KOMPSAT-6 (Korea Multi-Purpose Satellite-6) / Arirang-6
KOMPSAT-6 is the second SAR satellite of KARI (Korea Aerospace Research Institute) as a follow-on from KOMPSAT-5. The mission objective of KOMPSAT-6 system is the expedite provision of the spaceborne SAR (Synthetic Aperture Radar) images with high resolution required for the national demand in GIS (Geographical Information Systems), Ocean & Land management, Disaster monitoring, and Environment monitoring. The primary mission of KOMPSAT-6 system is to provide the SAR imagery of 0.5 m and 1 m resolution in the high resolution mode, 3 m resolution in standard mode, and 20 m resolution in the wide swath mode at an incidence angle of 45º. 1)
The spacecraft will be built by LIG Nex1 Co. Ltd. of Korea and Airbus Defence and Space's of Friedrichshafen, Germany. The contract was signed in Seoul on February 24, 2014 by Hyokoo Lee, CEO of LIG Nex1 Co. Ltd and Eckard Settelmeyer, Head of Earth Observation of Space Systems in Germany. Seoul-based LIG Nex1 Co., a division of the LG Group and a major Korean defense contractor, will use the KOMPSAT-6 work to enter into a long-term cooperation with Airbus Defence and Space in the domain of spaceborne radar. LIG Nex1 is accountable to KARI (Korea Aerospace Research Institute) for handling all aspects of the radar sensor. KARI plans to launch the satellite in 2020 as a replacement and successor to KOMPSAT-5 (launch on Aug. 22, 2013). 2) 3)
Figure 1: Illustration of the KOMPSAT-6 on-orbit configuration (image credit: KARI)
The spacecraft bus provides a number of functions to support the system operations. These functions consist of providing a stable platform for SAR payload, AIS payload, the state of health telemetry data, satellite timing, power and thermal management, attitude pointing and determination, and orbit determination. The spacecraft bus also provides the orbit maneuver capability necessary to maintain the orbit required for the KOMPSAT-6 mission.
The bus features the following subsystems: SMS (Structures and Mechanisms Subsystem), TCS (Thermal Control Subsystem ), AOCS (Attitude and Orbit Control Subsystem), PS (Propulsion Subsystem), TC&R (Telemetry, Command, and Ranging Subsystem), EPS (Electrical Power Subsystem), and FSW (Flight Software Subsystem). The key design features of KOMPSAT-6 spacecraft bus are described as follows:
- Integrated structure with hexagonal cross section
- Primary structure made out of CFRP (Carbon Fiber Reinforced Polymer)
- Fixed SAR antenna on a panel
- Fixed solar array with one deployable wing
- Panel-mounted units covered with MLI (Multi-Layered Insulation) tents
- 3-axis attitude control with zero momentum bias
- Dual-frequency GPS receiver for POD generation
- Passive & processor-controlled thermal control
- 1 N thrusters for precise orbit correction
- Solar array capability: 2.250 kW
The spacecraft bus uses the MIL-STD-1553B data bus interfaces for most of the spacecraft bus and payload to the IBMU (Integrated Bus Management Unit). The dual frequency GPS receiver provides the time, satellite position and velocity for on-orbit ephemeris, and L1/L2 frequency GPS pseudo-range and carrier phase data for post-processed precision orbit determination on ground. TC&RS includes S-band antennas, a RF assembly and redundant S-band transponders. AOCS provides 3-axis stabilization with off-pointing capability and yaw steering capability during imaging periods.
In addition, the spacecraft bus provides the mechanical and electrical interface to the launch vehicle, and the necessary strength and stiffness to deliver the satellite to mission orbit during launch, ascent, and insertion. For the case where the launch vehicle has placed the satellite into orbit within nominal dispersions from the desired mission orbit, the spacecraft bus provides the capability to correct the orbit through a series of orbit maneuvers to the final mission orbit.
Table 1: KOMPSAT-6 spacecraft parameters
Launch: A launch of the KOMPSAT-6 satellite of KARI is planned for 2020 with the Angara 1.2 vehicle of ILS (International Launch Services) from the Plesetsk Cosmodrome in Russia. 4)
Orbit: Sun-synchronous ground repeat track dawn-dusk frozen orbit, mean altitude = 505 km, inclination = 97.6º, LTAN (Local Time on Ascending Node) = 6 hours, the repeat cycle is 11 days.
