM-2 and HISUI, featuring panchromatic and multispectral imaging capability as well as hyperspectral imaging capability. The orbit is set such that PRISM-2 can observe the entire surface of the Earth in 50 km swaths without spacecraft body-pointing. For mission continuity, the local sun time will be at 10:30 hours which is same as that of ALOS. 5)
PRISM-2 (Panchromatic Remote-sensing Instrument for Stereo Mapping-2)
PRISM-2 provides high-resolution, wide-swath imagery with high geolocation accuracy, as well as precise DSMs (Digital Surface Models) using stereo pair images acquired by two telescopes, one nadir-looking and one backward-looking . To meet the mission requirements, some observation capabilities need to be upgraded from those of PRISM onboard ALOS. Table 1 compares the observation capabilities of PRISM and PRISM-2. 6) 7)
Table 1: Enhanced capabilities of PRISM-2 onboard ALOS-3
PRISM-2 is a two-line pushbroom-type stereo high-resolution instrument (nadir-looking and backward-looking of PRISM heritage on Daichi), featuring a telescope with a wide FOV (Field of View) and a large format FPA (Focal Plane Assembly) detector to realize a swath width of 50 km at a GSD (Ground Sample Distance) of 0.8 m on a swath of 50 km. An off-axis TMA (Three Mirror Anastigmat) design of the telescope is employed.
Figure 2: Prototype primary mirror for PRISM-2 (image credit: JAXA)
Figure 2 shows the prototype primary mirror for the telescope. An ultralow-expansion glass-ceramics material is employed for the mirrors. The primary mirror for the nadir-looking telescope is nearly 0.9 m x 0.6 m in size. Because it is an off-axis telescope, it needs a mirror that is much larger than the effective aperture. The use of an ultraprecise grinding technology before polishing can realize high-speed processing of a large aspheric mirror surface.
Figure 3: Schematic view of the PRISM-2 optical paths (image credit: JAXA)
To realize a wide swath without mechanical scanning, an array of ~ 65,000 pixels is needed for the FPA of the panchromatic nadir-looking detector. The detector configuration is comprised of eight CCD arrays, each of ~ 8,000 pixels. These CCD arrays are put side by side in the focal plane. The spectral selection is created by placing a filter in front of the CCD arrays. SiC (Silicon Carbide) is used for the focal plane. This material offers very good thermal conductance, a low CTE (Coefficient of Thermal Expansion) for acceptable dimensional stability, and a very high mechanical rigidity with a low mass. JAXA manufactured the SiC focal plane with good results.
The PRISM-2 instrument is a two-line imager with two independent catoptric systems for nadir- and backward-looking optics and FPAs to achieve along-track stereoscopy. The backward-looking panchromatic sensor will be installed on the instrument with a 23.8º backward offset from the nadir direction. The observation geometry of the nadir- and backward images will result in nearly simultaneous stereo imagery.
The backward-looking panchromatic FPA is of a similar configuration to the nadir-looking sensor. The spatial resolution of the backward-looking sensor is ~ 1.25 m (when the spacecraft is nadir pointed) and the swath is 50 km. About 40,000 pixels are needed for the FPA; the detector configuration is comprised of six CCD arrays, each of ~ 8,000 pixels.
Table 2: Main parameters of the PRISM-2 instrument
Figure 4: Schematic view of the image collection concepts of PRISM-2 (image credit: JAXA)
Legend to Figure 4: The left image shows the stereo observation concept for a two-line pushbroom stripmap imaging. The right image illustrates the pointing capability (±60º) of the very agile spacecraft. PRISM-2 enables quick access to any point in Japan within a day with spacecraft body-pointing.
HISUI (Hyperspectral Imager Suite):
HISUI, a hyperspectral and multispectral pushbroom imaging instrument, is of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) heritage of JAXA, flown on the Terra mission of NASA (launch Dec. 18, 1999) with a mission life of > 10 years obtained in 2010. The design/development of the HISUI instrument started in 2007, the project is funded by METI (Ministry of Economy, Trade, and Industry), managed by JAROS (Japan Resources Observation System Organization), and built by NEC Corporation. 8) 9) 10) 11) 12) 13) 14) 15)
The objectives of the HISUI mission are:
1) Global energy and resource related applications
- Oil, gas, metal,
- Observations for environmental assessments which are indispensable to resource developments
2) Other applications such as environmental monitoring, agriculture, and forestry
3) Promotion of domestic space and space utilization industry through wider applications of HISUI data.
