GCOM (Global Change Observation Mission)

GCOM is a JAXA (Japan Aerospace Exploration Agency) observation program consisting of a constellation of two medium-sized spacecraft with the provisional names of GCOM-W and GCOM-C. GCOM is seen as a follow-up program to ADEOS. with the overall objective to contribute to global change research through long-term (> 10 years) sustained observations with corresponding data sets. Three consecutive constellations of spacecraft are being planned, representing in particular the long-term Japanese contribution to the GEOSS (Global Earth Observation System of Systems) initiative. The prime goal of GEOSS is to achieve comprehensive, coordinated and sustained observations of the Earth environment. ^{1) 2) 3) 4) 5) 6) 7) 8)}

The mission of GCOM is to achieve the following objectives:

• The primary goal of GCOM-W, a sea surface observation mission, is to contribute to observations related to global water and energy circulation. The payload consists of an AMSR2 (Advanced Microwave Scanning Radiometer-2) instrument.
Note: The AMSR instrument was flown on ADEOS-II mission (Dec. 14, 2002 to Oct. 25, 2003 - a power failure accident ended ADEOS-II) of JAXA, and the AMSR-E (for EOS) instrument is being flown on the Aqua mission of NASA (launch May 2002).

• The primary goal of the GCOM-C mission is to contribute to surface and atmospheric measurements related to the carbon cycle and the radiation budget. The payload consists of a second-generation GLI (SGLI) instrument.

GCOM is expected to make the following achievements by the end of its mission (around 2023): ⁹⁾

1) Global warming

• Understanding of the global warming by global and long-term measurement data on various parameters.

• Separation between natural variability and trends using the data set covering the 27 year period from the launch of the ADEOS or the 21 year period from the launch of the ADEOS-II.

2) Change of land environment

• Understanding of global forest dynamics

• Understanding of snow and ice changes

3) Clarification of sink and source of greenhouse gases.

Table 1: Overview of GCOM series investigation themes

Table 2: Overview of some GCOM spacecraft parameters

GCOM-W1 spacecraft:

JAXA has decided to use medium-scale satellites for GCOM series observations. The GCOM-W1 spacecraft is 3-axis stabilized. Power of 4.05 kW is provided at EOL (End of Life). The spacecraft on-orbit dimensions are (deployed configuration): 5.1 m (X) x 17.5 m (Y) x 3.4 m (Z).

The spacecraft has a mass of about 1910 kg at launch (dry bus mass of 1324 kg, propellant mass of 151 kg, AMSR2 mass of 405 kg). The design life is 5 years. ¹⁰⁾

The PDR (Preliminary Design Review) of GCOM-W1 took place in March 2008. The CDR (Critical Design Review is scheduled for 2009.

Figure 1: Artist's view of the GCOM-W1 spacecraft (image credit: JAXA)

Launch: A launch of GCOM-W1 is planned for early 2012 on an H-IIA vehicle from TNSC (Tanegashima Space Center), Japan.

Orbit: Sun-synchronous orbit, altitude = 700 km, inclination = 98.19º, LTAN (Local Time on Ascending Node) at 13:30 hours (to continue the AMRS-E observations). Since the orbit is very similar to the one of the A-Train, there are good chances that it will join the A-Train constellation. ¹¹⁾

RF communications: Direct real-time downlink of payload data to receiving stations with agreement. Downlink in X-band (8245 MHz) with a data rate of 10 Mbit/s, modulation = OQPSK (Offset Quadrature Phase Shift Keying).

Sensor complement: (AMSR-2, a single instrument is flown)

AMSR2 (Advanced Microwave Scanning Radiometer-2):

AMSR2 is a follow-on JAXA radiometer of AMSR and AMSR-E heritage (passive instruments) installed on the ADEOS-II and the Aqua missions, respectively. The objective is to achieve measurement of: sea surface temperature (SST), soil water content (moisture), sea wind speed, water equivalent of snow cover, precipitation intensity, sea ice distribution, precipitable water, etc. The observables are the microwave emissions from the atmosphere, ocean, sea ice, and land which are being measured at multiple frequencies. ^{12) 13)}

The instrument employs a parabolic offset antenna (antenna aperture of 2 m diameter) providing a conical scan with a swath width of ~ 1450 km (from a 700 km orbit). The incidence angle is 55º. AMSR2 is a total power microwave radiometer with a two point external calibration method:

1) Deep space using a cold sky mirror

2) An on-board hot load.

For the absolute calibration, deep space observations will be done using the main mirror. AMSR2 is able to provide global observations in just 2 days.

The AMRS2 frequency channels are identical to those of AMRS-E except the 7.3 GHz channel which is being used for RFI (Radio Frequency Interference) mitigation.