Sensor complement: (XSAR, AIS)
XSAR (X-band Synthetic Aperture Radar)
XSAR is a multi-mode operations instrument consisting of the SSS (SAR Sensor Subsystem), the DLS (Data Link Subsystem) and the PLM (Payload Module). The SSS is equipped with an active phased array antenna, with electronic scanning capabilities in the azimuth and the elevation planes. The DLS is in charge of the storage and ground transmission of the SAR sensor data and of the ancillary data.
The following key characteristics are considered for XSAR payload architecture design: 5)
• X-band Active Phased Array Synthetic Aperture Radar with dual H/V polarized wideband slotted waveguide antenna and T/R modules
• Electronic beam steering capability in both azimuth and elevation plane
• Multi-mode operation over a wide access region by SAR techniques; High resolution mode by sliding spotlight, Standard mode by stripmap technique and Wideswath mode by TOPS (Terrain Observation with Progressive Scan) technique
• ATI/GMTI (Along-Track Interferometry/Ground Moving Target Indication) mode for experimental applications 6)
• High speed data rate downlink with two channels
Figure 2: Illustration of the KOMPSAT-6 observation modes (image credit: KARI)
The SSS comprises the following equipments:
• SCE (SAR Controller Electronics), which includes DTCU and DRU. The SCE is in charge of the command and control of the SSS to achieve the SAR mission. It provides wideband chirp signal generation, digitization, smart digital filtering and block adaptive quantization for data reduction. It also manages radar timing control and redundancy configuration of SSS.
• RFE (Radio Frequency Electronics). RFE performs frequency up-converting and bandwidth multiplication for transmitting signal, frequency down-converting and I/Q demodulating for receiving signal, and reference clock provision.
• FEI (Front End Interface). The FEI provides RF signal routing for nominal operation and calibration, and manages amplification and distribution RF signal.
• SAA (SAR Antenna Assembly). The SAA is an active phased array and it is able to operate different beam characteristics for transmission and reception. It manages electrical beam steering for horizontal and vertical direction. It consists of two leaves with 12 IFETs as basic elements and the fixed antenna structure.
• APS (Antenna Power Supply). The APS provides 100 V power to SAA and FEI which is received from the Platform PCDU (Power Control and Distribution Unit).
The SSS requires the internal calibration resources by hardware network and software tools in order to achieve radiometric performance.
The DLS (Data Link Subsystem) manages data storage, data handling and data transmission to ground station. DLS comprises the following equipments:
- DSHA (Data Storage & Handling Assembly). DSHA controls the SAR data acquisition and storage, data formatting and coding according to CCSDS standard, and data encryption.
- XTA (X-band Transmission Assembly). XTA provides baseband data modulation onto two carriers and power amplification of each downlink channel.
- XAA (X-band Antenna Assembly). XAA consists of two fixed wide coverage antenna for transmission to ground station of modulated data stream with an isoflux shaped radiation pattern.
PLM (Payload Module) consists of the structure, thermal control hardware and harness. The PLM structure is configured as one frame concept in the spacecraft structure without discrimination of platform and payload. This concept is able to use the fixed SAR antenna on structure without a deployment mechanism. The satellite structure is designed using CFRP panels to reduce the satellite mass and enhance the thermal characteristics. The payload thermal hardware is designed to maintain the payload component within the specified temperature limits during all mission lifetime using passive thermal control method such as SSM foil, heaters and sensors.
Figure 3: Functional block diagram of the XSAR payload including redundancy (image credit: KARI)
The SSS provides the following functions:
• Transmit equipment for signal generation and distributed transmit signal high power amplification
• Receive equipment for distributed receive signal low noise amplification, coherent down-conversion and digitization
• The SAA (SAR Antenna Assembly) with real-time (PRI to PRI) beam forming capability in azimuth and elevation via phase and amplitude control including temperature compensation and providing single phase as well as dual phase center
• Quad linear polarization(HH, VV, VH, HV) with dual coherent HH/HV and VV/VH
• Quad linear polarization(HH, VV, VH, HV) with dual coherent HH/HV and VV/VH
• Command and control functionality for agile and flexible operation.
The DLS comprises all the functions necessary for acquisition, storage and handling of data generated by the SSS, and for real-time or near real-time transmission to the ground station.
