RASAT (Earth Observation Satellite)
RASAT is a microsatellite imaging mission of TUBITAK-UZAY (Scientific and Technological Research Council of Turkey - Space Technologies Research Institute), Ankara, Turkey. The project is funded by the State Planning Office. Note: Prior to May 2006, TUBITAK-UZAY was referred to as TUBITAK-BILTEN.
RASAT is a Turkish word with a basic meaning of “observation”. There are connotations to Rasat as well, for instance the term “Rasathane” means 'astronomical observatory'. Hence, it is felt that “Earth Observation Satellite” is a most fitting interpretation of RASAT.
The overall objective is to establish and develop small satellite technologies in Turkey. It is the first remote sensing satellite that is being developed and manufactured in Turkey by Turkish engineers. The predecessor mission was BILSAT which was launched on Sept. 27, 2003, a cooperative mission of TUBITAK-BILTEN, Turkey, and of SSTL, Surrey, UK - involving on-the-job training of a Turkish team of engineers at SSTL. BILSAT was part of the DMC (Disaster Monitoring Constellation) - it was operational until August 2006 when it experienced a failure in its battery system.
Although the mechanical design of the RASAT platform is mainly based on the one of BILSAT heritage, the RASAT spacecraft will carry a number of new modules which are being designed, developed and integrated at the the TUBITAK-UZAY facilities. The main purpose of the RASAT program is to further improve the experience and skills acquired by the BILSAT program, as well as to further improve the infrastructure required to assemble, integrate and test small satellites in Turkey without receiving any foreign resources in terms of advisors or partners. 1) 2) 3) 4) 5) 6) 7) 8)
The RASAT project started in 2004. The CDR (Critical Design Review) took place in January 2008. The requirements call for an optical imager with a resolution of 7.5 m in the panchromatic band and 15 m in the multispectral bands. New components/payloads on RASAT are BILGE, ALP, GEZGIN-2, and the introduction of an X-band transmitter for an efficient downlink transmission rate.
Figure 1: Illustration of the RASAT spacecraft (image credit: TUBITAK-UZAY)
The RASAT microsatellite is based on the BILSAT bus and module heritage. The main stack uses the load carrying structure and employs trays for the various spacecraft subsystems. Unlike BILSAT’s honeycomb panels, the Earth and space facing facets are being built of aluminum parts with an internal web structure; the reason: the domestic suppliers aren't familiar so far with high-precision honeycomb manufacturing processes. 9)
AODCS (Attitude and Orbit Determination and Control Subsystem): The spacecraft is 3-axis stabilized. AODCS uses four reaction wheels and three magnetorquers as actuators. The attitude is being measured with four analog fine sun sensors, two fluxgate magnetometers, a MEMS gyroscope, a star tracker and a GPS reciever. The AODCS hardware (wheels, gyro assembly, and star tracker) is being controlled by the OBC using a dual redundant CAN bus. RASAT is a very agile spacecraft providing an off-nadir body-pointing capability of ±30º in all directions. 10)
Figure 2: Allocation of the ADOCS equipment on RASAT (image credit: TUBITAK-UZAY)
OBC (On-Board Computer): The legacy OBC is comprised of a 386 based computer of BILSAT heritage. It accommodates a solid-state data recorder based on a PowerPC processor. BILGE – a new generation advanced OBC - will actually serve either as the OBC, or as a data storage unit, or both (the legacy system is intended to serve as backup). All the subsystems on RASAT are connected to a dual redundant CAN (Controller Area Network) bus. Also, two of the new subsystems, the new OBC (BILGE) and new SSDR (Solid-State Data Recorder) are furnished with the experimental MIL-STD-1553B bus. The heritage high-speed data links consist of raw data transmission over the LVDS layer. These run at speeds of 20 Mbit/s. In addition, BILGE features radiation-hardened SDRAMs (Synchronous Dynamic Random Access Memory).
The new high-speed data links onboard RASAT make use of the ESA standard SpaceWire connections with data rates of 130 Mbit/s. The latter interface runs between the new OBC, SSDR, the signal processing subsystem, and a new X-band transmitter module. 11)
Figure 3: Block diagram of the BILGE architecture (image credit: TUBITAK-UZAY)
BILGE is basically a COTS PowerPC based computer of Freescale. The popular PowerQUICCII communications processor is employed at the heart of the system running at 50 MHz bus speed and at 200 MHz core speed. BILGE uses the RTOS (Real Time Operating System) of Integrity with the feature of virtual address space. BILGE can communicate through various communication links. They can be listed as follows:
- 2 ports of 1 Mbit/s CAN bus
- 2 ports of 1 Mbit/s MIL-STD-1553B bus
- 3 ports of 130 Mbit/s SpaceWire bus
- Up to 8 duplex ports of 8 Mbit/s HDLC over opto-isolated or LVDS (Low Voltage Differential Signaling) channels
- 3 ports of 25 Mbit/s DATA-CLK-DV raw data over LVDS.