Table 3: Preliminary specification of the AMSR2 instrument

Figure 2: Three views of the AMSR2 instrument (image credit: JAXA)

Figure 3: Conical scanning configuration of AMSR2 on GCOM-W1 (image credit: JAXA)

AMSR2 calibration:

AMSR2 has two calibration targets which named HTS (High Temperature noise Source) and CSM (Cold Sky Miller). The configuration of these targets are shown in Fig.5.

Figure 4: Configuration of calibration assembly; TCP and Sun Shade are installed on HTS/AMSR2 only (image credit: JAXA)

HTS/AMSR2 thermal design: The accuracy and the reliability of AMSR2 has been improved compared with the designs of AMSR and AMSR-E. There were major problems in HTS thermal design of AMSR and AMSR-E,and they caused large temperature gradient on the absorber surface of HTS. For the thermal design of HTS/AMSR2, the following two major items were requested to improve: (Ref. 13)

1) Uniformity of the absorber surface (2.5ºC_p-p)

2) Thermal measurement accuracy (0.4ºC (3σ)).

Uniformity of the absorber surface of HTS/AMSR2.

To improve the temperature uniformity, thermal design of HTS/AMSR2 has been improved. The thermal design of HTS/AMSR, AMSR-E, shown in Figure , has the following features:

- Controlling the absorber temperature using heater rods inside the absorber corns

- The surface at the facing side of the absorber covered with MLI whose temperature was not controlled.

- The thermal connection between the absorber surface and outside(space, sun etc.) was large because there were big chinks between HTS and the MLI of the facing side. This design caused the large temperature gradient on the absorber surface.

The improved thermal designed was analyzed using on-orbit thermal analysis and two thermal vacuum tests. As a result of the on-orbit thermal analysis with the AMSR2 model, which adopted these improvements, the temperature uniformity of the absorber is satisfying the request.

Figure 5: Heater control of HTS/AMSR-E (image credit: JAXA)

To improve the temperature uniformity of its surface, The thermal design around HTS/AMSR2 had been improved in the following items (shown in Figure 6):

- Changing thermal control methodology; covering all aspects of the absorbers with panels whose temperatures are uniformly controlled. (instead of the Heater rod in the AMSR-E/HTS)

- Installing the TCP (Thermal Control Panel) to control the temperature of the facing side of absorber surface

- Installing “Sun Shade” to minimize the sunlight heat input which comes into HTS through the chinks between HTS and TCP.

Figure 6: Heater control of HTS/AMSR2 (image credit: JAXA)

Data products:

The following parameters are part of the GCOM-W1 standard products: ¹⁴⁾

• Brightness temperature

• Total vapor power

• Total cloud liquid water

• Precipitation

• SST (Sea Surface Temperature)

• Sea surface wind speed

• Sea ice concentration

• Snow amount

• Soil moisture

GCOM-C1 spacecraft:

The GCOM-W1 spacecraft is 3-axis stabilized. Power of > 4.25 kW is provided at EOL (End of Life). The spacecraft on-orbit dimensions are (deployed configuration): 4.6 m (X) x 16.3 m (Y) x 2.8 m (Z).

The spacecraft has a mass of about 1950 kg at launch (dry bus mass of 1374 kg, propellant mass of 176 kg, SGLI mass of 400 kg). The design life is 5 years.

Figure 7: Artist's rendition of the GCOM-C1 spacecraft (image credit: JAXA)

Launch: A launch of GCOM-C1 is planned for 2013 on an H-IIA vehicle from TNSC (Tanegashima Space Center), Japan.

Orbit: Sun-synchronous orbit, altitude = 798 km, inclination of 98.6º, LTDN (Local Time on Descending Node) at 10:30 hours.

RF communications: Direct real-time downlink of payload data to receiving stations with agreement. Downlink in X-band (8105 MHz) with a data rate of 138.76 Mbit/s, modulation = OQPSK (Offset Quadrature Phase Shift Keying).

Sensor complement: (SGLI, a single instrument is flown)

SGLI (Second-generation Global Imager):

SGLI is an advanced multi-purpose visible/infrared (VNIR, SWIR, TIR) imager of GLI heritage, flown on ADEOS-II. The objective is to measure ocean color, SST (Sea Surface Temperature), land use and vegetation, snow and ice, clouds, aerosols and water vapor, etc.

• The prime goal of SGLI is to retrieve global aerosol distributions. To achieve this target, SGLI will have 2 polarization channels with 3 directions

• SGLI is mainly focused to land and coastal areas. There are 11 channels with an IFOV of 250 m. GLI on ADEOS-II had only 6 channels of 250 m resolution.

The SGLI assembly features two separate sensors (radiometers) labeled VNIR (Visible Near Infrared) and IRS (Infrared Scanner). Note, the VNIR device is also referred to as VNR in the text.