Figure 5: HISUI project structure (image credit: JAROS, NEC)
HISUI is comprised of two elements: a multispectral imager, also referred to as MSS (Multispectral Sensor), with excellent spatial resolution and swath width, and a hyperspectral imager, also referred to as HSS (Hyperspectral Sensor), with a high identification capability thanks to a high wavelength resolution (total of 185 bands). 16)
The hyperspectral imager (or radiometer) covers the spectral range from the visible to the SWIR (Short-Wavelength Infrared) region (0.4 – 2.5 µm). To satisfy high SNR (Signal-to-Noise Ratio), the diameter of the telescope is designed with an aperture of 30 cm for the ground sampling distance of 30 m. An off-axis three mirror anastigmat type (TMA) telescope is used with an F-number of 2.2. The ground footprint is projected to the slits with the gap of 30 µm. The light entering the slits is introduced to two spectrometers, one for the VNIR (Visible and Near Infrared) radiometer, and the other for the SWIR radiometer. Both spectrometers adopted a reflective grating system.
Spectrograph: For the hyperspectral radiometer application, aberration corrected Offner-type spectrographs are selected. Since the wavelength region of the hyperspectral radiometer is wide, two spectrographs are manufactured and each of them is optimized for the VNIR and the SWIR region, respectively.
Detectors: For the VNIR region, backside illuminated silicon CMOS detectors are selected since a CMOS-type detector implementation features low smear and blooming. For the SWIR region, a PV-MCT (Photovoltaic Mercury Cadmium Telluride) type linear array is selected due to its high and uniform quantum efficiency throughout the SWIR region and its low dark current behavior at temperatures of <150 K.
The SWIR detector is mounted inside a dewar and cooled down to ~145 K by using a Stirling cryocooler to decrease the dark current down to the negligible level.
A HELU (Hyper Electronics Unit) functions a a signal processor of the hyperspectral data stream. HELU makes the correction of PRNU (Photo Response Non-Uniformity), non linearity, offset and smile distortion. In order to reduce the data size, data binning in spectral direction and lossless data compression are implemented at HELU. The detector pixels are binned in the spectral direction, over 4 pixels for the VNIR case and over 2 pixels for the SWIR case, in order to produce unity image pixels.
Table 3: Specification of the Hyper-Multispectral Sensor and comparison with ASTER
The multispectral radiometer covers the VNIR spectral region with the ground sampling distance of 5 m, which satisfies the user requirement for high spatial resolution. The blue band is added for those users who investigate seawater regions. The radiometric accuracy is almost the same as that of the ASTER, and a better MTF (Modulation Transfer Function) performance is expected using the new scanning technique. The wide swath of 90 km is achieved using the three-mirror off-axis telescope, which increases the observation frequency of the specified target areas. The optical components are arranged to minimize stray light caused by reflection at the elements. A back-illuminated Si-CCD detector array is adopted with a filter assembly for four bands. 17)
Since the smile and keystone phenomena distort the spectrogram, which is difficult to correct by data processing, a fine spectrogram is obtained at the interval of 2.5 nm for VNIR and 6.75 nm for the SWIR range that are decimated on-board by shifting data in the spectrum direction to correct the smile distortion. Therefore, the smile distortion is minimized and the response function is improved. The resulting SNR is 450 and 300 for the VNIR and SWIR ranges, respectively.
The VNIR spectral range employs a Si-CMOS detector array while the SWIR range uses an HgCdTe detector array. The SWIR detector is cooled with a Stirling type cryocooler.
The hyperspectral imager of HISUI features a pointing capability in cross-track of ~±3º, corresponding to a ±30 km shift from nadir (Figure 12).
TMA (Three Mirror Anastigmat) optics: The MSS and HSS devices use TMA optics. The mirrors are made from glass ceramic material Zerodur (Schott, Germany), as well as from NTSiC (New Technology Silicon Carbide) developed in Japan. NTSiC is a high strength reaction sintered SiC material developed by combining key technologies and expertise of several Japanese manufacturers (NEC Toshiba, ). 18) NTSiC is pore-free and can be polished without cladding material like CVD (Chemical Vapor Deposition) SiC. Such additional layer enables obtaining excellent polishing quality but requires large special furnaces. Another concern is the bimetallic effect that may appear when applying such polishing layer and the risk of going through this layer during the lapping and polishing process (Ref. 16).
The customer decided to have M1 and M3 of the MSS TMA be made from NTSiC material. This choice was also dictated by the fact that these two mirrors were quite of the same dimension of nearly 300 mm x 500 mm. Each of the six mirrors of the MSS and HSS TMA instruments is lightweighted, polished coated, equipped with electrical grounding straps, and fitted with pads and MFDs (Mirror Fixation Devices).
