THEOS (Thailand Earth Observation System)
THEOS is an Earth observation mission of Thailand, under development at EADS Astrium SAS, Toulouse, France. In July 2004, EADS Astrium SAS signed a contract for delivery of THEOS with GISTDA (Geo-Informatics and Space Technology Development Agency) of Bangkok, Thailand. GISTDA is Thailand's leading national organization (i.e., space agency) in the field of space activities and applications. The Thai Ministry of Science and Technology is funding the program.
The THEOS cooperative agreement includes the production and launch of one optical imaging satellite, as well as the development of the ground segment necessary to operate and control the satellite directly from Thailand. The contract also specifies on-the-job training of Thai engineers as part of the EADS Astrium development team. Also as part of the THEOS program, GISTDA earned the right to receive data from the SPOT-2, 4 and 5 spacecraft of CNES in Thailand, which have many features similar to those of THEOS. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
The prime objective of THEOS is to provide Thailand with an affordable access to space (i.e., a state-of-the-art Earth observation satellite for the near future), and to spawn through this program's operational experience the country's own capability and infrastructure an indigenous potential for the development of future space missions.
The science objectives call for the provision of:
1) Panchromatic (2 m) and multispectral (15 m) imagery from THEOS observations, and
2) The generation of geo-referenced image products and image processing capabilities for applications in the fields of cartography, land use, agricultural monitoring, forestry management, coastal zone monitoring and flood risk management.
The Thai government has also expressed its intention to offer THEOS data to the disaster mitigation efforts under the International Charter.
Figure 1: Artist's rendition of the THEOS spacecraft (image credit: EADS Astrium, GISTDA)
The THEOS satellite is based on the new generation of EADS Astrium Optical Earth Observation high performance satellites using the medium-sized AstroSat-500 platform. In particular, the spacecraft design/structure is of FormoSat-2 (formerly ROCSat-2) heritage, a spacecraft built by EADS for NSPO of Taiwan with a launch on May 20, 2004. 14) 15) 16)
The THEOS satellite consists of two parts, the payload for the imaging (with cameras and associated electronics) and the bus, in charge of all service functions. The bus structure is of size: 2.1 m x 2.1 m x 2.4 m (height). The satellite is three-axis stabilized. The upper deck of the platform carries the payload and also the attitude sensing devices of AOCS (Attitude and Orbit Control Subsystem), namely the star and sun sensors, gyroscopes, and GPS receiver. The lower deck contains the actuation devices: magnetic torquer, the four reaction wheels and the autonomous propulsion module.
The fixed solar array uses GaAs cells and consists of two deployable flaps providing a power of 840 W. The entire S/C architecture is designed in such a way as to provide a low roll inertia, a key factor for satellite agility and instrument line-of-sight stability. The S/C is very agile providing a body-pointing capability of ±45º in roll and pitch (45º pitch in 60 s, 10º roll in 25 s, 30º roll in 60 s, respectively). The S/C wet mass is about 750 kg with 82 kg of hydrazine propellant mass. The design life is five years or better.
Figure 2: Illustration of the THEOS spacecraft (image credit: EADS Astrium)
Table 1: Overview of some THEOS parameters
Figure 3: Designation of some elements of the THEOS spacecraft (image credit: GISTDA, EADS Astrium)
Launch: The THEOS spacecraft was launched on a Dnepr vehicle from the Yasny/Dombarovsky launch center (a town in Orenburg Oblast, Russia, 51.0º N, 58.0º E) on October 1, 2008. The launch provider was ISC Kosmotras of Moscow, a Russian-Ukrainian joint venture company.
Orbit: Sun-synchronous near-circular orbit, altitude = 822 km, inclination = 98.7º, period = 101.4 min, local equator crossing time at 10:00 hours on a descending node, repeat cycle = 26 days (14 5/26 orbits per day). Accessibility: 2 days with 50º tilting angle, and 5 days with 30º tilting angle.
The THEOS spacecraft mission orbit has the same repetivity (14 + 5/26) as the SPOT spacecraft, i.e. the same altitude of 822 km, but a different mean local solar time. THEOS and SPOT satellites follow the same grid on Earth.
Figure 4: Argument of latitude phasing (image credit: GISTA, EADS Astrium)
The 26 positions shown on Figure 4 correspond to the possible positions in argument of latitude to match the SPOT ground track reference grid. In order to avoid simultaneous visibility of the SPOT-5 and the THEOS spacecraft from the Thai ground station, the THEOS argument of latitude shall be selected in the interval shown on Figure 4. This choice takes into account the end of life drift in local solar time of SPOT-5 and ensures a minimum separation with SPOT-2.
The local solar time (LST) at descending node is 10:00 hours for the THEOS mission and 10:30 hours for SPOT.
• The THEOS spacecraft and its payload are operating nominally in 2012.