• VNIR is a pushbroom instrument providing 14 channels in the VNIR spectral region (actually also in the UV), 11 channels are termed VNIR-NP (VNIR Non-Polarized), and 2 channels are called VNIR-P (VNIR-Polarized). The VNIR-P channels of the polarimeter provide 3 polarization angles at: 0º, 60º, and 120º.

The VNIR-NP channels are divided into three 24º pushbroom type telescopes configured in the cross-track direction to realize the wide FOV (70º) requirement with wide spectral range (380 nm to 865 nm). Each telescope has refractive telecentric optics and 11 channels CCD on which the '11 channel bandpass filter assembly' is mounted.

To realize the VNIR-P polarization observation, three linear polarization channels (0º, 60º and 120º) are set for two pushbroom telescopes which are dedicated for 670 nm and 865 nm observation. A tilting operation around the Y-axis of ±45º is required for VNIR-P to observe aerosols (scattering angle requirement). The scattering angle observation is calculated using the satellite orbital position, sun and observation target direction. A scattering angle direction between 60º and 120º is required for the aerosol retrieval over the land surface.

• IRS is a whiskbroom type scanning radiometer (mechanical method) covering the SWIR (Shortwave Infrared) and TIR (Thermal Infrared) spectral regions.

Figure 8: Schematic view of the SGLI instruments (image credit: JAXA)

The key VNIR observation channels such as 670 nm and 865 nm are being observed with both low and high dynamic range independently according to the requirements (Table 5). The total spectral channels for SGLI are optimized to 19 channels including tilting polarization observation (there were 36 channels for GLI instrument). On the other hand, the SGLI standard products are increased from 22 products of GLI to 29 products.

The basic IFOV (Instantaneous Field of View) is set to 250 m - compared to GLI’s 1 km requirement. Using this higher resolution with a wide FOV (1150 km for VNR and 1400 km for IRS), it is expected that the human activity influence on Earth's environment can be studied.

Table 4: Key parameters of the SGLI instrument

Table 5: Radiometric specification of the VNIR channels of SGLI

Table 6: Specification of the IRS (SWIR and TIR) channels of SGLI

The optical SGLI instrument is being designed and developed at NEC Toshiba Space, Tokyo, Japan. In turn, NEC Toshiba Space selected Sofradir of France to provide the infrared detectors for SGLI. As of 2008, Sofradir is providing concept studies for the cooled infrared MCT (HgCdTe)focal plane array detectors of the SGLI instrument. The two TIR arrays are centered on 10.8 and 12 µm wavelengths respectively, which are hybridized on a single readout circuit for accurate registration. ^{15) 16)}

Figure 9: Illustration of the SGLI VNR instrument (left) and its inner configuration (right), image credit: NEC Toshiba, JAXA

The IRS whiskbroom scanner features six channels in the region of 1.05 µm to 12 µm (Table 6). The 45º tilted scan mirror is rotated around the X-axis continuously to realize a scan of 80º for Earth observation; in addition, the onboard calibrator (blackbody, solar diffuser, and inner light source) and deep space are being scanned on each scanner revolution. Compared with the double-sided mirror employed on GLI and MODIS, the constant incident angle to the IRS scan mirror represents an advantage for the calibration function.

The observation light is directly focused onto the focal plane using a Ritchey-Chretien type telescope without any relay optics. The infrared spectral range is divided by the dichroic filter for the SWIR and TIR regions in order to optimize the detection process.

The four SWIR channels employ an InGaAs photodiode detector array cooled to -30ºC using a Peltier thermo electronic cooler. The two TIR channels use a photovoltaic type HgCdTe (PV-MCT) detector array cooled to 55 K by a Stirling-cycle cooler. The bandpass filters corresponding to the spectral channels are mounted on the focal plane in the detector packages.

The solar diffuser (made of Spectralon), the inner light source using LEDs (Light Emitting Diodes) for the SWIR channels and a high-emissivity blackbody for the TIR channels, are used as the onboard calibrator. These calibration sources and a deep space window, arranged around the scan mirror, make it possible to obtain calibration data on every scan.

Figure 10: Illustration of the SGLI IRS radiometer (top) and its inner configuration (bottom), image credit: NEC Toshiba, JAXA

GCOM ground segment and data distribution:

There will be two categories of observation data from the GCOM payload instruments: a) the global observation data set, which will be downlinked to the Svalbard station (Spitzbergen, Norway) on every orbit; b) the regional observation data around Japan (a subset), which will be downlinked to the JAXA domestic station on every pass of station visibility.

• The global observation data will be sent from Svalbard to TKSC (Tsukuba Space Center). They will be archived and processed at TKSC and be delivered to researchers and practical fields users.

