ASNARO (Advanced Satellite with New system ARchitecture for Observation)
ASNARO is a Japanese optical high-resolution Earth imaging mission under development by the NEC Corporation and USEF (Institute for Unmanned Space Experiment Free Flyer). The project is funded by NEDO (New Energy and Industrial Technology Development Organization), a Department of METI (Ministry of Economy, Trade and Industry) of the Government of Japan.
The ASNARO project was initiated by USEF in 2008. The overall objective is to develop a next-generation high-performance minisatellite bus system based on open architecture techniques and manufacturing methodologies to drastically reduce the cost and the development period with adoption of up-to-date electronics technologies. The observation requirements call for the provision of high-resolution imagery [Pan of < 0.5 m, and MS of 2 m GSD (Ground Sample Distance) on a swath of 10 km]. This implies also the development of a high-performance imager and to demonstrate new technologies in space. 1) 2) 3) 4) 5) 6)
Figure 1: On-orbit illustration of the ASNARO spacecraft (image credit: NEC)
The new NEC standard minisatellite bus system under development, the result of a joint study by JAXA/ISAS and NEC, is highly adaptive for various missions. As the payload interfaces are standardized - including mechanical, thermal, electrical and RF interfaces - the bus system can be applied not only for optical observation mission but also for other payloads like the SAR, hyperspectral, and infrared instrument applications. Due to these features, NEXTAR (Next Generation Star) is highly adaptive for various missions and can reduce non-recurrent cost and development time. NEXTAR series consists of NEXTAR-100L/300L/500L, etc. corresponding to a size of satellite bus. 7) 8) 9) 10) 11) 12)
The ASNARO project selected the NEXTAR-300L bus developed by NEC. NEC intends to apply this bus system for commercial and EO uses. The goals of NEXTAR-300L system are as follows:
• To provide a high performance advanced minisatellite (100-500 kg) system at a low cost and in short time
• To provide an end-to-end solution (including the satellite, ground system, data network, satellite application and technology transfer) as a package
• Use of the SpaceWire network architecture.
The performance features of ASNARO in GSD (Ground Sample Distance) are comparable to those of other advanced commercial imaging missions like GeoEye-1, WorldView-2 and Pleiades-HR, with a considerable smaller spacecraft mass.
Figure 2: Performance comparison of ASNARO with first class commercial imaging satellites (image credit: NEC)
Figure 3: The basic idea of the NEXTAR system (image credit: NEC)
Figure 4: Standard bus concept of the NEXTAR system (image credit: NEC)
Figure 5: Illustration of the standard bus portion of the spacecraft (image credit: USEF, NEC)
Figure 6: Photo of the EM (Engineering Model) of the NEXTAR bus (image credit: NEC, USEF)
Figure 7: Summary of the innovation of the spacecraft development architecture (image credit: USEF, NEC)
The establishment of the “bus standards” to innovate the spacecraft development architecture is expected to reduce the hurdle heights for a small business enterprise to enter the “space market” to induce the competition, and reduce the cost and development time of future spacecraft, and finally excite whole space industries (Ref. 4).
The minisatellite bus system is comprised of subsystems which can be arranged in building-block style with the SpaceWire network, using high-performance COTS components and improved verification test methods. The SpaceWire RMAP (Remote Memory Access Protocol) technology is adopted as one of the standardization features of NEXTAR.
- Versatile onboard computer(SpaceCube2)
- Telemetry/Command interface module
- SpaceWire router
- Network data recorder
- Standard middleware
- Software development kit
These components are implemented with universality by improving previous satellite subsystems on the following points:
1) Radiation-resistance devices: To prevent soft errors caused by space radiation, such as data inversion or latch-up due to excessive current, the main devices, including microprocessors and routers, are manufactured using the SOI (silicon on Insulator) process, which features excellent radiation resistance. This leads to the error-free operation of the devices in the radiation environment inside the solar system.
2) High-speed, universal network: SpaceWire NEXTAR uses the SpaceWire RMAP standard, which is the embedded network protocol standardized internationally by the EU, USA, Japan and Russia. By using this technology, the communication at up to 400Mbit/s per channel can be achieved with a simplified architecture.
