1. Introduction
Vietnam Space Center project started in 2011 in order to build a hi-tech space center at Hoa Lac Hi-tech Park, Hanoi, Vietnam. It is a project under cooperation in space technology between Vietnamese and Japanese government. Within the framework of the project, 22 researchers of VNSC have been sent to five Japanese universities (The University of Tokyo, Keio University, Tohoku University, Kyushu Institute of Technology, and Hokkaido University) and 14 more researchers are going to be sent in 2015. They are taking master courses about satellite technology in order to work for Vietnam Space Center when it is completed. During the time in Japan, they also participate in an educational satellite project.
The result of this project will be the first Vietnamese micro satellite called MicroDragon (MDG). It is developed by VNSC researchers under instruction of the Japanese professors come from these universities. The main missions of MDG is assessing coastal water quality of Vietnam to develop aquaculture and locating living resources by observing ocean color.
2. Mission Definition
2.1. Mission Statement
Viet Nam has a coastline of 3 260 km that crosses 13 degrees in latitude, from 8°23’N to 21°39’N. There are four main fishing areas that are composed of Gulf of Tonkin shared with China, Central Vietnam, Southeastern Vietnam, and Southwestern Vietnam (part of Gulf of Thailand) shared with Cambodia and Thailand. Marine catches are the highest in the Central and the Southeast of Vietnam. Apart from these geographical zones, the fishing areas can be divided in inshore-coastal fishery and offshore fishery. Inshore water is considered as the water of less than 30 m deep in the Tonkin Gulf and the South and less than 50 m deep in the center of Vietnam.
Vietnam has huge potential of coastal aquaculture with shrimp culture being dominant. Brackish-water shrimp (Penaeus species) is the main species raised along the coast. The enormous increase of the brackish-water aquaculture has some negative impacts as the silting of the inland area as the aquaculture areas are up to 10 km inland and the reduction of the mangrove area.
The Government has put forward the following policies: Use the state capital to invest for scientific research, building centers for aquatic seed production, human capacity building, establishing stations for environment monitoring and forecast, fishery extension activities, aquatic feed production and medicine for aquatic animals.
The fisheries and aquaculture sectors are significant contributors to the economy of Vietnam. Seafood is always in top 10 of main exporting goods of Vietnam. There are about 10% households in the fishery. Vietnamese fisheries have been growing considerably and promoted by the government, aiming hunger elimination and poverty reduction and at the same time increase people’s income.
However, Aquatic natural resources are dwindling because of the increasing exploitation. Therefore, the development of aquaculture is essential for sustainable economics. In order to develop aquaculture, we have to know water quality, which is suitable for each species. However, monitoring of the coastal water quality using only water samples taken in vast coastal area, it’s not only time and labor consuming but also lack of the necessary temporal and spatial information. Thus, a system with integration of remote sensing technique and seawater sampling is more effective to do that.
2.2. Mission Objective
Ocean color observation to assess coastal water quality and locate living resources is a main mission of MicroDragon satellite. The system will provide data which researchers and scientists in fishery field and oceanography use to analysis and distribute information to fishermen and environmental managers.
3. Development Plan
According to the plan, all of project works should be completed by September 2017 and the satellite is assumed to be launched in 2018. The MDG’s orbit type is assumed as Sun Synchronous, altitude 666 km and Local Ascending Node is 13 pm.
4. Current Design (till Preliminary Design Review (PDR))
4.1. System Architecture
The satellite consists of seven subsystems namely Payload Subsystem, Structure Subsystem, Thermal Subsystem, Attitude Determination and Control Subsystem (ADCS), Command & data Handling (C&DH) Subsystem, Electric Power Subsystem (EPS) and Communication (COM) Subsystem.