• In Dec. 2011, Orbit Logic Inc. of Greenbelt, MD, USA signed a contract with GISTDA of Thailand for the delivery of a collection feasibility solution for the THEOS imaging satellite. Under the terms of the contract Orbit Logic will host the off-the-shelf, web-based CFT (Collection Feasibility Tool) configured for the THEOS satellite on an Orbit Logic server for exclusive use by GISTDA. An enhanced version of CFT will be delivered to GISTDA in early 2012 for hosting on a GISTDA server for operational use by GISTDA and THEOS imagery customers. 17)
• On December 15, 2010, an emergency orbital maneuver was conducted to avoid a close approach of space debris. 20)
The THEOS satellite project represents a significant milestone in Thailand’s space activities; it brought about a major progress in terms of facility, infrastructure and human resources developments for the country. Developments for future THEOS global online data service provision are underway.
Figure 5: Timeline of THEOS operations (image credit: GISTA, EADS Astrium)
• On June 1, 2009, official announcement of THEOS data service opening to domestic users. This occurred after successful in-orbit tests, further calibration-validation for about 3 months and preparation for operational data service.
• By the end of 2008, all space segment and ground segment functions were verified. In January 2009, there was a successful in-orbit review and the system was declared “operational”. 24)
• The IOT (In Orbit Test) phase started on Oct. 17, 2008 consisting of two cycles of 26 days each. All specified system performances of the spacecraft and its payload were tested, validated and calibrated in this period. In parallel, imagery of certain regions was acquired for image calibration and validation.
• In the period Oct. 4-14, 2008, a series of orbit transfer maneuvers was conducted to raise the orbit from 680 km to the final nominal orbit of 822 km.
• On Oct. 3, 2008, THEOS took the first set of images over Bangkok, Phuket Island and some regions outside Thailand. 25)
• LEOP (Launch and Early Operations Phase): After the satellite was separated from the launcher and placed into the initial orbit at an altitude of 680 km, the communications system was activated and the spacecraft came into first contact with the Kiruna ground station of SSC (Swedish Space Corporation) in Sweden. A series of checks was conducted followed by the switch-on of several subsystems. Then the spacecraft was commanded into normal mode; the first mission plan was uploaded on the next day, Oct. 2, 2008.
Figure 6: First MS image of THEOS of Phuket Island (Thailand) taken on Oct. 3, 2008 (image credit: GISTDA)
Sensor complement: (PAN Camera, MS Camera)
The optical instruments are composed of a panchromatic and a multispectral camera. Both instruments employ an electronic unit in support of the following functions: gathering of video data, numerical conversion, compression, formatting, etc. Both cameras are of the pushbroom imaging type using linear CCD arrays in the focal plane. The primary mirror and the focal plane are made of silicon carbide (SiC) to ensure light weight and good thermo-elastic stability (alignment). 26) 27)
Figure 7: Assembly of the THEOS sensor complement at EADS (image credit: EADS Astrium)
PAN Camera (Panchromatic Camera):
This instrument is providing a spectral range of 0.45 to 0.90 µm. The spatial resolution is 2 m on a swath width of 22 km.
Figure 8: Illustration of the Pan Camera (image credit: EADS-Astrium)
MS Camera (Multispectral Camera):
The camera is a dioptric sensor type with 4 filters - providing 4 bands in the spectral range of 0.45 to 0.90 µm. The spatial resolution is 15 m on a swath width of 90 km.
Table 2: Characteristics of the optical instruments
THEOS features 2 telescopes in parallel: 1 catadioptric telescope of 600 mm aperture diameter (Korsch telescope design) for Pan alone, and one refractive telescope (aperture diameter of ~100 mm) for MS alone. The two telescopes on the spacecraft, providing different swaths, can be seen in Figures 2 and 3. Note: In the RSI imager design on FormoSat-2 (of EADS Astrium SAS), a single catadioptric telescope with 600 mm aperture is being used for both, Pan and MS observations (providing the same swath of 24 km).
Figure 9: Nominal observation configuration of THEOS (image credit: GISTDA, EADS Astrium)
The high agility of the spacecraft provides a wide FOR (Field of Regard) to image various targets as illustrated in Figures 10 and 11. A FOR (or accessible corridor) of 1000 km can be provided within a roll maneuver of ±30º. Oblique viewing can be used to increase the viewing frequency for a given point during a given cycle.
Stereo imaging: Stereo pair can be used for relief perception and elevation plotting (Digital Elevation Modelling). Thesestereo images can be acquired by THEOS with 2 different methods:
1) The programming of two images of the same areaon the ground acquired at different roll viewing angles on successive satellite passes
2) The pitch agility allows acquiring a stereo pair in thesame pass at less than 5 minute delay.
Figure 10: Stereo observation scenario (image credit: GISTDA, EADS Astrium)
Repeat viewing capability: The THEOS oblique viewing capability allows for the imaging of any area within a 1000 km swath (for 30° roll). Oblique viewing can be used to increasethe viewing frequency for a given point during a givencycle. The frequency varies with latitude: over Thailand, a given area can be imaged 9 times duringthe same 26-days orbital cycle. This means a yearly revisit of 126 times and an average of 3 days, with an interval ranging from a minimum of 1 day to a maximum of 5 days.