Table 2: Specification of the XSAR instrument
SAR payload development:
The KOMPSAT-6 payload system design and development is performed by the leading of KARI payload team. Korea industries are involved in the development of the SSS (SAR Sensor Subsystem), DLS (Data Link Subsystem) and PLM (Payload Module). The overseas partners are engaged in the payload unit development, in a mutual cooperative fashion.
The core development equipment of the SAR payload are the SCE (SAR Controller Electronics) and the DSHA (Data Storage& Handling Assembly). The core units are being developed by the KARI payload team and Korea industries based on SAR technology and the KOMPSAT program heritage. The PGSE (Payload Ground Support Equipment) are also being developed in accordance with payload verification and payload test plan, respectively. The payload AIT (Assembly, Integration and Test) activities will be performed at KARI facilities. For the next step, the detailed payload system design and units development will be executed according to its specification. In particular, the engineering model of the SCE (SAR Controller Electronics) and DSHA (Data Storage& Handling Assembly) are under development.
S-AIS (Satellite-Automatic Identification System) payload
AIS is a maritime wireless system designed to identify basic parameters relating to the position, heading, destination and cargo of larger vessels with the main purpose of collision avoidance. The AIS payload is the secondary payload of the KOMPSAT-6 satellite. The AIS payload receives and provides the AIS signal data for ship collision avoidance and traffic management. The AIS objectives are: 7)
• AIS signal collection from vessels on sea
• Raw data sampling for OGP (On Ground Processing) mode
• Demodulation of AIS burst signals for OBP (On Board Processing) mode.
The AIS payload for KOMPSAT-6 consist of the AIS receivers and the AIS antennas. The satellite AIS receivers are able to disentangle these multiple collision signals of the ground target region and reconstruct the original messages. The AIS receivers are able to operate in both frequency spectrum capture modes (Raw/OGP) and on-board processing (OBP) mode allowing simultaneous operation.
VHF band antennas are needed for the AIS signal reception. Multiple VHF antennas can be used for better AIS signal reception. An antenna deployment mechanism is applied for deploying the antennas after the satellite deployment into orbit.
Table 3: Parameters of the AIS payload
Electrical Interface Design: The IBMU (Integrated Bus Management Unit) primary (RCL, PPS and COM) is connected to the AIS receiver primary (hot), and the IBMU redundant is connected to the AIS receiver redundant (cold), as shown in Figure 4. The AIS payload has the following electrical interface characteristics.
- RS-422/RS-485: 115.2 kb/s, 230.4 kb/s, 460.8 kbaud/s and 921.6 kbaud/s (baud rate is selectable)
- PPS input: RS-422/RS-485
- Redundancy control: RS-422/RS-485
- Connectors: Antennas: SMA 50 Ohm female
- Power: High performance micro D-subminiature
- Serial: High performance micro D-subminiature
This configuration offers complete redundancy on all parts and quadruple redundancy on the decoder boards and the interface boards. The Interface Board contains an RF front-end filter, one LNA (Low Noise Amplifier) for each antenna, external interface connectors, board connectors, power supply, and communication transceivers.
Figure 4: Electrical interfaces between the AIS receiver and the PCDU/IBMU (image credit: ETRI)
Mechanical Interface Design: The mechanical interfaces design has been carried out for the AIS receiver and the AIS antennas mounting on the spacecraft. The unit accommodation design for KOMPSAT-6 is in a preliminary design phase so far. Mounting configurations of AIS receiver and locations of the antennas on the spacecraft are shown in Figure 5 for the AIS receiver and Figure 6 for the AIS antennas, respectively. 8)
Figure 5: AIS receiver mechanical configuration for accommodation (image credit: Kongsberg Seatex)
Figure 6: AIS antenna accommodation on the KOMPSAT-6 spacecraft (image credit: ETRI)
The AIS receiver consists of three interconnected boards in a housing with all external connectors mounted at a recessed top front. The mounting points to the satellite structure are placed on a single plane at the bottom of the receiver. The enclosure is made as a non-magnetic metallic case which forms an all-enclosing electromagnetic and radiation shield. The mechanical design assumes a spacecraft manufacturing tolerance of ±0.1 mm.
ETRI (Electronics and Telecommunications Research Institute) of Daejeon is to design and develop the S-AIS system for KOMPSAT-6.