EPS (Electric Power Subsystem): Spacecraft power is provided by 3 outside panels that are being covered with surface-mounted solar cells ( ±X and +Y sides). The 4th panel on the -Y side has an elliptical cutout of the solar panel to accommodate the star sensor. A maximum power of 65 W is provided (average on-orbit power of 42-52 W depending on the season. The power is distributed at 28 V and at 5 V. For heritage reasons, 6 Ah NiCd batteries at 28 V will be used on RASAT. 12) 13)
In addition, new lithium-ion batteries, referred to as ALP, have been developed in the framework of the RASAT program.
RF communications: The TT&C links are being provided in S-band as well as in UHF/VHF bands, the latter subsystem is being used as a backup due to its long heritage in small space missions. Since a UHF/VHF implementation features better omni directional characteristics than S-band, the UHF/VHF subsystem is also being used for the commissioning phase as the prime TT&C system.
The RASAT spacecraft has a total mass of ~95 kg and a design life of 3 years.
Figure 4: Illustration of the RASAT structural configuration (image credit: TUBITAK-UZAY)
The following items were introduced with the RASAT project:
• System level design
• High performance flight computer (BILGE) and SpaceWire router
• All flight software
• All ground station software
• A 100 Mbit/s X-band transmitter
• Real-time processing, compression and encryption board (GEZGIN-2). GEZGIN-2 includes the total integration of the JPEG2000 algorithm and new image processing and encryption capabilities.
• Li-ion battery, ALP (Intelligent Li-ion battery charge regulator).
Table 1: Overview of spacecraft parameters
Launch: The RASAT spacecraft was launched as a secondary payload on August 17, 2011 on a Dnepr-1 launch vehicle using the SHM (Space Head Module) configuration from the Yasny/Dombarovsky launch site located in the Orenburg Region, Russia. The launch provider is ISC Kosmotras. 15) 16) 17)
The primary payload of the cluster launch will be the Sich-2 spacecraft of NSAU (National Space Agency of Ukraine) with a launch mass of 175 kg.
The secondary payloads on this flight are:
• NigeriaSat-2 of NASRDA, Abuja, Nigeria, spacecraft mass = 270 kg
• NigeriaSat-X of NASRDA,, Abuja, Nigeria, spacecraft mass = 87 kg
• EduSat of the University of Rome (sapienza), Italy, spacecraft mass = 10 kg
• RASAT of Tubitak Uzay, Turkey, spacecraft mass = 95 kg
• AprizeSat-5 and AprizeSat-6 of AprizeSat, Argentina built by SpaceQuest, Fairfax, VA, USA, each microsatellite features a next generation AIS (Automatic Identifictem System) payload. Each spacecraft has a mass of 14 kg. 18)
• BPA-2 (Blok Perspektivnoy Avioniki-2 — or Advanced Avionics Unit-2) of Hartron-Arkos, Ukraine. The BPA-2 experimental payload remained attached to the upper stage of the Dnepr-1 launch vehicle.
Figure 5: Photo of some payloads in the payload bay of the Dnepr Launch Vehicle (image credit: GAUSS)
Orbit: Sun-synchronous orbit, altitude altitude = 675 km x 700 km, inclination = 98.25º, the orbital period is about 98 min, the equatorial nodal crossing time is at 10:15 LTAN (Local Time on Ascending Node).
Status of the mission:
• February 2014: The RASAT spacecraft and its payload are operating nominally. So far, the satellite captured images from all around the World with a total coverage area of 4,650,000 km2, corresponding . to about 6 times the area of Turkey. The imagery can be used for urban regional planning, forestry, agriculture, disaster management and many other purposes. The operational life of the spacecraft is expected to exceed the design life of 3 years. 19)
• In January 2013, the RASAT spacecraft and its payload are in good condition. One of the 2012 operational activities was to create a mosaic map of Turkey; the satellite accomplished a coverage of 98% of the area in 2012 (Ref. 20).
• The spacecraft and its payload are operating nominally in 2012.
- Being the first domestically built satellite, RASAT was very instructive for the TUBITAK- UZAY engineers and Turkey’s aerospace sector. During the design, manufacturing, testing, commissioning and operation of the spacecraft, many important lessons were learned including the AODCS related ones. The project learned that it is useful to account for smooth controller and flight computer switching. The experience has stressed the importance of hardware-in-the-loop tests which simulate satellite’s flight in space. The project learned also that in-orbit calibration is absolutely necessary and that using analog sensors can be very cumbersome and time-consuming as it complicates the calibration procedures (Ref. 10).
• The commissioning phase of RASAT ended in late December 2011. The spacecraft is operational since the start of 2012 (Ref. 20).