Figure 11: Event monitoring scenario of various targets (image credit: GISTDA, EADS Astrium)
GISTDA is the operating agency of the THEOS spacecraft. The ground segment consists of a control center at Sriracha (Chonburi Province, Thailand), along with an S/X-band station to control and monitor the satellite from Thailand and to receive the payload imagery within its range of coverage. The Sriracha complex is located about 100 km from Bangkok featuring two 4.5 m antennas for S-band and a 13 m antenna for X-band data acquisition (Ref. 10). 28) 29) 30)
In addition, there is a payload processing center to acquire, process, archive, and exploit the imagery. The THEOS image acquisition is performed on user requests. The THEOS Image Ground Segment is installed at the Thailand Ground Station Compound in Lad Krabang, Bangkok.
The THEOS ground segment consists of two parts:
• CGS (Control Ground Segment)
• IGS (Image Ground Segment)
Figure 12: Architecture of the THEOS ground segment (image credit: GISTDA)
The CGS (Control Ground Segment) consists of 4 main subsystems which are:
• MCP (Mission Planning Center)
• FDS (Flight Dynamics System)
• SCC (Satellite Control Center)
• S-band station as shown in Figure 13.
Figure 13: Architecture of the THEOS CGS, (image credit: GISTDA)
Figure 14: Thailand ground station coverage circle of 2000 km radius (image credit: GISTDA)
GISTDA-NSPO framework of cooperation:
The failure of the S-band antenna due to lightning strike brought about the initiative for the Inter-operability project. For several weeks continually, GISTDA’s engineers had to interface with SSC’s (Swedish Space Corporation) S-band system in Kiruna in order to communicate with the THEOS spacecraft. This incident instigated a discussion to optimize the system to reduce the probability of failure of the system. The conclusion that was arrived at was that other agencies must also be facing similar problems and that brought about the quest to find global partners for inter-agency cooperation that would cut costs and increase resources. 31)
On account of this incident, GISTDA got into ground segment cooperation discussions with NSPO (National Space Organization) of Taiwan. NSPO is currently operating the FormoSat-2 mission and is in the process of developing FormoSat-5.
After the initial discussion in Taiwan in June 2010, GISTDA and NSPO started to work hand-in-hand to improve the inter-operability of their ground stations in order to create a common pool of resources under the banner of the mutual coordination project called ICE (Inter-Operability, Cooperation and Engineering). It turns out there are quite a few commonalities and characteristics between both projects. By sharing the resources, both parties aim to learn from each other and increase the knowledge pool of its employees.
In the fall of 2010, an agreement for long-standing cooperative relations between GISTDA and NSPO, was signed. It covers cross-support in the following areas:
• Satellite system health review
• Ground station support including Bi-directional TT&C services
• Satellite performance enhancement
• Satellite system life extension
• Mission operations and Ground Data Systems services.
The agreement means GISTDA and NSPO can provide each other network support and space operations services more quickly; this is considered as very significant. The sharing of resources is a sensible and efficient way to achieve enhanced space science value in an era of tight budgets; particularly, the bi-directional sharing of TT&C services will enhance effectiveness and reduce risk for both agencies. This cooperation will benefit both parties by providing back-up in case a mission's ground station is not available due to maintenance, weather or disasters, by ensuring additional station support during critical mission phases and by expanding station resources.
The real-time network communications is provided through secure communication channel via VPN (Virtual Private Network) that links the SCC (Satellite Control Center) of GISTDA to the RTS (Remote Terminal System) of NSPO and the SOCC (Satellite Operation and Control Center) of NSPO to the SBS (S-band Subsystem) of GISTDA (Figure 15).
Figure 15: Interlinking between components of NSPO and GISTDA (image credit: GISTDA)
The satellite control systems and antenna systems support end-to-end data security when using the communications services. Security implementation includes IP address authentication, confidentiality of socket port numbers, IPSec encryption, tunnel encryption and physical access control to the mission systems. Communication via the VPN may take place in real-time or non real-time. 32)
On February 9, 2011, the GISTDA operations team completed the first successful operation of the FormoSat-2 satellite through the GISTDA antenna subsystem. Just one day later, on February 10, 2011, the first successful test of operation of the THEOS spacecraft was conducted by the NSPO operations team.
From the first successful test onwards, the aim of the project moved from just the success of each pass to reducing possible human errors and removing system limitations. Automation processes were implemented and efforts were implemented to make the operation through the secondary antenna station just like nominal operations support.
Future plans: The ICE project holds a considerable potential for expansion. With the success of the ICE project, GISTDA aims to extend this project to more partners and finally, start a consortium. This cooperation sets in motion a possibility for new small Earth observation and small scientific satellite operators to participate and gain from a group similar to the Landsat International Ground Station network.
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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.