KOMPSAT-6 Ground Segment (KGS)
The ground segment of the KOMPSAT-6 system is under design to comply with the requirements assigned to the ground segment. According to the requirement analysis, the ground segment is designed to be comprised of three elements, which consist of the MCE (Mission Control Element) , IRPE (Image Reception and Processing Element) and the CE (Calibration Element), respectively. Both internal and external interfaces of the ground segment are under design considering all operational aspects. As a result, the ground segment is expected to have functionalities such as monitoring the state of health of the satellite, programming and control for the operation, receiving and processing SAR observation data to provide standard products to the user community. 9)
The KOMPSAT-6 ground segment is to be operated based on mission operation procedures in support for mission scheduling and planning activities and observation data reception and processing activities. Detailed procedures for mission scheduling and planning activities consist of the following support functions:
1) Reception of product orders from users
2) Generation of image collection proposal based on the order analysis result
3) Generation of image collection plan
4) Generation of mission plan including satellite operation timelines
5) Command generation and transmission to the satellite.
The KGS is located at the KARI (Korea Aerospace Research Institute) site; it is comprised of the MCE (Mission Control Element), IRPE (Image Reception and Processing Element) and CE (Calibration Element). The satellite interfaces with ground station via both S-band and X-band communications capabilities. The launch segment is comprised of the launch vehicle, launch related equipment, and launch services/operations required for delivery the satellite into mission orbit.
The KOMPSAT-6 external interfaces include the King Sejong station on Antarctica to support urgent imaging request with looking mode change and contingency support, additional ground stations in support of LEOP (Launch and Early Orbit Phase), and the IGS (International GPS Service) data center providing high accuracy ground based GPS data products to KARI for POD (Precision Orbit Determination) processing.
Figure 7: System architecture of the KOMPSAT-6 mission (image credit: KARI)
MCE (Mission Control Element): The MCE provides the satellite control capabilities that allow operators to carry out the KOMPSAT-6 mission with mission planning, command generation and transmission, telemetry processing and SOH (State of Health) monitoring, space flight dynamics support, and simulation for satellite operation. The MCE is designed to provide the S-band RF communication capability which links it with the KOMPSAT-6 satellite in space for the command uplink, ranging and telemetry downlink including AIS (Automatic Identification System) data. These key functions of the MCE are summarized as follows:
• Communication with KOMPSAT-6 satellite via S-band RF link
• Satellite Control and Monitoring
• Mission Activity Planning and Command Procedure Planning
• Orbit Determination, Prediction, and Maintenance
• Satellite simulation
Figure 8: Architecture and interfaces of the KOMPSAT-6 ground segment (image credit: KARI)
The MCE is designed to consist of five subsystems. The TTC (Telemetry, Tracking & Command ) subsystem provides the functionality for telecommand and telemetry communications with the KOMPSAT-6 satellite via S-band RF link. The SOS (Satellite Operation Subsystem) provides the functionality for command generation and transmission to the satellite, monitoring SOH of the satellite and data processing for the AIS as well as the housekeeping telemetry. The MPS (Mission Panning Subsystem) provides overall satellite mission planning, incorporates requests, defines KOMPSAT-6 configurations, and prepares the operation timeline. The FDS (Flight Dynamics Subsystem) provides space flight dynamics support such as orbit determination, orbit prediction, and orbit maneuvers. The SIM (Simulator) provides the function of command/telemetry simulation, operator training and satellite visualization. Figure 9 depicts the architecture and interfaces of the MCE.
Figure 9: Architecture and interfaces of KOMPSAT-6 MCE (image credit: KARI)
IRPE (Image Reception and Processing Element): The IRPE is designed to provide the capability of receiving RF signal from KOMPSAT-6 satellite via X-band, retrieving SAR observation data, generating Level 0 products, standard products and value-added products if needed, and providing level products to users. The IRPE consists of five subsystems, which are UIS (User Interface Subsystem), ICPS (Image Collection Planning Subsystem), DIS (Direct Ingestion Subsystem ), PMS (Product Management Subsystem ), and PPT (Post Processing Tool). Figure 10 shows the architecture and interfaces of the IRPE.
