The International Telecommunications Union (ITU) divides satellite communication services into three categories: fixed-satellite service (FSS), mobile satellite service (MSS), and broadcasting satellite service (BSS). The traditional emergency satellite communication is mainly an FSS system, which uses a C/Ku high orbit satellite to construct the VSAT satellite communication system. However, in recent years, the MSS system has played an increasingly important role in major natural disasters, especially within 72 h of the disaster. When the ground network cannot be used, the rescue organization needs to use the MSS system to understand the real-time messages in the disaster area, so as to organize effective rescue activities.
At present, most communication satellites in the world are mainly GEO satellites, which have wide coverage and a relatively stable location. However, with the increasingly crowded orbital space and the development of cellular communication, multiple access, spot beam, and other small satellite technologies, LEO satellites have gradually been application in space-based communication systems; these satellites have short transmission delay, small path loss, and more effective frequency reuse. A number of countries have successively launched large-scale LEO satellite systems.
Inmarsat was the first global mobile service satellite communication system in the world. The user link uses the L-band to provide low speed digital voice, data communication, facsimile, and telegraphy services. Inmarsat’s first to fourth generation satellite user links all adopted the L-band, while the Inmarsat fifth-generation satellites use the Ka-band, based on DVB-S2 technology, and support broadband services having uplink of 5 Mbit/s and downlink of 50 Mbit/s.
Iridium utilizes on-board processing, on-board switching, and interplanetary link technologies, and uses TDMA multiple access for direct communication between handheld terminals and satellites, providing users with voice, data, paging, fax, and other services. “Iridium NEXT” mainly aims at IP broadband networking and the scalability and upgrading of load capacity, which will enable it to meet the complex needs of future spatial information applications.
Starlink adopts a new connection protocol that is more lightweight than IPv6 and supports P2P. It is a decentralized network similar to blockchain. Satellites can accurately understand the longitude and latitude of each user’s physical location and reasonably distribute their data links. Starlink’s second generation system will employ inter-satellite laser links to provide seamless network management and service continuity. Starlink users access the Internet by converting the satellite signal into a Wi-Fi signal through a Wi-Fi router.
The Oneweb system has set up more than 50 gateway stations around the world, and its user link adopts the Ku-band. The user terminal can be used as a local Internet access point, and users can access the Internet through 3G, LTE, Wi-Fi, or satellite links.
The BeiDou satellite system is an active satellite positioning and communication system independently developed by China. It is the third satellite navigation system covering the world after the Global Positioning System (GPS) of the United States and the Global Navigation Satellite System (GLONASS) of Russia. BeiDou satellites can not only provide rapid positioning and timing services for ground users worldwide, but also enable short digital message communication, which is one of the means of ensuring communication in an emergency.
The Hongyan global satellite constellation communication system is planned to be composed of 300 low orbit small satellites and a global data service processing center, with all-time two-way communication capability under complex terrain conditions. It will provide users with global real-time data communication and comprehensive information services. The Hongyan constellation will integrate a number of satellite application functions. Its satellite data acquisition function can realize large-scale regional information collection and meet the needs of monitoring data information transmission in the fields of oceanography, meteorology, transportation, environmental protection, geology, and disaster prevention and reduction. According to the existing information, it is expected that the system will include a communication network of 60 artificial satellites by 2023. By 2025, the second phase will be completed, and a mobile satellite communication and network access system having space-air-ground-sea integration will be built.
The Hongyun project is a “star chain plan” independently developed by China. It is expected that 156 satellites will be launched into orbit 1000 km above the ground. The project is committed to building a satellite-borne broadband global mobile Internet that can provide integrated services for communication, navigation, and remote sensing for all types of users around the world. Because of its extremely low communication delay, extremely high frequency reuse rate, and real global coverage, the Hongyun project can meet the needs of China, underdeveloped Internet areas, and large-scale user units to share broadband Internet access. At the same time, it can also meet the application requirements of high real-time information interaction, such as in emergency communication, sensor data acquisition, industrial Internet of Things, and remote control of unmanned equipment.
