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An introduction of ASTER (The Advanced Spaceborne Thermal Emission and Reflection Radiometer) for the Earth Observation

Masatane Kato, Yuichi Maruyama, Makoto Tsukada
Earth Remote Sensing Data Analysis Center (ERSDAC)
3-12-1, Kachidoki, Chuo-ku, Tokyo 104-0054. Japan
Tel: (81)-3-3533-9380 Fax : (81)-3-3533-9383
E-mail: kato@ersdac.or.jp , maruyama@ersdac.or.lp , tsukada@ersdac.or.ac.jp 


Abstract
ASTER, the successor to the Japanese Earth Resources Satellite-1 (JERS-1) launched in 1992, is a high-resolution multi-spectral imaging sensor, developed by MITI (Ministry of International Trade and Industry) , covering from visible to thermal infrared region. ASTER, Project is a point carried out by MITI/ERSDAC and NASA to contribute the extensive understanding of the Earth as a system through NASA's EOS (Earth Observing System) Program. ASTER was successfully launched from Vandenberg Air Force Base on December 18th , 1999 as a one of the sensors of the 'TERRA' satellite of EOS Flagship. After launch, ASTER ICO(Initial Check-Out) were conducted by ASTER three projects, Instrument/Science/GDS(Ground Data System) Project. The function check for ASTER sensors were carried out by ASTER Instrument project, the calibration/validation for the data processing/scheduling algorithm by ASTER Science project, and the performance check for all ground data systems by ASTER GDS. ERSDAC is involved with the ASTER Mission operation and data processing and distribution. The acquired data are processed in ASTER GDS (Ground Data System) at ERSDAC as various data products from level-1 A to level-4. Users of ASTER can confirm the data acquisition and make a request through the following web site. http://www.ersdac.or.jp 
 
ERSDAC coordinated the public solicitation of researchers to use ASTER data, called ASTER Announcement of Opportunity (ASTER AO). Any and all researchers, scientists, and research organizations are welcome to submit research proposals for non-profit and peaceful purposes to use ASTER data. They will be privileged to obtain a certain amount of ASTER data and to submit the data acquisition requests (DAR). ASTER data will be distributed to all users on a nondiscriminatory basis for peaceful purposes in consistent with national laws and regulations.

Introduction
ASTER Program is to make contribution to extend the understanding of local and regional phenomena on the Earth surface and its atmosphere through EOS Program, a comprehensive each observation program led by NASA of USA and is executed jointly by the Ministry of International Trade and Industry, Japan (MITI) and NASA, MITI has the responsibility of the development and operation of a sensor which is called ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and the ASTER Ground Data System, while NASA has the responsibility for the development and the operations of the platform. Both parties have the responsibility for the ASTER data processing, distribution and archive.

In order to execute this joint project, ASTER Science Project along with ASTER Science Team, ASTER Instrument Project, and ASTER Ground Data System Project have been established.

ASTER Instrument
ASTER is a high-resolution multi-spectral imaging sensor with visible-near infrared, short wavelength infrared and thermal infrared spectral bands. ASTER was launched on December in 1999 as the one of the sensors of the NASA's 'TERRA's ', which was the first satellite of the EOS program.

Table 1 'TERRA' Orbit Parameters
Item  Specification
Orbit  Sun Synchronous
Local time at equator  10:30 + 15 min. am
Altitude range  705 km at equator 
Inclination  98.20 + 0.150
Repeat Cycle  16 days (233 revolutions / 16 days)
Distance between adjacent orbits  172 km 
Orbit period  98.9 min
Mission life  6 years
Sensors on 'TERRA'  ASTER : Advanced Spaceborne Thermal Emission and Reflection Radiometer

CERES : Clouds and the Earth's Radiant Energy System

MISR : Multi-angle imaging Spectrometer 

MODIS : MODerate-resolution Imaging Spectrometer 

MOPITT : Measurements Of Pollution In The Troposphere


ASTER is comprised of 3 subsystems, VNIR (Visible and Near Infrared Radiometer), SWIR (Short Wavelength Infrared Radiometer), TIR (Thermal Infrared Radiometer ). The main characteristics of ASTER are;

High spectral resolutions: 3 bands over VNIR region, 6 bands over SWIR region and 5 bands over TIR region.
High spatial resolutions: VNIR 15m, SWIR 30m and TIR 90m.
Along track stereoscopic data acquisition capability: Base/Height = 0.6.
Cross track pointing capability : VNIR + 240, SWIR + 8.550. The swath of ASTER is 60 km and narrower than the orbital spacing (172 km). It can, however, observe the entire globe by using this pointing capability. Although the repeat cycle of 'TERRA' is 16 days , the global observation cycle of ASTER is 48 days due to its swath. ASTER can perform an emergency observation within 5 days (worst case) using the +240 pointing capability of VNIR.

Although the mission operation of ASTER is mainly in daytime because of an optical sensor, TIR can observe the thermal emissive in nighttime and if temperature of a target such as lava is high enough, it is observed by SWIR as well. Table 2 shows main characteristics of ASTER. 

