Thailand Earth Observation System (Theos) Development Program
The program is developed by Geo-Informatics and Space Technology Development Agency (Public Organization) (GISTDA) under the Ministry of Science and Technology, with European Aeronautic Defence and Space Company – Astrium (EADS Astrium SAS), in France, as a prime contractor. It consists of design, development, test, launch, and commissioning phases of THEOS satellite, as well as integration with associated control and data exploitation ground facilities.
This paper gives an overview of system architecture, satellite architecture, its performance and main applications, as well as technology transfer and trainings for Thai engineers and operators. THEOS satellite is a compact and agile satellite with a mass of 750 kilograms and a power consumption of 800 Watts. It has two push-broom scanning optical instruments, providing worldwide imagery in Panchromatic and Multispectral modes, covering the Visible and Near Infrared spectrum. The Panchromatic instrument has a resolution of 2 metres and a swath width of 22 kilometres.
The Multispectral instrument has a resolution of 15 metres and a swath width of 90 kilometres. The satellite will be launched into a sun-synchronous low earth orbit in July 2007 and will have a design lifetime of at least 5 years. 1. Introduction After many years of using remote-sensing data from foreign satellite systems, Thailand has decided to have its own earth observation satellite system for rapid and sustainable country development. Thailand Earth Observation System (THEOS) Development Program was then begun in July 2004.
The program is developed by Geo-Informatics and Space Technology Development Agency (Public Organization) (GISTDA) under the Ministry of Science and Technology, with European Aeronautic Defence and Space Company – Astrium (EADS Astrium SAS), in France, as a prime contractor. THEOS system, which is fully owned by Royal Thai Government, will be operated by GISTDA. 1 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 2. Architecture of THEOS System THEOS system is a stand-alone earth observation system.
The architecture of THEOS system has been developed in order to satisfy needs and requirements from GISTDA, Ministry of Science and Technology, as well as other relevant Ministries and imagery users in Thailand. These requirements include resolutions, swath width, coverage area, revisit frequency, as well as image quality and applications. THEOS system comprises of the following main constituents, as shown in Figure 1. THEOS Satellite (Space Segment) Ground Facilities (Ground Segment) • • Control Ground Segment (CGS) Image Ground Segment (IGS) !
Figure 1: Architecture of THEOS System 2 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 3. Performance of THEOS Satellite The payloads of THEOS Satellite are Panchromatic and Multispectral pushbroom scanning optical instruments. The radiometric and spatial performances of the instruments have been optimised between resolutions and swath width in order to satisfy imagery application requirements in Thailand. The spectral range of each imagery band of Panchromatic and Multispectral instruments is shown in Table 1 and Figure 2.
Table 1: THEOS Spectral Bands Name PAN B0 B1 B2 B3 Band Panchromatic Multispectral / Blue Multispectral / Green Multispectral / Red Multispectral / Near-Infrared Band Width 0. 45 – 0. 90 µm 0. 45 – 0. 52 µm 0. 53 – 0. 60 µm 0. 62 – 0. 69 µm 0. 77 – 0. 90 µm 100 PANCHROMATIC B0 BAND B1 BAND B2 BAND B3 BAND WHITE LIMESTONE 80 REFLECTANCE (%) 60 SAND 40 VEGETATION DRY SOILS 20 WATER 0 0. 40 0. 50 0. 60 0. 70 0. 80 0. 90 WAVELENGTH (µm) Figure 2: THEOS Spectral Bands The spatial performance of Panchromatic and Multispectral instruments is shown in Table 2 and Figure 3.
THEOS satellite has a tilting capability of up to ± 50°, although some high accuracy applications might limit this to ± 30°. It can be tilted both in fore and aft, as well as east and west direction of satellite ground track. 3 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 Table 2: Spatial Performance of THEOS Satellite Parameter Ground Sampling Distance Number of Pixels Swath Width Accessible Corridor (at ± 30° Tilting Angle) Panchromatic 2m 12,000 pixels 22 km at Nadir 1000 km Multispectral 15 m 6,000 pixels 90 km at Nadir 1100 km
Figure 3: Spatial Performance of THEOS Satellite The orbit of THEOS satellite will have parameters as shown in Table 3. With this orbit and its tilting capability, THEOS satellite will have worldwide imagery area, as well as short revisit and access time, as shown in Figure 4 and Figure 5. It can be seen from Figure 5 that if the satellite is tilted to its maximum capacity of 50°, accessible corridor will already cover 90% of the whole earth surface within 1 day.