3) Versatile multi-purpose embedded computer: With previous satellites, it had been a common practice to develop a dedicated embedded computer in each subsystem. For the NEXTAR standard platform, the project developed the SpaceCube 2, a versatile multi-purpose computer application for the control of any subsystem, by reflecting recent improvements in microprocessor performance and advances in LSI integration. By concentrating all efforts for the development of the single type of computer, implementing ASICs extensively and developing basic software, the project succeeded in reducing the size to 1/8 of previous computers, thereby contributing to the size reduction of the satellite bus. This computer was mounted on the SDS-1 (Small Demonstration Satellite-1) of JAXA (launch Jan. 23, 2009), and completed the in-orbit demonstration without any problem (Ref. 11). 14) 15)
Table 1: Space Cube 2 specification
Figure 8: Photo of the SpaceCube 2 computer system (image credit: NEC)
4) Standard middleware/software design kit: The SpaceCube 2 uses a RTOS (Real-Time Operating System) based on TRON as its basic software. The RTOS is based on the TRON (The Real-time Operating system Nucleus) specification which was developed by a collaboration of Japanese industry and universities. The TRON package is used about 60% of the RTOS implementations in the embedded equipment market. It is low-price, but high-quality applications can be built easily. And it is blessed with a substantial technical education environment for fostering quality software development engineers. 16)
The project integrated the high-reliability technology assets as the standard middleware on the RTOS, achieving integration of the development environment across all subsystems. The RTOS and standard middleware are integrated into SDDS/E (System Director Developer’s Studio/Embedded) that is provided as an open platform of an Eclipse-based embedded software integrated development environment. This development environment is provided exclusively through the NEC portal site based on cloud computing technology in order to ensure version control and configuration control. It also enables business deployment based on joint collaboration with a third-party vendor or enterprise.
The implementation of a standard platform in this way makes it possible to adapt flexibly to the remote sensing target, operation scale, operational mode and system operation period of each operator, to quicken integration and to deliver the spacecraft system to customers promptly in operation on the orbit (Ref. 11).
EPS (Electrical Power Subsystem) and SAP (Solar Array Paddle subsystem): The project developed power supply units and SAP that are compatible for use in low Earth orbit to deep space exploration missions; they feature a reduced number of units, compact size, light weight and low cost.
Electrical power is supplied using the series topology, in which the SAP outputs are connected in series to an APR (Array Power Regulator), which performs step-down regulation. This makes it possible to regulate the SAP output voltage, which varies greatly depending on the distance from the sun. The project also adopted the battery bus topology, in which the BAT (battery) and other satellite onboard equipments are connected directly to the APR output, so that the SAP output and BAT charging can be controlled by adjusting the APR output alone. The SAP uses triple junction high-efficiency solar cells and the BAT uses large (50 Ah capacity) lithium-ion battery cells.
The PCU (Power Control Unit) has a SpaceWire type data bus interface and achieves high functionality, compact size and low mass by means of high-density packaging (Figure 9).
Figure 9: Illustration of the EPS components (image credit: NEC)
AOCS (Attitude and Orbit Control Subsystem): Since the functional and performance requirements for the AOCS are highly dependent on each satellite’s mission requirements, the functions, specifications, and configuration of devices such as sensors and actuators are variable between the satellite missions. Consequently, the AOCE (Attitude and Orbit Control Electronics) system, which is the computer handling the integrated control of these devices, is difficult to produce by repetitive manufacturing, making it necessary to develop and verify this subsystem independently for each project.
The project introduced SpaceWire technology in the AOCS and succeeded in implementing a “standard platform AOCS”, which is a standardization of hardware and software at the module level that makes it possible to cope with any mission requirement using a catalog menu selection method. This new AOCS offers the following features:
1) The interfaces of the AOCE are implemented as individual interface modules called ACIMs (Attitude Control Interface Modules). The ACIMs can be combined according to the equipment configuration requirements of each satellite, so the AOCE can be manufactured by repetitive manufacturing. The SpaceWire connections between the ACIMs and the AOCE make design changes unnecessary resulting from additions or modifications. The AOCE itself is implemented using a universal computer [SMU ( Satellite Management Unit)] of the same type as those used in the data processing subsystem (Figure 10).
Figure 10: Variable AOCE configuration capability with ACIM (image credit: NEC)
The AOCS of ASNARO includes an RCS (Reaction Control Subsystem). The spacecraft is very agile providing a body-pointing event monitoring capability of ±45º in any direction from nadir. The precise and swift control for the Earth observation maneuvers are performed by RWA (Reaction Wheel Assembly), IRU (Inertial Reference Unit), STT (Star Trackers), GPSR (GPS Receiver) and the control S/W in AOCS.