Table 2 Main characteristics of ASTER

Item  Specification
Spatial resolution VNIR: 15m (Bands 1-2, 3N&3B*) 0.52-0.86mm

SWIR: 30m (Bands 4-9) 1.60- 2.43mm

TIR: 90m (Bands 10-14) 8.125-11.65mm
Base-to-height ratio of stereoscopic capability  0.6(along-track)
Swath width  60 km
Pointing function in cross-track direction VNIR: +24 deg. (Bands 1-3)

SWIR: +8.55 deg. (Bands 4-9)

TIR: +8.55 deg. (Bands 10-14)

Mission Operation
The operation of the ASTER will be affected by various constraints such as duty cycle (average 8 minutes/orbit) and pointing frequencies. Moreover, many possible combinations of the observation modes complicate the data acquisition during the 6-years mission period. There are three data acquisition request categories ; DAR (Data Acquisition Request), STAR (Science Team Acquisition Request) and ETR (Engineering Team acquisition Request).
  1. DAR: DARs correspond to the local observations requested from individual investigators selected through authorized process by ASTER Announcement of Opportunity (AO).
  2. STAR: STARs correspond to the observations requested from ASTER Science Team and are divided into three sub-categories; Global Mapping (GM), Regional Monitoring (RM) and Local Observation.
  3. Global Mapping (GM) is to make a global data set comprised of a single set of images covering the entire global land surface using all channels including stereo, of each subsystems. For GM, a global prioritization map is shown in Fig. 1.
  4. Regional Monitoring (RM) is to make regional data sets necessary to perform a large area and/or multi-temporal analysis of a region such as a study over several years of the advance.
  5. Local Observations is to observe an spatial and time limited target (Ex Volcano eruptions of Mt. Usu/Mt. Miyake, Flooding at Nagoya in Japan this year etc.). 
    6.
  6. ETR: ETRs correspond to the observations requested from ASTER Instrument Team for the check of the health and safety and the performance of the Instrument including on board calibration data acquisition. 
The scheduling for mission operation is established once a day using the ASTER Scheduler algorithm based on the prioritization of the data acquisition requests. Prioritization is done considering the factors such as user category and observation category. The schedule is fixed 27 hours before the start of the observation day.

ASTER Data Products
The ASTER data products, such as Radiance registered at sensor. Surface radiance, Surface temperature, Surface emissive, surface reflectance. Radiance registered at sensor with ortho-photo correction, Digital elevation model (Relative) and so on, are developed by ASTER Science Team and distributed by ASTER GDS (Ground Data System).

ASTER Ground Data System
ASTER Ground Data System (ASTER GDS), located at ERSDAC in Japan, is the ground system for ASTER operation, data processing, data archiving and distribution. the ASTER GDS plays an important role in the sensor operation by ASTER Operation Segment (AOS) and data processing by Science Data Processing Segment (SDPS) , and network and manage entire ASTER GDS by Communication and System Management Segment (CSMS) and requires close coordination with NASA/GSFC (Goddard Space Flight Center) / EOSDIS (EOS Data and Information System). ASTER GDS has important interfaces with the users by Information Management Subsystem (IMS) as well as with the United States. Configuration of ASTER GDS is comprised with 3 segments that are shown in Figure 1.



ASTER Images Observed in ICO Periods
After launch, some ASTER images were observed in the initial check -out phase. Two ASTER images, one VNIR image at san Francisco in USA and another TIR image at Rift valley in Ethiopia are shown in figure 2 and figure 3 respectively. VNIR images in San Francisco, whose ground resolution is 15m, make it possible to discern Golden Gate Bridge, buildings, streets, and golf courses, etc. On the other hand, TIR images in Rift valley show distinction between rocks such as andesite and basalt based on the emissiveity. 

ASTER Announcement of Opportunity
ERSDAC coordinates the public solicitation of researchers to use ASTER data, called ASTER Announcement of Opportunity (ASTER AO) .Any and all researchers, scientists, and research organizations are welcome to submit the research proposals for non- profit and peaceful purposes to use ASTER data .They will be privileged to obtain a certain amount of ASTER data and to submit the data acquisition request (DAR). ASTER AO has already started and proposals will be accepted over the ( http://www.astwed.ersdac.or.jp/ao/ ) and through regular mail (Post)

Data Policy 
ASTER data is distributed according to IEOS (International Earth Observing System) Data Exchange Principles in consistent with national laws and regulation, i.e.:
  • All ASTER Data is available for peaceful purposes to all users on a non-discriminatory basis and in timely manner. 
  • All ASTER Data is available for the designated users at the lowest possible cost for non-commercial use on the following categories: Research Application, and operational Use for the Public Benefit. For purposes other than above, the specified data is made available in accordance with terms and conditions to be the established by the Data providing Agency.

Conclusion
ASTER Program is executing ASTER GDS Project as well as NASA and ASTER Instrument Project intensively in comparison with ASTER Science Project along with ASTER Science Team. The contribution to making clear the process of an environmental change on global scale and to forecasting the future of a global environment by ASTER data observed after launch, depends on the results of research in various fields by not only academic users but also practical users.

Acknowledgements
The author is also grateful to NASA and MITI for their support in the EOS and ASTER programs.

References
T. Kawakami, "ASTER Program Overview" , Proc. SPIE, Vol. 3502, pp. 23-29, 1998.