The space segment consists of a spacecraft bus and an electro-optical payloadScroll Down
The space segment consists of a spacecraft bus and an electro-optical payload. The electro-optical payload is a push-broom camera with TDI sensors (1 panchromatic and 4 multi-spectral bands). The spacecraft has a Ground Sampling Distance of 1m for Panchromatic and 4m for Multi-spectral, with a 12.2 km Swath Width. DubaiSat-2 has the capacity to store approximately 17,000 km2 of image data. The modules in the satellite use two CAN Bus networks to communicate with each other. DubaiSat-2 also includes an experimental propulsion system for orbit correction and maintenance. The satellite’s expected lifetime is at least five years.
Figure 1 shows DubaiSat-2 system architecture.
DubaiSat-2 has a hexagonal shape with a philosophy of separating the Bus from the Payload. The mechanical bus consists of 2 decks and an upper Sun shield. The electronics are distributed on the decks and on the side panels. Four solar panels are attached to the sides of the satellite. Longerons and rails make the bus structure frame. On the top, CFRP struts hold the Sun shield at the baffle of the payload HiRAIS.
HiRAIS is attached to the bus at the internal deck. The mechanical configuration of the satellite has < 2000 mm height and < 1500 mm in diameter. The total mass of the satellite doesn’t exceed 300 Kg.
Figure 2 DubaiSat-2 Structure
Figure 3 DubaiSat-2 Mechanical Configuration
DubaiSat-2’s Electrical Power System (EPS) supplies and controls the required voltage levels and current that is essential for the satellite operation during its long life mission. The EPS uses a rechargeable Li-ion battery to provide power for the satellite’s payload and other subsystems. The system is divided into two stages, first is the charging stage and the second is the discharging stage.
The charging stage is made of power generator (solar panels) and power regulator. DubaiSat-2 generates more than 450W of power using four Solar Panels (SP). Each SP contains 6 arrays and each array consists of 26 cells. The solar panels charge the batteries. The battery charging process is handled and regulated by the Battery Charging Regulators (BCR) modules. DubaiSat-2 has three BCRs with hot redundancy configuration. The spacecraft can function normally with only two BCRs.
The second stage of the EPS is the discharging stage. In this stage, the batteries outputs are converted and distributed to other subsystems. The conversion process occurs in Primary Power Conditioning Module (PPCM) and Secondary Power Conditioning Module (SPCM), where discharged voltage produced from the batteries (26V – 32V) is converted into unregulated -15V, -12V, +5V, +12V and +15V. These voltages then go to the Primary Power Distribution Module (PPDM) and Secondary Power Distribution Module (SPDM), which control and distribute these voltages to the related spacecraft modules via power switches. The On-Board Computer (OBC) through Primary Power Interface Board (PPIB) and Secondary Power Interface Boards (SPIB) controls these power switches.
DubaiSat-2 EPS also includes Power Safety and Separation Module (PSSM) to insure the stability and reliability of EPS system. The PSSM was designed to save and maintain battery voltage level prior to spacecraft launch. Once the satellite separates from the launcher, PSSM allows the power to flow into the satellite. Figure 4 shows EPS configuration in DubaiSat-2.
Figure 4 EPS Configuration
The Command and Data Handling Subsystem (C&DH) handles all telecommands received by the spacecraft and collects telemetries from all subsystems. It also provides the required environment for the satellite’s onboard software. Two separate CAN networks, CDH and ACS CAN, are used to exchange data between C&DH and other subsystems. Each CAN network has a 500 Kbps data rate. ACS CAN is dedicated to ACS modules and CDH CAN to all other modules. The C&DH is shown in Figure 5 and consists of the following modules:
Figure 5 C&DH Subsystem
Figure 6 C&DH Mechanical Configuration
4.1. Telemetry and Telecommand Module (TMTC)
DubaiSat-2 has two TMTCs with hot redundancy configuration. Both TMTCs share the same board. The TMTC performs all CCSDS telecommands decoding and CCSDS telemetries encoding, reducing the load on the on-board computer. It receives telecommands sent by the ground station through the S-band demodulator, and sends telemetries to the ground station through S-band modulator. The TMTC also acts as a watchdog to the DubaiSat-2 OBC. It has an autonomous reconfiguration capability, which insures normal spacecraft operation in the event that the primary OBC fails.
4.2. On-Board Computer (OBC)
The On-Board Computer (OBC) module is the main central processing unit in the satellite. It communicates with all modules through CAN bus network. All the telemetries generated by the different subsystems go to OBC were they are recorded and processed. OBC also handles all decoded telecommands received from the TMTC. Based on the received telecommands or the collected telemetries, the OBC decides and commands different DS-2 modules. OBC is directly connected to both CDH and the ACS CAN network buses. DubaiSat-2 has two On-Board Computers with cold redundancy configuration. The primary and redundant OBCs could be reconfigured either through a ground station command or autonomously by TMTC.
4.3. Interface Boards (IBs)
The Interface Boards (IB) are used to interface satellite modules and unites to the two CAN bus networks. The IBs performing the following tasks:
All interface boards do the same tasks, but each board is designed to accommodate the needs of the device it's interfacing with:
The real time Flight Software (FSW) of DubaiSat-2 has high reliability and robustness. It controls all autonomous operation of the satellite in Space. The backbone of the FSW is the VxWorks real time operating system. Several application tasks are running concurrently to perform satellite health monitoring, control and data logging (HouseKeeping Task), HiRAIS management, imaging and data download operations (Payload Task), internal and external network management, Onboard SW management (Executable Task), large data handling (Large Data Transfer Task), flight attitude and orbit control (Flight Control Task). The FSW runs on OBC that has the capacity to save data for up to 3 days. It also has the redundancy in EEPROM boatloading operation. The FSW performs memory wash operations and is also supported by hardware EDAC to detect and recover from Single Event Upset (SEU).