Table 2: Overview of spacecraft parameters 17)
Project status (Ref. 6):
• The ASNARO project started in 2008 and the program phase has shifted to its system-level testing
• ASNARO bus structure was completed in 2010. SpaceWire component interface tests were performed from December 2010, and fundamental electronic tests including SpaceWire network tests at system-level were completed in March 2011.
• Proto-flight tests of the optical sensor were completed in 2011, followed by proto-flight tests of the spacecraft to be completed by Q2 of 2012, preparing for the launch campaign in December 2012.
Figure 11: Photo of the ASNARO proto-flight model (image credit: NEC, Ref. 11)
Figure 12: Alternate view of the deployed ASNARO spacecraft (image credit: NEC)
Launch: A launch of the ASNARO-1 spacecraft is scheduled for August 2014 on a Dnepr-1 vehicle from Dombarovsky (Yasny), Russia. The launch provider is ISC Kosmotras.
Originally, the launch of ASNARO-1 was planned on Japan’s new Epsilon solid rocket in 2011, according to METI’s initial development schedule. However, both the Epsilon and ASNARO projects were hit by budget shortfalls in 2010, delaying both projects, and making the Epsilon unavailable for a 2012 launch. 18)
Orbit: Sun-synchronous orbit, nominal altitude = 504 km, inclination = 97.4º, LTDN (Local Time on Descending Node) = 11:00 hours.
RF communications: The TT&C communications are in S-band. The stored image data are transmitted to the ground station by the X-band transmitter and the directional antenna mounted on the APM (Antenna Pointing Mechanism). The transmitting data rate is 800 Mbit/s with 16-QAM (Quadrature Amplitude Modulation).
APM is a small and low-mass device, developed and manufactured by Shinshu University and Tamagawa-seiki. Co, LTD, Japan. The objective is to achieve high speed data transmission to the ground station. 19) 20)
The low mass requirement was implemented with an aluminum structure and a cable tray system (instead of rotary joint mechanism to save costs). The mass of the APM is 4.4 kg, including the X-band antenna; the pointing accuracy of the unit is better than 0.1º. The maximum slew rate is 4º/s and the acceleration rate is 0.4º s-2.
Figure 13: The APM X-Y gimbal model (left) and an illustration of the two-axis motion scheme (right), image credit: Shinshu University
Sensor complement: (OPS)
The payload of ASNARO consists of OPS (Optical Sensor), data recorder, and X-band data transmitter subsystems.
OPS (Optical Sensor):
OPS is a compact pushbroom instrument developed by NEC and NTSpace (NEC Toshiba Space Systems Ltd.). The design introduces the following technologies: 21)
• Optics subsystem: Use of a TMA (Three Mirror Anastigmat) telescope
• The primary mirror is made of NTSIC (New Technology Silicon Carbide). SiC is considered the most suitable material of spaceborne telescope mirrors, because of high stiffness, low thermal expansion, high thermal conductivity, low density and excellent environmental stability. Newly developed high-strength reaction-sintered SiC, which has two to three times higher strength than a conventional sintered SiC, is one of the most promising candidates in applications such as lightweight substrates of optical mirrors, due to being fully dense and having small sintering shrinkage (±1 %), and low sintering temperature. 22)
Figure 14: Illustration of the NTSIC primary mirror substrate (image credit: NEC, USEF)
Table 3: OPS performance requirements
In Feb. 2009, the Goodrich Corporation (Charlotte, N. C., USA) was awarded a contract to provide optical sub-assemblies for the new OPS telescope.
Figure 15: Schematic view of the OPS pushbroom instrument (image credit: NEC, USEF)
The spacecraft AOCS provides the following observation modes:
2) Snap shot mode: In this mode it is possible to acquire the nominal 10 km x 10 km area’s image. At the moment of imaging, the satellite body is controlled to be fixed to inertial space.
3) Wide view mode: This mode is being used to provide wide area images along with a few sets of neighboring snap shot images.
4) 3D mode: This mode is used to acquire the stereo imagery of the target area. In this mode, the observations are performed from two different orbital positions to obtain 3-dimensional information of the target area.
5) Strip map mode: In this mode, it is possible to acquire zonal imagery - a continuous image with a maximum length of 850 km and a width of 10 km.