• The regional observation data around Japan will be sent from the JAXA domestic station to TKSC. They also will be archived and processed at TKSC and be delivered to researchers and practical fields users.

JAXA will provide JMA (Japan Meteorological Agency) and JAFIC (Japan Fisheries Information Service Center) with the observation data of AMSR-2 and SGLI, respectively. JMA and JAFIC will use them for weather forecast and sea condition information, respectively.

The TT&C (Telemetry Tracking & Command) data will be downlinked to Svalbard via X-band, and to the JAXA ground network via S-band, and be sent to TKSC. TKSC is in charge of spacecraft monitoring and control including operations planning.

Figure 11: Overview of the GCOM ground segment elements (image credit: JAXA)

2) Information provided by Keizo Nakagawa of JAXA, Tokyo, Japan

3) K. Imaoka, “Status of the GCOM-W mission and AMSR follow on instrument,” AMSR-E Joint science team meeting ,University of Hawaii at Manoa, August 13, 2005 URL: http://www.ghcc.msfc.nasa.gov/AMSR/meetings2005/imaoka_status_gcomw.pdf

4) H. Shimoda, “Global change observation mission,” URL: http://www.cosis.net/abstracts/COSPAR2006/00204/COSPAR2006-A-00204.pdf?PHPSESSID=7d14646d7a0631d1dc351ad1f55d038b

5) http://www.jaxa.jp/projects/sat/gcom/index_e.html

6) H. Shimoda, “Global Change Observation Missions,” Proceedings of the 32nd ISRSE (International Symposium on Remote Sensing of Environment), San José, Costa Rica, June 25-29, 2007

7) H. Shimoda, “Global Change Observation Missions,” Proceedings of IGARSS 2007 (International Geoscience and Remote Sensing Symposium), Barcelona, Spain, July 23-27, 2007

8) http://www.jaxa.jp/pr/brochure/pdf/04/sat25.pdf

9) H. Shimoda, “Overview of Japanese Earth observation programs,” Proceedings of SPIE, `Sensors, Systems, and Next-Generation Satellites X,' edited by Roland Meynart, Steven P. Neeck, Haruhisa Shimoda, SPIE Vol. 6361, Stockholm, Sweden, Sept. 11-14, 2006, doi: 10.1117/12.691981

10) K. Imaoka, A. Shibata, N. Ebuchi, T. Igarashi, S. Fukui, K. Tanaka, T. Kimura, T. Sezai, Y. Tange, H. Shimoda, “Overview of the GCOM-W mission and AMSR follow-on instrument,” Proceedings of SPIE, Vo. 5978, 2005, pp. 49-56, `Sensors, Systems, and Next-Generation Satellites IX. Edited by R. Meynart, S. P. Neeck, and H. Shimoda,' doi: 10.1117/12.631493

11) Haruhisa Shimoda, “Overview of Japanese Earth Observation Programs,” 9th PORSEC 2008 (Pan Ocean Remote Sensing Conference), Dec. 2-6, 2008, Gngzhou, China, URL: http://ledweb.scsio.ac.cn/cd2/%E9%97%AD%E5%B9%95%E5%BC%8F/JAXA%20program08.ppt

12) http://www.ghcc.msfc.nasa.gov/AMSR/meetings2006/pdf/yamanashi2006amsre_jtsm_gcom.pdf

13) Takamasa Itahashi , Keizo Nakagawa, “Thermal design overview of AMSR2/GCOM-W1 satellite,” Proceedings of the 27th ISTS (International Symposium on Space Technology and Science) , Tsukuba, Japan, July 5-12, 2009, paper: 2009-f-02, URL: http://www.senkyo.co.jp/ists2009/papers/html/pdf/2009-f-02.pdf

14) Global Change Observation Mission 1st Research Announcement, “AMSR2 on GCOM-W1 Algorithm, Validation, and Application,” Jan. 14, 2008, URL: http://sharaku.eorc.jaxa.jp/AMSR/AMSR2_RA/documents/GCOM_RA1_E.pdf

15) P. Chorier, A. Dariel, L. Vial, A. Poupinet, M. Vuiliermet, P. Tribolet, “Sofradir advances in Infrared detectors for Space Applications,” Proceedings of the 26th ISTS (International Symposium on Space Technology and Science) , Hamamatsu City, Japan, June 1-8, 2008

16) Yoshihiko Okamura, Kazuhiro Tanaka, Takahiro Amano, Masaru Hiramatsu, Koichi Shiratama, “Design and bread-boarding activities of the Second-generation Global Imager (SGLI) on GCOM-C,” Proceedings of the 7th ICSO (International Conference on Space Optics) 2008, Toulouse, France, Oct. 14-17, 2008, URL: http://www.icsoconference2008.com/cd/pdf/S4%20Imagers%20-%20Okamura.pdf