• As of late October 2011, the spacecraft is still in the commissioning phase; however, images are being downloaded successfully. The satellite is a test bed for the Turkish indigenous satellite flight computer, flight software, X-band transmitter, the mission planning software and image encryption and compression module designed by TUBITAK UZAY. Hence, the imagery is only a byproduct of the RASAT mission. The main goal is to test the equipments and to obtain flight heritage for the follow-on Turkish LEO missions. 20)
Figure 6: Sample image of the city of Izmir, Turkey, as observed by RASAT in 2011 (image credit: TUBITAK-UZAY)
• The first contact with RASAT (on August 17, 2011) was made by TUBITAK UZAY’s additional ground station in Andoya, Norway. The second contact was made by TUBITAK UZAY’s main ground station in Ankara, Turkey.
Sensor complement: (OIS)
OIS (Optical Imaging System):
OIS is a panchromatic (Pan) and multispectral (MS) snapshot imager designed and developed by SI (Satrec Initiative), Daejeon, Korea. The OIS instrument is a variant of the basic EOS-A system of SI to meet the requirements of the customer. 21)
OIS is a pushbroom imaging system that is comprised of two configurable units, namely EOU (Electro-Optical Unit) and the PCU (Processing & Control Unit). EOU consists of the telescope and the FPA (Focal Plane Assembly). In turn, the PCU consists of the SPM (Signal Processing Module), the CSM (Control & Storage Model), and the PSM (Power Supply Module).
The key system features of OIS are summarized in Table 2. The imaging system is capable of taking Pan images with a resolution of 7.5 m and MS images (RGB) with a resolution of 15 m from an orbital altitude of ~700 km.
• The detector of a Pan image scene has a size 4096 pixels x 4096 lines. This corresponds to volume of 16 MByte.
• The detector of a MS image scene has a size of 2048 pixels x 2048 lines. This corresponds to 4 MByte for a single MS frame (i.e. blue, green or red) or 12 MB for a full MS frame.
A total of 7 Gbit is available for memory allocation: 4 Gbit for Pan and 3 Gbit for MS imagery. In terms of “number of image scenes per orbit”, it can be concluded that the local memory is capable of storing approximately 31 scenes of Pan data and 31 scenes of MS data.
One Pan scene as well as one MS scene corresponds to an area of 30.7 km x 30.7 km on the ground.
Table 2: Parameters of the OIS instrument
The telescope is based on the Maksutov design with all spherical optical components made of Fused Silica. Its entrance pupil diameter is 104 mm and the effective focal length is about 466.7 mm. The optical design gives near diffraction-limited performance with a MTF (Modulation Transfer Function) value higher than 0.3 at the panchromatic pixel Nyquist frequency. Three baffles are used for stray-light control.
The metering structure is made of Invar and other materials. It will protect optical components from static and dynamic loads during transportation and launch. It is also designed to minimize the deformation of optical surfaces and to maintain the optical tolerances during optical alignment / integration, integration with the satellite bus, and operation in orbit. A cutaway solid model of EOU is shown in Figure 7. EOU is designed to have the first natural frequency higher than 300 Hz.
Figure 7: Cut-away view of the EOU (image credit: SI)
FPA houses a linear detector array for simultaneous imaging in one panchromatic and three multi-spectral channels. Its proximity electronics provides clock pulses and biases for the operation of the detector array. The pre-amplified analog video signals are transmitted to SPM for processing.
PSM generates all internal power from an unregulated +28 V of the satellite bus. When +28 V power in turned on, PSM automatically turns on CSM for communication with the satellite bus. PSM includes solid-state power switches for other modules and it supports CSM for the internal telemetry data collection.
SPM generates timing pulses for the operation of FPA and processes analog video signals from FPA. It performs gain control for video signals, analog-to-digital conversion, and correlated double sampling. The digitized image data are transmitted to CSM in high-speed for storage. SPM has an auxiliary real-time image data port for the alignment of FPA.
Within CSM, a high-performance DSP (Digital Signal Processor) performs the overall management of operation and the storage / transmission of image data. The total image data storage capacity is 7 Gbit (4 for panchromatic image data). For communication with the satellite bus, primary and redundant CAN buses are used. CSM has four LVDS (Low Voltage Differential Signaling) image data ports dedicated for image data transmission for each channel and they operate at 25 MHz.
PCU EQM (Engineering Qualification Model): This was developed with an identical configuration with the flight model (FM) in order to verify its full functionality and performance. Along with functional test, environmental tests performed are as follows:
• EMI/EMC test per MIL-STD-461E
• Thermal cycling test
• Thermal vacuum/cycling test per ECSS-E-10-03A
• Random vibration test per ECSS-E-10-03A.