Figure 16: Schematic view of the various observation modes (image credit: NEC, USEF)
The ground segment consists of the data center, the integrated mobile station and the ground stations. The ASNARO spacecraft will be operated and controlled by mainly the data center and two ground stations, located in Okinawa and Hokkaido, Japan. Additionally, a rental ground station network will be utilized to support the LEOP (Launch and Early Orbit Phase) of the spacecraft and to develop to provide commercial services of the ASNARO data all over the world. The ground segment is being developed by PASCO Corporation of Tokyo, Japan. 23)
Data Center: In the data center, monitoring and control of orbit and attitude of the satellite, planning and management of the satellite bus operation plan, reception and management of imaging plan, processing of the image data and storage and management of the product will be performed. The data center is being newly constructed for ASNARO system with the concepts of compactness, self-automation, high speed and user oriented aspects.
Table 4: Development concept of the data center
Figure 17: ASNARO system configuration (image credit: PASCO)
Configuration of the Data Center: The data center is composed of 7 systems (Figure 18). These are:
1) Order Handling and Distribution System: This system accepts and handles acquisition request and send them to Mission Planning System as data acquisition requests. The status of the requests is provided to end-users. Based on the acquisition requests, it performs planning of the acquisition and the planning is sent to Mission Data Processing System. The processing status is notified to end-users.
2) Mission Planning System: The system receives the acquisition requests from end-users via Order Handling and Distribution System and plans the acquisition planning.
3) Mission Data Processing System: The system performs the geometric correction of the image data according to the processing planning from Order Handling and Distribution System and the stores the data into Mission Data Archiving System.
4) Mission Data Archiving System: The system manages the archiving and backup of all processed image data.
5) Spacecraft Operation Control System: The system receives the telemetry data demodulated at the ground station and provides the information for monitoring the status of the satellite to the operators. Also, it receives the planning of the satellite bus operation and the planning of image acquisition and transforms them into the satellite commands, which are transmitted to the ground station for uplink. Status of the command uplink can also be monitored.
6) Orbit Dynamics System: The system calculates the orbit of the satellite with high accuracy using the data of GPS receiver on the satellite and performs the orbit control for keeping the satellite to the nominal orbit. Based on the calculated orbit data, it also calculates and provides the information necessary for the satellite operation (such as AOS/LOS times of each ground station, maneuver planning, event information, position of the other interfering satellites, Sun and Moon) to each system.
7) Spacecraft Operation Planning System: The system performs the planning of the satellite orbit control by using orbit parameter, etc. from Orbit Dynamics System and the planning of the maintenance of the solar array paddle and the battery by using the position of the Sun. It also performs the operation planning of each ground station by using availability of ground station and the control operation based on the satellite telemetry data. Besides, it has functions to check satellite resources.
The ground network for ASNARO system will consist of ground stations in Hokkaido, Okinawa, Singapore and Svalbard (Figure 17). Each ground station will be equipped with receivers which can demodulate the 16 QAM signal used by ASNARO. Each ground station will be connected to the data center with high speed network. In addition to this ground network, customer ground stations and integrated mobile stations will also be available as part of ASNARO ground stations’ network.
Figure 18: Functional diagram of the data center (image credit: PASCO)
Figure 19: Communication cones of the ASNARO ground stations for ASNARO system projected onto Earth (image credit: PASCO)
Features of Integrated Mobile Stations: An integrated mobile station is comprised of a ground system including the 4.6 m diameter antenna on the truck. It is an all-in-one system with all functions necessary for the satellite operations, e.g. mission control, mission planning, ground station, mission data processing and GIS (Geographic Information System). The main objective is to provide emergency response in case of disaster and image data reception all over the world. It’s a cost effective, compact in size and high performance system.
To meet these requirements, the integrated mobile station is being developed with the concept of being compactness, labor saving, adaptive environment, modularized, low cost and quick data delivery.
Table 5: Concept of the development of the integrated mobile station (image credit: PASCO)
Configuration of an Integrated Mobile Station: An integrated mobile station has 4.6m diameter antenna for satellite tracking and shelter room having houses processing systems on the truck. Roughly, it has functions of the data center and the ground station. The newly developed outrigger dedicated for the antenna reduces the swift of the antenna and enables the tracking on the satellite even under strong wind condition. In the shelter room, devices for antenna system and processing servers are stored in 3 racks which allows the operator large working space.