The UIS provides functionalities for the reception, management of product order, generating image collection proposal based on feasibility study and image collection request as the confirmation on image collection proposal and submitting product generation request for the generation of standard products. The ICPS is designed to have functionalities such as the generation of image collection plan which are essential in detailed command parameters for the SAR payload operation, management of image collection planning status. To make the image collection plan optimal in view of resources, the ICPS provides the capability of managing resources related to the SAR payload operation. The DIS, in connection with the X-band antenna system, provides functionalities such as scheduling X-band RF signal reception, X-band RF signal receiving and status monitoring, analog to digital conversion to extract CADU (Channel Access Data Unit) frames for retrieving SAR source packet data. The PMS is designed to have functionalities such as catalog data generation, product generation status control and monitoring and standard product generation. Finally the PPT is designed to be used for SAR application such as interferometry, ATI/GMTI (Along-Track Interferometry/Ground Moving Target Indicator), the fusion of SAR data with AIS data, and so on.
Figure 10: Architecture and interfaces of KOMPSAT-6 IRPE (image credit: KARI)
CE (Calibration Element): The SAR calibration encompasses the activities or processes to link the values of acquired SAR images to reflectivity of ground targets (back scattering coefficient) by achieving the highest instrument observation accuracy of measurements. The CE, as part of the KOMPSAT-6 ground segment, provides the capabilities or tools to achieve efficient SAR calibration by providing the following functions: 1) range & azimuth antenna pointing offset measurement, 2) geometric range & azimuth offset measurement using point target, 3) product coverage location error measurement, 4) antenna pattern verification, 5) absolute radiometric calibration factor evaluation, 6) polarimetric effects (channel imbalance & cross-talk) compensation, 7) long-term & daily calibration image scheduling.
To fulfil these functions, the CE is designed to consist of five subsystems. The PCS (Pointing Calibration Subsystem) is used in calibrating the range and azimuth antenna pointing offsets by using notch beam pattern. The GCS (Geometric Calibration Subsystem) is to be used in calibrating pixel localization offsets by using point target and measures image product coverage location errors. The RCS (Radiometric Calibration Subsystem) is prepared for verifying reference antenna pattern with the measured antenna pattern from images and ground receiver, and also used in calculating absolute radiometric calibration factors. The MCS (Multi-polarimetric Calibration Subsystem) provides a means to compensate polarimetric distortions such as channel imbalance and cross-talk. Finally, the CSS (Calibration Scheduling Subsystem) is used for organizing the long-term and daily calibration image collection schedule. Figure 11 shows the architecture of CE with internal and external interfaces.
Figure 11: Architecture and interfaces of the KOMPSAT-6 CE (image credit: KARI)
1) Seon-Ho Lee, Jae-Cheol Yoon, Jin-Hee Kim, "KOMPSAT-6 Mission, Operation Concept, and System Design," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
2) "Airbus Defence and Space sign contract to deliver space radar to the Republic of Korea,", Airbus DS Press Release, March 14, 2014, URL: https://airbusdefenceandspace.com/newsroom/news-and-features/airbus-defence-and-space-sign-contract-to-deliver-space-radar-to-the-republic-of-korea/
3) Peter B. de Selding, "German-Korean Industrial Team To Build KOMPSAT-6 Radar Imager," Space News, Feb. 26, 2014, URL: http://www.spacenews.com/article/financial-report/39625german-korean-industrial-team-to-build-kompsat-6-radar-imager
5) Yong Chul Hwang, Chang Ho Nam, Ui Young Pak, Se Young Kim, Jeong Ho Lee, "KOMPSAT-6 SAR Payload System Design," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
6) Dochul Yang, Okchul Jung, Donghan, Lee, "KOMPSAT-5/6 SAR Interferometry," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
7) Yong-Min Lee, Jin-Ho Jo, Byoung-Sun Lee, "Preliminary Design of S-AIS Payload for KOMPSAT-6," SPACOMM 2016 : The Eighth International Conference on Advances in Satellite and Space Communications, Lisbon, Portugal, Feb. 21-25, 2016, URL: https://www.thinkmind.org/download.php?articleid=spacomm_2016_2_20_20016
8) Design Definition File Summary Report, Issue 1A, ASRx50-KSX-DDF-014, Kongsberg Seatex AS, 2015
9) Chiho Kang, Okchul Jung, Taebong Oh, Dochul Yang, Gabho Jeun, "Operation concept of KOMPSAT-6 ground segment," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
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 (firstname.lastname@example.org).