Table 3: THEOS Orbit Parameters Parameter Orbit Type Altitude Number of Orbits per Day Local Equator Crossing Time Orbital Period Orbit Inclination Orbit Cycle Distance Between Passes (at Equator) Coverage Area Visibility Area Access time Value Circular sun-synchronous low earth orbit 822 km 14+5/26 orbits per day 10:00 am 101 minutes98. 7 98. 7° 26 days (369 orbits) 108 km (Between 2 closest passes) 2800 km (Between 2 consecutive passes) All earth surface Radius of more than 2000 km from ground station (at 5° elevation angle) 2 days with 50° tilting angle 5 days with 30° tilting angle 4
The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 Figure 4: Accessible Corridor Covered in 5 Days (with 30° tilting angle) ° Figure 5: Accessible Corridor Covered in 2 Days (with 50° tilting angle) ° 4. Architecture of THEOS Satellite THEOS satellite consists of two main parts, the optical instrument payload and the platform or bus. That platform is based on AstroSat 500 Bus, which has already been designed and developed by EADS Astrium. All the technologies used in THEOS Satellite are space qualified and have been used in other French earth observation programs.
This can ensure the minimum risk and guarantee the success of the THEOS mission. 5 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 THEOS platform consists of the following subsystems. On-board management unit Image processing unit S-band unit for telemetry and telecommand link X-band unit for payload data downlink Power distribution and regulation unit Thermal control system for both the payload and the platform Attitude and orbit control system, which has the following sensors and actuators • • Magnetometer, sun sensor, star sensor, gyroscope, and GPS receiver Magnetotorquers, reaction wheels and thrusters.
The specifications of THEOS satellite are summarised in Table 4. Table 4: Specifications of THEOS Satellite Parameter Total Mass Size Power Consumption Nominal Life Time Fuel and Capacity Data Processing Data Downlink Rate Obit Determination Payload 750 kg 2. 1 m x 2. 1 m x 2. 4 m 800 W > 5 Years Hydrazine 80 kg Compression ratio of 2. 8 or 3. 7 120 Mbit/s (X-band) GPS Panchromatic and Multispectral cameras Value Payload Data Recorder 40 Gbit solid-state memory Satellite Ground Speed 6. km/s The AstroSat 500 Bus has a shape of a hexagonal with a length of 1. 5 metre on each side, as shown in Figure 6. The size is determined by the area for equipment installation on each panel as well as to be compatible with small launcher fairing with a diameter of 2 metres. The top of the platform is attached to payload interface plate. This is where optical instrument payload as well as star sensor and gyroscope are mounted, for high accuracy pointing. At the bottom of the platform are 4 eaction wheels and 4 thrusters for attitude and orbit control. In the middle of the platform is propulsion module and fuel tank. All other subsystem units are mounted on each panel for the reasons of balance, thermal control, electrical and mechanical aspects, as well as flexibility of integration and test. 6 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 Panchromatic telescope Multispectral Camera Star Sensors Gyroscopes Solar Array X-band Antenna S-band Antenna
Thermal Radiator Figure 6: AstroSat 500 Bus Once in the orbit, the operation can be divided into 3 main modes. • • • Acquisition and Safe Hold Mode, which will be automatically activated when the satellite is first injected to the orbit by a launcher or when an anomaly occurs on the satellite. Normal Mode, which is used in idle and imaging operation Orbit Control Mode, which is used when propulsion module has to be activated in order to correct the orbit, due to air drag and other perturbations.
After the satellite is released from launcher, solar array will be deployed, acquisition sequence will be activated, and first ground contact will be made. When the satellite is switched to Acquisition and Safe Hold Mode from Normal Mode due to anomaly, satellite will be reinitialised and recovered. During this time, the satellite will have to be able to rely on its autonomy, without any ground contact.
This is achieved by switching off all unnecessary equipments to ensure sufficient power, and use only the most reliable equipments, which are sun sensor, magnetometer, magnetotorquers, and reaction wheels in order to ensure sun pointing configuration during sun light for power generation, and earth pointing configuration during eclipse for ground contact. In Normal Mode, the satellite will be kept in the sun pointing configuration during sun light, and optical instrument will be in stand-by mode.
When imaging is required, the satellite will be earth pointed with some tilting in order to access the target area. During eclipse, the satellite will be kept in earth pointing configuration for telecommand uplink as well as telemetry and data downlink. 7 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 5. Applications of THEOS Data Main applications of THEOS data in Thailand are in many different fields, including cartography, agriculture, forestry, land use, coastal monitoring, geology, irrigation, drought and flood management, as well as national security.
Required performance of satellite data in different fields are shown in Table 5, in terms of scale, coverage area, and revisit period. Also, since THEOS satellite has a tilting capability of up to ± 50°, both in fore and aft, as well as east and west direction of satellite ground track, images of the same area from different angles can be processed to create digital terrain model of the area. This adds more use in many fields and creates even more applications.