It has a Data Integration and Solution System, which integrates the satellite image data with other data sources and provides the analyzed data. It is also equipped with an A0 plotter to print out the images and analyzed data for the onsite information sharing. For the communication with external, it has mobile data communication system and satellite communication system. The shelter room protects the operator working inside from the radio transmission from the antenna. Inside the shelter room is fully air-conditioned to keep the operational temperature and humidity of computers and equipments. It is equipped with lightning rod to protect the antenna and the truck from lightning and anemometer to monitor the wind speed (Figure 20).
Table 6: Main functions of an integrated mobile station (image credit: PASCO)
Figure 20: Integrated mobile station for ASNARO system (upper: transportation position, lower: operational position), image credit: PASCO
Innovative connectivity of the space segment and the ground segment:
The developed innovative connectivity of the space segment and ground segment will enhance the operation capability and comparable to large scale satellites.
Improved acquisition capability: For attitude control of the satellite, 3-axis reaction wheels are utilized. In addition to the agility by using reaction wheels, the ground segment will have improvements for the acquisition efficiency, acquisition chances and success rate of the acquisition.
1) Acquisition efficiency: The ASNARO spacecraft has several acquisition modes. The basic acquisition mode is the Snapshot mode (10 km x 10 km). However, depending on the largeness or shape of the area of interest, it is more efficient to use the StripMap mode or the Skew mode. In the ground segment, based on the area of interest (AOI) requested from the end-user, the optimum acquisition mode is automatically selected and a most efficient acquisition plan is programmed. In the planning, the satellite resources (storage and power) are considered to optimize the acquisition planning in mid to long term.
2) Acquisition chances: By optimizing and speeding up the process from mission planning to the command generation, it is possible to accept the acquisition request from end-users until 30 minutes before uplink. This enables to implement an acquisition plan just before the image acquisition in case of an emergency and leads to the improvement of acquisition chances. From mission planning to the uplink, every process is automated and nominally no operator action is required.
3) Acquisition success rate: In the mission planning process, target candidate areas which are likely to be covered by less cloud are preferably selected according to the weather forecast and observatory data to improve the acquisition success rate. As mentioned above, the final deadline for the image acquisition request is 30 minutes before uplink. By utilizing high latitude ground station, in nearly all around the world it is possible to refer the weather information 30 minutes to 2 hours before image acquisition. By using global numerical weather prediction models as reference information, prediction accuracy for one hour later weather is better than 90% and the improvement of acquisition success rate is expected. It is also possible to add different kinds of weather data as add-on.
Improved data delivery: To reduce the power consumption of the satellite, image acquisition and downlink are separately performed. The communication speed of ground network and image processing speed are improved which lead to a highly efficient data delivery.
1) Communication speed of ground network: By connecting ground stations and the data center by high speed Wide Area Ethernet of up to 1 Gbit/s which is as fast as the downlink rate of ASNARO (16QAM, ~800 Mbit/s), real-time data transmission from the satellite to the data center is realized.
2) Image processing speed: To perform distributed processing of raw image data utilizing the multi-core CPU, products required by end-users can be generated in short time. Overview of the product generated by this system and targeted processing time is shown in Table 7. For any product, the target time for the delivery to end user is less than one hour from image acquisition.
Product accuracy: In the ground segment, in addition to orbit and attitude information from the satellite, external information is used to perform correction processing with high accuracy and pixel localization accuracy as good as large sale satellites. The targeted localization accuracy is less than 10 m (CE90) for Level 1B product. The approach for that is described in the following.
1) Introduction of “Precise Position Determination System”: The star tracker calibration, estimate of optical sensor directional alignment and precise attitude determined value processing is performed in the ground segment to reduce the error between the attitudes of optical sensor, internal error of optical sensor and error of attitude determination. This enables the improvement of pixel localization accuracy.
2) Initial calibration and periodical calibration: During commissioning phase and nominal operation phase, evaluation result of the product accuracy will be statistically processed and by correcting the parameter the product accuracy will be improved. For the product evaluation, GCPs (Ground Control Points) prepared for the calibration will be used as true value. The evaluation using GPSs will be performed over the whole earth. This is to provide the product in any area with same accuracy.
3) Construction of reference GCP database: By constructing reference GCP database over the entire earth and performing fully automatic matching with acquired image data and image correction, localization accuracy of the product will be drastically improved.
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24) Processing time for one Snapshot scene (10 km x 10 km)
25) Cumulative processing time from raw data
26) In case of Level 1 B pan-sharpen
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.