The Tiantong-1 satellite mobile communication system is a satellite mobile communication system independently developed and constructed by China. It is also an important part of China’s space information infrastructure, and solves the problem of seamless signal coverage in a particularly complex environment. The Tiantong-1 01, Tiantong-1 02, and Tiantong-1 03 satellites were successfully launched in 2016, 2020, and 2021 respectively. After the Tiantong-1 03 satellite was launched into orbit, it was networked in orbit to the Tiantong-1 01 and Tiantong-1 02 satellites. The Tiantong-1 satellite mobile communication system will achieve regional coverage in the Asia-Pacific region. It has the advantages of strong reliability, strong real-time performance, strong confidentiality, wide coverage, and long communication distance. It can provide stable and reliable voice, short message, video, and other mobile communication services, and improve the service level of satellite communication and emergency communication support capacity. The Tiantong-1 satellite system is a narrowband mobile satellite communication system. In case of natural disasters, the emergency communication capability of Tiantong-1 can play a significant role.
3.3. Hybrid Networks
Emergency communication networks are typical complex networks, involving a variety of communication technologies. Their network composition is diverse and changes dynamically. As a new type of self-organizing wireless network, WMN can not only effectively combine with emergency communication methods such as satellites and short-wave, but can also be used as an extension of mobile communication networks and metropolitan area networks to provide more extensive wireless access coverage and comprehensively improve the quality of emergency communication.
Under 5G with high bandwidth and low delay, WMN has more technical advantages, improving the synthesis, application, and systematization. The unique advantages of the ad hoc network enable it to occupy a position in the field of emergency communication. In the future, ad hoc networks will have an ideal space for improvement in miniaturization and portability. In the application of emergency communication, the integration of mesh networks with other private network communication and satellite communication systems will also be the focus of future development. Future development will also include the integration of 5G with satellite networks, 5G with UAV integration, and the space-ground convergence network.
3.4. 5G Networks
5G network technology includes network slicing technology, ultra-dense heterogeneous network technology, and multi-access edge computing technology. In emergency rescue work, the network slicing technology can be used to realize the interconnection of various devices, thus comprehensively improving the efficiency of emergency rescue command and promoting the development of rescue work. With the support of 5G network technology, all regions need to speed up the construction of emergency rescue communication equipment, especially that used for the network control center, satellite ground station, and converged communication platform. At the same time, it is also necessary to use emergency communication equipment to expand network coverage and promote the development of emergency rescue work.
4. Future Development Directions
With the rapid development of network technology and information technology, future emergency networks will develop in the direction of intelligence, integration, popularization, and low cost. This will mainly introduce network intelligence and integration, and lead to the development trend of space-air-ground-sea integration networks.
Due to limitations in the radio spectrum, service geographic coverage, and operating costs, 5G ground communication networks alone cannot provide high-quality and reliable services at any time and in any place. This is especially the case in remote areas such as oceans, forests, and deserts. In order to provide truly ubiquitous wireless communication services on a global scale, it is necessary to develop an integrated space-air-ground-sea network to achieve global connectivity. The rapid development of wireless, broadband, fusion, and three-dimensional network technology provides the construction foundation for the integration of the space-air-ground-sea network with information sharing, interconnection, and unified command. Integrated use of big data, cloud computing, AI, satellite communications, multimedia communications, mobile communications, and other advanced technologies, and integration of space-based, air-based, ground-based, and sea-based platforms, can be used to construct the space-air-ground-sea integrated emergency communication network architecture. This architecture can realize the overall emergency resource planning and collaborative services of multiple platforms. The network architecture is shown in Figure 7. The integrated space-air-ground-sea network is based on a ground network, extended by air-based and sea-based networks, and connected and supplemented by a space-based network; thus, it covers the natural space including space, the sky, the ground, and the ocean. The air-based network consists of satellites, which can be divided into GEO, MEO, and LEO satellites. The space-based network is composed of aerial platforms such as near-space airships, airplanes, UAVs, and hot-air balloons. The ground-based network is mainly composed of LANs and cellular mobile networks, including all wired and wireless network facilities on the ground. Currently, this mainly relies on 5G network technology. The sea-based network consists of coastal base stations, surface ships, buoys, etc.
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Figure 7. System topology of the space-air-ground-sea integrated emergency communication network.