Figure 6: Schematic view of the HISUI assembly on ALOS-3 (image credit: JAROS, NEC)
Figure 7: Block diagram of the HISUI instrument (image credit: JAROS, NEC)
Figure 8: Optical layout of the HISUI hyperspectral imager (image credit: JAROS, NEC)
Figure 9: Functional block diagram of the HiSUI instrument (image credit: Japan Space Systems, NEC Corporation, Ref. 15)
Table 4: Specification of the HISUI instrument (Ref. 15)
Figure 10: Optical layout of the HISUI multispectral imager (image credit: JAROS, NEC)
Figure 11: Operation support scenario of HISUI (image credit: JAROS, NEC)
Figure 12: Cross-track pointing capability of the HISUI hyperspectral imager (image credit: JAROS, NEC)
HISUI instrument calibration: The National Institute of Advanced Industrial Science and Technology (AIST) is responsible for the radiometric calibration of HISUI. The long-term radiometric calibration is indispensable for quality assurance and control of data products. There is an onboard calibration system in the hyperspectral radiometer and the multispectral radiometer. 19)
The onboard calibration system for the hyperspectral radiometer is able to perform radiometric and spectral calibration. This calibration system has two halogen lamps, some photodiode detectors, some band pass filters and spectral calibration filter. The types of photodiode detectors are Si photodiodes for the VNIR region and InGaAs photodiodes for the SWIR region.
The onboard calibration system for the multispectral radiometer is able to perform radiometric calibration. The system has two halogen lamps and some photodiode detectors. The types of photodiode detectors are Si photodiodes with and without bandpass filter. The wavelength regions of these bandpass filters are of the same wavelength region that is used for detectors for observation.
Figure 13: Schematic view of the onboard calibration system for the multispectral radiometer (image credit: NEC)
In the evaluation model of the hyperspectral radiometer, the spectral calibration accuracy had been evaluated in the air environment and in the thermal vacuum environment. The tests under air environment verified the results when comparing with each wavelength calculated by spectral calibration filter and Hg-Ar lamp. The result of these tests was < 0.16 nm in the VNIR region, and < 0.4 nm in the SWIR region (Ref. 15).
Several on-orbit calibration methods have been conducted for previous and ongoing satellite missions, such as onboard calibration using equipped onboard lamps as reference light sources, vicarious calibration comparing brightness of terrestrial targets with in-situ measurements, cross calibration comparing the brightness with other satellite sensors, and Lunar calibration treating Moon as a known brightness light source. Since satellites in space can observe the Moon without considering the atmosphere and aerosols, it enables to provide more accurate radiometric calibration especially in the near infrared wavelength region where strong water absorptions exists. 20)
New lunar reflectance model: The HISUI lunar calibration requires a reflectance model which is composed of enough spectral bands along with sufficient spatial resolution. A new lunar reflectance model has been proposed based on the hyperspectral data of the SP (Spectral Profiler) onboard the SELENE mission which was a Japanese Moon satellite operated in the period 2007 – 2009. The model covers a wavelength range from 500 nm to 1600 nm and it also involves lunar surface photometric properties depending on incident, emission and phase angles.
The values of the model are given in sizes for every 0.5º x 0.5º in lunar longitude and latitude, which is comparable to the resolution of a lunar observation by the HISUI hyperspectral imager; this method enables to evaluate the brightness of every pixel. This new SP-based lunar calibration model is useful not only for HISUI, but also for other ongoing and future multispectral and hyperspectral missions; the model will be made available to the public.
HISUI operations plan and coverage:
The high source data rate of HISUI (1 Gbit/s + 0.4 Gbit/s) at 70% compression requires a duty cycle of the instrument as well as an allocation of the downlink communication resources of the spacecraft.
The cumulative land observation time of each ALOS-3 orbit is shown in Figure 14. Although time necessary for land observation in an orbit varies from 48 to 2030 seconds, its average is about 15 minutes which is close to HISUI's maximum total observation time in an orbit. In addition, HISUI can separate its observation time to several segments to avoid open oceans and observe land and coastal areas efficiently. According to the simulation, five segments will be enough for most of daytime land observation. 21)
Figure 14: Cumulative land observation time of ALOS-3 orbits (image credit: AIST, ERSDAC, JAROS)
The global mapping of the HISUI hyperspectral imager is limited by its narrow swath (30 km) and huge data amount. Table 5 shows the simulation results of global mapping achievement by HISUI hyperspectral imager for various observation days. If HISUI hyperspectral imager can downlink 300 GByte of data per day, which is about 20% of ALOS-3's downlink capacity, more than 40% of the global land surface will be observed at least once in four months and more than 97% in ten months.
Table 5: HISUI global mapping (one time) achievements and observation days, the downlink allocation is 300 GByte/day
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.