Table 5: Required performance of satellite data for main applications Application Cartography Agriculture Agricultural map Plant growth monitoring Illegal plantation Forestry Forest monitoring Forest management Forest fire control Land Resources Land use Urban development Coastal monitoring Geology Spatial Requirements Revisit Period (Scale, Coverage Area) Whole country in less than 5 1:25,000 (1) years Map/monitoring: Area of 1 hectare (2) Illegal plantation: Area of 0. 2 hectare Forest monitoring: Area of 9 hectare Forest management: Area of 0. hectare 1:50,000 for general use 1:25,000 (1) for local applications Structural geology: Area of 100 m2 For rivers with a width of less than 15 m For checking change of water level of more than 10 m Sample area more than 6 times a year (2 times for each growth cycle) Local area, as requested, in less than 1 month Whole country in less than 2 years Local area, as requested, in less than 1 month Whole country in less than 5 years Local area, as requested, in less than 1 month Whole country in less than 5 years Local area, as requested, in less than 1 month World water network every year Every 6 days
Irrigation Flood risk management : Image data in Panchromatic mode has a resolution of 2 m, which can generate 1:25,000 scale maps. : Area of 1 hectare equals 10,000 square metres 8 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 6. Control Ground Segment There will be only one control ground segment in Thailand. This will consist of all necessary equipments for Thai operators to monitor and control the satellite automatically and safely, in order to guarantee the success of the mission.
Control ground segment of THEOS system consists of 3 main parts S-Band Telemetry, Telecommand and Control Centre • Detect and check contact link when the satellite enters visibility area • Receiving and sending telemetry and telecommand through S-Band link Satellite Control Centre • Check and monitor health status and control the satellite • With Flight Dynamic System, perform orbit determination, orbit propagation, and required manoeuvres Mission Planning Centre • Collect user requests • Issue daily optimised work plan, by taking into account user requests and satellite utilisation 7.
Image Ground Segment Receiving and processing ground facilities have been designed and developed to be compatible with the existing high performance X-band station of GISTDA, which includes a 13-metre diameter antenna. By using simple architecture, and all qualified components, the success and smooth operation can be ensured. Image ground segment of THEOS system consists of 2 main parts X-band receiving station • Receive image data through X-band downlink •
Demodulate and base-band process Image processing facility • Frame synchronise and reformat • Store image data flow • Create and update catalogue • Store image data for future use or when requested • Perform image correction at different levels, including radiometric, geometric correction, with and without reference map or ground control points, as well as digital terrain model generation Image exploitation facility • Process image for specific applications The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 8. Technology Transfer and Trainings One important part of this program is technology transfer and trainings for Thai engineers and operators, which cover theory and on-the-job training on design, development, test, and control of the satellite, as well as receiving and processing image data. Different components of technology transfer and training program are as follow.
THEOS program training, with the objective for the 20 Thai engineers to be key people of Thailand’s long-term space activities, as well as to supervise and support THEOS in-orbit operations • Theoretical technology transfer on satellite technology, design, integration and test of satellite system, as well as control and operations at EADS Astrium Space School, with a duration of 3 months for Introduction Courses, and 6 months for Advanced Courses.
On-the-job training, where 20 Thai engineers will have the opportunity to get involved in actual activities of THEOS development from the beginning of the development to the launch date. The 20 Thai engineers can be divided into 3 groups as follow. o Ground system (5) ? ? ? ? ? • • System and ground segment development engineer System and ground segment engineer Control ground segment engineer Image ground segment engineer Image and products engineer o Satellite Operation and Control (4) ? ? System operation engineer (2) Attitude and orbit control engineer (2) Space Segment (11) ? ? ? ? ? ? ? ? ? ? Satellite engineer (2) System engineer Electrical engineer Mechanical engineer Thermal engineer Software engineer Instrument engineer Command and data handling engineer Assembly integration and testing engineer Product assurance engineer 10 The 2nd Asian Space Conference, Hanoi, Vietnam, 8-11 November 2005 THEOS operational training, for 20 Thai operators to be able to operate, control and maintain THEOS satellite, as well as receive and process image data.
Yearly technical seminar, to provide key issues about space technology, satellites, earth observation, and space program management to wider audience in Thailand Scholarships for master and doctoral degrees in space and satellite technology in France 9. Conclusion Thailand Earth Observation System (THEOS) Development Program comprises of THEOS satellite as well as associated control and data exploitation ground facilities. The program also includes a set of technology transfer and trainings for Thai engineers and operators.
THEOS Satellite is a compact and agile satellite with a mass of 750 kilograms and a power consumption of 800 Watts. It has two push-broom scanning optical instruments, providing worldwide imagery in Panchromatic and Multispectral modes, covering the Visible and Near Infrared spectrum. The Panchromatic instrument has a resolution of 2 metres and a swath width of 22 kilometres. The Multispectral instrument has a resolution of 15 metres and a swath width of 90 kilometres. The satellite will be launched into a sun-synchronous low earth orbit in July 2007 and will have a design lifetime of at least 5 years.