Figure 8: Photo of the PCU (image credit: SI)
EOU (Electro-Optical Unit): For compact high-resolution imaging systems like OIS, careful attention must be paid during opto-mechanical design and analysis. The metering structure must be designed to protect sensitive optical components from static and dynamic loads and to meet the optical tolerance requirements for ground and space operation. EOU EQM was developed with the objectives to test its performance and to verify its optomechanical design against dynamic loads.
Figure 9: Photo of the EOU EQM telescope (image credit: SI)
1) Gökhan Yüksel, Altu? Okan, U?ur Murat Lelo?lu, “First LEO Satellite Built in Turkey: RASAT,” Proceedings of the 3rd International Conference on Recent Advances in Space Technologies (RAST 2007), Istanbul, Turkey, June 14-16, 2007
2) A. Okan, “Feasibility study on thermal survivability of X-band subsystem on RASAT microsatellite,” Proceedings on the 2nd International Conference on Recent Advances in Space Technologies (RAST 2005), Istanbul, Turkey, June 09-11, 2005
3) Information provided by Baris Gencay of TUBITAK-Uzay (Space Technologies Research Institute), Ankara, Turkey
5) U. M. Lelo?lu, “The Small Satellite Odyssey of Turkey,” Proceedings of the International Workshop on Small Satellites, 'New Missions, and New Technologies,' SSW2008, Istanbul, Turkey, June 5-7, 2008, URL: http://www.bilten.metu.edu.tr/tubitakuzay/yayinlar/SSW08_Manuscipt_Leloglu.pdf
8) “First micro satellite designed and built in Turkey: RASAT,” URL: http://www.uzay.tubitak.gov.tr/tubitakUzay/tr/aboutUs/brochures/enRasat.pdf
9) S. Ontaç, S. Da?, M. I. Gökler, “Structural Finite Element Analysis of Stiffened and Honeycomb Panels of the RASAT Satellite,” Proceedings of the 3rd International Conference on Recent Advances in Space Technologies (RAST 2007), Istanbul, Turkey, June 14-16, 2007
10) Farid Gulmammadov, Ça?atay Yavuzyilmaz, Özgür Kahraman, “The Lessons Learned from the Commissioning and Operation Phases of RASAT: An AODCS Perspective,” Proceedings of the UN/Japan Workshop and The 4th Nanosatellite Symposium (NSS), Nagoya, Japan, Oct. 10-13, 2012, paper: NSS-04-0319
11) Mehmet Durna, Hakan Urhan, Onur Turhan, Ömer Kozal, Melike Gürün, “A New Generation On-Board Computer and Solid State Data Recorder suitable for SpaceWire Platforms,” Proceedings of the 3rd International Conference on Recent Advances in Space Technologies (RAST 2007), Istanbul, Turkey, June 14-16, 2007
12) “Clyde Space wins contract for RASAT ,” 2008, URL: http://newsweaver.co.uk/aerospace/e_article000989473.cfm?x=b11,0,w
13) Clyde Space Delivers Battery Charge Controllers For RASAT,” Space Mart, Oct. 7, 2008, URL: http://www.spacemart.com/reports/Clyde_Space_Delivers_Battery-Charge_Controllers_For_RASAT_999.html
14) D. Icoz, N. D. Kahyaoglu, M. Bolucek, E. Oncu, T. Kirilmaz, C. Dudak, H. Sunay, V. Akan, O. Sen, “Communication subsystem of RASAT,” Proceedings of 5th International Conference on Recent Advances in Space Technologies (RAST), Istanbul, Turkey, June 9-11, 2011
17) “Turkey's first national satellite launched,” World Bulletin, Aug. 17, 2011, URL: http://www.worldbulletin.net/?aType=haberYazdir&ArticleID=77632&tip=
18) “Dnepr launches AprizeSat-5 and AprizeSat-6,” SpaceQuest, August 17, 2011, URL: http://www.spacequest.com/Articles/AprizeSat_Launch_8-17-11.pdf
19) The information was provided by Aziz Koru of TUBITAK UZAY, Ankara, Turkey.
20) The information was provided by Bar?? Gençay of TUBITAK UZAY, Ankara, Turkey.
21) Ee-Eul Kim; Young-Wan Choi; Woong Choi; Hyun-Gu Kim; Sang-Jin Park; Ji-Ho Yun; Myung-Seok Kang; Seong-Keun Jeong; “Integration and Testing of Optical Imaging System for RASAT ,” Proceedings of RAST (Recent Advances in Space Technologies) 2007, Istanbul, Turkey, June 14-16, 2007, URL: http://www.satreci-us.com/documents/papers/RAST%202007%20-%20Integration%20and%20Testing%20of%20RAST2007-%20Optical%20Imaging%-20System%20for%20RASAT%20%-28EE%20Kim%20et%20al.%29.pdf
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.