4.2 Subsystems Design
4.2.1 Payload Subsystem
Payload Subsystem includes Space-borne Multispectral Imager (SMI) and Triple Polarization Imager (TPI) cameras for taking images and assessing coastal water quality of Vietnam. Besides, secondary missions of the MDG such as: Store&Forward (S&F), Antimony Tin Oxide Coating Solar Cell (ATOCSC) and Atomic Oxygen Sample (AOS) are also implemented for scientific purposes.
4.2.2 Command & Data Handlin (C&DH) Subsystem
Onboard computer is a hardware that is installed in satellites to carry out Command & Data Handling functions including: Gathering telemetry and mission data, process and send them to the ground; Receiving commands from the ground, process and distribute them to designated components; Managing system clock and process timing; Monitoring satellite states and take proper actions if necessary without commands from the ground.
4.2.3 Structure Subsystem
Figure 6. External structure of MicroDragon
Structure of the MDG satellite is a 50cm cube and T shaped panel supports its structure mainly while keeping accessibility to all of components and ease of integration. Fig.6 shows the external structure of the MDG satellite with several components.
4.2.4 Attitude Determination and Control Subsystem (ADCS)
The subsystem employs an onboard computer, called the attitude control unit, and a wide range of the sensors and actuators in order to control the satellite’s attitude. There are five types of sensors installed in MicroDragon: six non-spin solar aspect sensors (NSAS), a geomagnetic aspect sensor (GAS), a star tracker (STT), a fiber optical gyroscope (FOG) and a GPS (antenna and receiver). Four reaction wheels (RW) are utilized as actuators for attitude control. MicroDragon also has three magnetorquers (MTQ), used to de-tumble the satellite during the de-tumbling period and to de-saturate the reaction wheels. All of these components are controlled by the onboard computer (OBC).
4.2.5 Communication (COM) Subsystem
The COM subsystem includes S-band transmitter and receiver (STRX), S-band antenna (S-ANT), X-band transmitter (XTX) and X-band antenna (X-ANT). By using two S-band antennas, the S-band transceiver is responsible for transmitting telemetry of the MDG satellite from Onboard Computer (OBC) and receiving tele-commands from the ground. While the S transceiver is connected to (OBC), the X-band transmitter has a data line with Science Handling Unit (SHU). Mission data which is stored in SHU is transmitted to the X-band transmitter, then the iso-flux X-band antenna to the ground. All of specifications of COM components are listed as in Table 1.
| STRX
(S-band transmitter and receiver) |
|
| S-ANT |
|
| XTX
(X-band Transmitter) |
|
| X-ANT |
|
| S-HYB |
|
Table 1. COM Specifications
4.2.6 Electrical Power Subsystem (EPS)
EPS is responsible for generating and providing power for the satellite to operate in the orbit. EPS includes components such as: Solar Array Paddle/Solar Cell Panel (SAP/SCP), Li-Ion Battery Module (LIBM), Power Control Unit (PCU), Power Distribution Unit (PDU), three separation switches and flight pin. The configuration of solar cells of the MDG satellite is shown in Fig. 10.
5. Summary of MDG satellite parameters
| Satellite mass, size | ~ 50 kg, 50 cm x 50 cm x 50 cm |
| Main Payload | Space-borne Multispectral Imager (SMI) and Triple Polarization Imager (TPI) cameras |
| OBC (Onboard Computer) | RS422 Interface |
| ADCS (Attitude Determination and Control Subsystem) | Nonspin Solar Aspect Sensor (NSAS) (6), Geomagnetic Aspect Sensor (GAS), Star Tracker (STT), Fiber Optics Gyroscope (FOG), GPS Receiver
– Magnetic Torquers (3), Reaction Wheels (4) |
| RF Communication (COM) | – S-band: downlink = 64 kbit/s, uplink = 4 kbit/s; 2 S-band patch antennas
– X-band transmitter: 10 Mbit/s, CCSDS compliant; X-band iso-flux antenna |
| Electrical Power Subsystem (EPS) | SAP generation: 140W (max)
Li-Ion battery: 22.0-33.6 V, capacity: 5800 mAh |
Table 2. Overview of satellite parameters
