Abstract: Based on the functional module division of the AOS system, this paper introduces a crucial component of the system: the downlink controller. This scheme utilizes the Ark series CPU chip, a core device with independent intellectual property rights in China, and designs and implements the downlink controller under the Linux operating system, presenting its main workflow. Experimental results show that the downlink controller effectively achieves the packaging and multiplexing of multiple data streams. Keywords: AOS; downlink controller; Ark; packaging; multiplexing 1. Introduction With the further development of aerospace technology, the Advisory Committee on Space Data Systems (CCSDS), established in 1982, formulated the Advanced On-Orbit System (AOS) framework and standards aimed at meeting the needs of complex spacecraft. Currently, this standard is still being revised, supplemented, and developed. It is a product of the era of international cooperation in aerospace telemetry, tracking, and command (TT&C) and communication, and also an inevitable development in aerospace TT&C and communication technologies. Space missions are increasingly characterized by a strong international nature, and international cooperation is essential for the rational development and utilization of space resources. For China's aerospace technology to go global, aligning with the AOS standard is the best choice. According to the relevant requirements of the standard, the AOS system can be divided into the following main parts: data source, link controller, combiner/splitter, etc. The downlink controller packages and multiplexes the data to be transmitted into standard-compliant packets at the data source end and sends them to the combiner for transmission. Based on the above functional division, the downlink controller is a key device in the entire system, belonging to the hardware entity in the space link layer. Its role is equivalent to the data access process in the virtual channel link control sublayer. 2. Controller Design The AOS system is mainly used in the data systems of complex spacecraft. Complex spacecraft need to transmit many types of data, including images, voice, data, files, emails, etc., with varying bit rates, from a few bits/s to hundreds of megabits/s, resulting in a large total information volume. The characteristic of the AOS system is that it can use different services for different types of data, and different services have different data processing methods and formats. The spacecraft data system using the AOS system ultimately merges numerous data of different types and rates into a unified data stream, which is then organized and sent to the space physical channel for transmission. In a spacecraft data system employing the AOS architecture, the downlink controller resides in the VCLC layer of the space data link layer. Its main functions include: packaging non-CCSDS packets (such as undelimited strings, IPv6, etc.) generated by the data system to generate Packed Protocol Data Units (E_PDUs), then multiplexing them to generate Multiplexed Protocol Data Units (M-PDUs), and using the VCA service of the Virtual Channel Access sublayer to transmit the M-PDUs to the combiner. User-generated data (such as CP_PDUs, IPv4, etc.) is directly multiplexed by the controller to generate Multiplexed Protocol Data Units (M-PDUs) and sent to the combiner. The functional block diagram of the downlink controller is shown in Figure 1. [align=center]Figure 1 Functional Block Diagram of Downlink Controller[/align] 2.1 Several Main Data Unit Formats 3. Controller Implementation For the implementation platform of the controller, we used the GT2000 embedded development board from ARCA Technology Co., Ltd., a Chinese company. The GT2000 is an embedded CPU product based on the ARCA II core, and is China's first CPU with independent intellectual property rights. It has a maximum power consumption of only 360 milliwatts when running at a 400MHz clock speed, featuring high performance and low power consumption. This is of great significance for researching the underlying interface devices, key modules, and equipment of AOS with independent intellectual property rights in China. The GT2000 provides two serial ports, one Ethernet port, and other peripheral interfaces. The link controller uses the Ethernet port for data transmission. Furthermore, the GT2000's embedded Linux operating system makes the inter-process communication mechanism very flexible, and the socket communication interface and various rich asynchronous time processing methods provided by the Linux system greatly facilitate our rapid data processing. Based on the functions performed by the controller, we implemented it on the Linux platform using standard C language. The overall workflow of the downlink controller is shown in Figure 2. First, the system listens on the Ethernet port. If a user requests to send data, the port is opened for reception. Then, the data type is determined. If the data type is not in CCSDSS format, the controller's packaging module encapsulates the data into a Packet Protocol Data Unit (B_PDU) and sends it to the multiplexing module. Other data, such as CP_PDU, IPv4, etc., are sent directly to the multiplexing module. The data sent to the multiplexing module is multiplexed to generate a Multiplexed Protocol Data Unit (M_PDU) which is then sent to the combiner. [align=center] Figure 2 Overall Workflow of the Controller[/align] The packaging function in the controller's operation is relatively simple and will not be elaborated here. However, during the multiplexing module's processing, it is necessary to set the first header pointer of the M_PDU header field to indicate the position of the first complete packet in the M_PDU, so that the receiving end can easily extract the various data packets in the M_PDU packet area. Simultaneously, when user data is multiplexed, since the multiplexed data is of variable length, the generated Multiplexed Protocol Data Unit (M_PDU) is of fixed length. If insufficient data units are obtained from the user, the multiplexing module needs to generate padding packets of appropriate length (in CCSDS protocol standard data units) and fill them into the M_PDU. The shortest padding packet length is 7 bytes (6 bytes header, 1 byte padding data). If the padding data required in an M_PDU is less than 7 bytes, a 7-byte padding packet is generated, which fills the M_PDU and then overflows into the next M_PDU. The header pointer assignment and padding workflow is shown in Figure 3. [align=center] Figure 3 Header pointer assignment and padding workflow[/align] 4. Testing and Verification The link controller can support multiple data streams. First, some real data is simulated on a PC: unbounded strings, IPv6, IPv4, and CP_PDU. The PC and controller are connected via Ethernet. Then, the various simulated data from the PC are sent to the controller, which receives and processes the data. Test results show that this controller can package and multiplex multiple data streams well, with no packet loss, and operates stably and reliably. 5. Conclusion This paper introduces the downlink controller of an AOS system designed based on domestically produced chips. This controller can effectively process simulated spacecraft data, and some key technologies in data transmission were verified, laying the foundation for further research. The author's innovation lies in the fact that current applications of AOS systems in China mainly rely on purchasing complete sets of equipment from developed countries or using key components with foreign intellectual property rights. This paper's development of AOS core devices with independent intellectual property rights using domestically produced chips is of great significance to the future development of China's aerospace industry. References [1] Gao Ke, Shan Jiayuan. Design of general real-time control software based on LINUX [J]. Microcomputer Information, 2006, 7-2: 61-63. [2] CCSDS 701.0-B-3: Advanced Orbiting Systems, Networks and Data Links: Architectural Specification. Blue Book. 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