Figure 7 Flight Software Overview
The Attitude and Orbit Control Subsystem (AOCS) is designed to advance agility and stability performance of the satellite during mission operations. With a relatively high Moment of Inertia (MoI) values, the satellite is designed to perform Single Strip Imaging, Multi-Strip Imaging, and Single-Pass Stereo Imaging. The agility of the satellite can reach up to 60° maneuver within 90 sec. The pointing accuracy is <0.12° (3 σ) and stability is < 0.009°/sec, the AOCS is designed to satisfy TDI sensor operations requirements. It consists of 5 Reaction Wheels (RW) in constant operation with 4 Fiber Optic Gyros (FOG). For the measurements, Fine Sun Sensors and Magnetometers are used and Star Trackers for fine measurements. Also, Hall Effect Propulsion System (HEPS) is used to perform Orbit maneuvers in Space. It uses electric propulsion with Xenon gas and utilizes a Microwave Cathode similar to the one used for HAYABUSA ion propulsion system.
Figure 8 DubaiSat-2 Imaging Scenarios
Figure 9 Fast Multi-Strip Imaging Operation
The Telecommunications Subsystem (TS) provides the communication link between DubaiSat-2 and the ground station, which is used for TT&C. The uplink channel on the spacecraft consists of S-band Receiver and a 32kbps BPSK Modulator. The downlink channel includes a 32kbps BPSK demodulator and S-band Transmitters. The receivers have a hot redundancy configuration, while the transmitters use a cold redundancy configuration. Figure 10 below shows DubaiSat-2 Telecommunications Subsystem configuration.
Figure 10 DubaiSat-2 Telecommunications Subsystem
High-Resolution Advanced Imaging System (HiRAIS) is Dubaisat-2 primary payload. It consists of three main units:
The Electro Optical Subsystem (EOS) consists of the telescope, Auxiliary Camera Module (ACM) and Focal Plane Assembly (FPA) all integrated into one system. It’s an advanced imaging system with a Korsch optical design of 5 mirrors. The optical design of five mirrors includes the main mirror (M1) of 415 mm diameter, 3 mirrors to increase the focal length up to 5.7 m, and a flat mirror to reflect light rays onto FPA. Light-weighted Zerodur is used for the mirrors, while CFRP material was used to design the main opto-mechanical structure of HiRAIS. The temperature balance of the mirrors surface, the distances between the mirrors and the FPA are all actively controlled by a feedback heating system, which includes thermostats and heaters. In addition to passively with heat dissipative materials.
Figure 11 HiRAIS Structure
Figure 12 HiRAIS Optical Design Overview
ACM is a power-conditioning unit, which provides the necessary voltage levels for FPA. It also includes the CAN interface to control FPA imaging operation and to provide EOS electronic telemetries to the main On-board Computer (OBC). The FPA includes the push-broom CCD TDI sensor, which collects light from the telescope and converts it into an analog signal. It then converts these analog signals into 10bits digital data and sends it to SSR.
Figure 13 ACM
The processing, storage and maintenance of image data is then handled by the Solid State Recorder Unit (SSRU), which consist of 2 Control and Interface Boards (CIB), 4 Storage Boards (SB), 2 Power Boards (PB) and a Backplane Board (BP). All boards in SSRU are connected through the Backplane Board (BP). The CIB and PB have a cold redundancy configuration.
Figure 14 SSRU
The CIB include 8051 microcontroller responsible for the overall management and operation of SSR. It communicates with OBC through the CAN bus network. The CIB receives the image data from the FPA and bypasses the data to the dedicated memory location in SB. Moreover, CIB also handles the real-time compression and CCSDS formatting of image data before sending it to the XTU.
Storage Board (SB) is where the image data is stored and maintained. Each SB consists of 64Gbit of data capacity. The total storage capacity in SSRU is 256Gbits, which is capable of storing approximately 17,000 km2 of image data. The Power Board (PB) provides power to all boards in SSRU. It receives power directly from EPS and uses DC-DC converters to generate the required voltages.
The X-band Transmission Unit (XTU) receives image data from CIB in SSRU and downlinks it to the ground station. The XTU has a 160 Mbps data rate and uses a one-axis (-90 to +90 degrees) steerable antenna.
Figure 15 X-band One-Axis Gimbal Antenna
The Hall Effect Propulsion System (HEPS) is an electrical propulsion system with Xenon gas fuel and microwave cathode. It will be used for orbit correction and maintenance. The system consists of Xenon Fuel Unit (XFU), Thruster Head Unit (THU), Power Processing Unit (PPU) and the Cathode Unit (MCU). It has a force of > 7 mN and a specific impulse of > 1000 sec. it will be used mainly when the satellite is close to the South Pole in a descending SSO orbit, due to its high power consumption of 300 W during its operation.
The XFU includes a main tank of 150-bar pressure and 2 Kg capacity of Xenon. It also includes pressure valves and orifices to control the flow of the fuel into two small tanks of 3 bars each for both the anode and cathode. The THU consists of a thruster head that includes magnets for ion motion direction. The PPU distributes different power voltages for HEPS system like 250, -100, 15 and 5 V. It also has the switches for the XFU control. The MCU is provided by JAXA as a cooperative project to measure its performance, since a similar unit was used for the deep space mission HAYABUSA.
Figure 16 DubaiSat-2 HEPS
Figure 17 HEPS Operation
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