Communication system for maritime satellite phones

The Inmarsat system is the world's first commercial satellite mobile communication system managed by Inmarsat. The original Chinese name was "International Maritime Satellite Communications System" and now it has been renamed "International Mobile Satellite Communications System". °In the late 1970s and early 1980s, Inmarsat leased American Marisat, European Marecs and Intelsat-V satellites (all GEO satellites) to form the first generation of Inmarsat systems to provide marine vessels with Global maritime satellite communication services and necessary safe rescue channels for maritime disasters. The three second-generation Inmarsat satellites were deployed in the early 1990s.

For the early first- and second-generation Inmarsat systems, communication could only be carried out between ship stations and shore stations. Communication between ship stations should be transferred by the shore stations to form "two-hop" communication. The operating system is third-generation Inmarsat with spot beams, direct communication between ships and stations, and support for portable telephone terminals. The space segment of the Inmarsat system (third generation) consists of four GEO satellites, covering the Pacific Ocean (the satellite is positioned at 178° east longitude), the Indian Ocean (65° east longitude), the eastern Atlantic Ocean (16° west longitude) and the western Atlantic Ocean (longitude west). 54°). The system's Network Control Center (NOC) is located at the Inmarsat (International Mobile Satellite Organization) headquarters in London. It is responsible for monitoring, coordinating and controlling the operations of all satellites in the network, including satellite attitude, fuel consumption, on-board working environment parameters and equipment. Monitor the working status, supervise the operation of various earth stations (shore stations), and assist the network coordination station in coordinating relevant operational matters.

The system has some earth stations (customarily called shore stations) near the coast of each ocean area, and at least one network coordination station (NCS). The shore station is owned by the competent authorities of the Inmarsat signatory countries. It is not only the interface with the ground public network, but also the control and access center of the satellite system. Its functions include: responding to user calls (from ship stations or land users): identifying ship stations code to identify, allocate and establish channels; register calls and generate billing information; monitor and manage channel status: monitor and receive maritime disaster information: compensate for satellite transponder frequency deviation; pass satellite self-loop testing and basic monitoring of ship stations Testing etc. Typical shore station antenna diameter is 1lm ~ 13m. The network coordination station manages and coordinates the channels in the entire ocean area, allocates and controls satellite circuits according to the requirements of shore stations for calling telephone circuits; monitors and manages channel usage, and in emergencies, forcibly inserts the conversation channel and issues a call for help Signal.

The composition of the Inmarsat system is shown in Figure 7-21. The ground segment includes network coordination stations, shore stations and ship stations (mobile terminals). The link between the satellite and the ship station uses the L-band, uplink 1.636GHz ~ 1.643GHz, downlink 1.535GHz ~ 1.542GHz; between the satellite and the shore station, C and L dual-band operation. When transmitting voice signals, the C-band (uplink 6.417GHz ~ 6.4425GHz, downlink 4.192GHz ~ 4.200GHz) and L-band are used for user telegrams, data and distribution channels.

For "maritime" from satellites to sea vessels Channel, since the elevation angle of the ship station to the satellite is usually greater than 10°, and the sea surface is rough enough for the electromagnetic waves in the L-band, there is no specular reflection component. Therefore, in addition to the direct component, the received signal only contains diffuse reflection multipath. Component, this "maritime" channel is the Rician channel. Inmarsat's ship stations mainly have three standard types: A, B, and C. The main parameters are shown in Table 7.2.

Type A station<. /p>

It is the main large-scale ship terminal in the early days of the system (1980s). It adopts analog FM mode and can support voice, fax, and high-speed data (user telegram). It adopts BPSK modulation/demodulation mode with a rate of 56 /64kb/s, and has emergency communication services in distress.

Type B station

It is a digital replacement product of type A station. It supports all services of type A station. The digital voice rate is 16kb/s, which is higher than that of type A station. The frequency and power utilization rate (the bandwidth of type A station is 50kHz, and the bandwidth of type B is 20kHz; the satellite power used by type B station is only half of that of type A), the cost of the space segment is greatly reduced, and the volume and mass of the terminal station are smaller than those of type A. The number of sites has been reduced a lot.

Type C station

is a small terminal for global store-and-forward low-speed data. Ship-mounted or vehicle-mounted C-type terminals use omnidirectional antennas and can communicate while traveling. Portable or fixed terminals use small directional antennas. The channels of the C system include information channels and signaling channels, etc., with a rate of 1200b/s, of which the information channel transmission rate is 600b/s (also the transmission rate of type C terminal). It supports data and fax services, and is also widely used in group call safety networks, vehicle and ship management networks, telemetry, remote control and data collection, as well as distress alarms, etc.

D-type terminal

It is a ground terminal used in the Inmarsat global satellite short message service system. It supports two-way short messages between the headquarters and personnel in remote areas, unattended equipment and sensors. Information communication. The terminal can receive messages of 128 characters, and can also send short messages (less than 3 bytes) and long messages (less than 8 bytes). The terminal can have a built-in GPS receiver.

E-type terminal

It is a satellite emergency radio position indicating beacon terminal and a special equipment for global maritime distress warning. When a ship is in distress, the E-type terminal will float on the sea surface and immediately send out an alarm signal (including position coordinates, ship's class, etc.), which will be transmitted to Inmarsat's emergency radio position indication beacon processor via satellite. Typically, distress information can be transmitted to the search and rescue center within one minute.

Type M terminals

are small digital telephone (4.8kb/s), fax and data (2.4kb/s) terminals. For the fourth generation

Inmarsat-3 spot beam system, the M-type terminal evolved into the smaller Mini-M or Inmarsat-Phone type, whose size and quality are equivalent to that of a notebook computer. This terminal has been widely used.

Aviation terminal

(Inmarsat-Aero) is used for communication between aircraft and between aircraft and the ground. There are many types of aviation terminals. Aero-C type is the aviation version of Inmarsat-C. It receives and sends data and messages in a store-and-forward manner. The information rate is 256b/s. The terminal uses a blade antenna with a gain of 0dBi. Aero-H terminals are mainly used for long-range commercial large aircraft. The terminal has 6/12 voice/data channels, and the terminal has a high-gain antenna with a gain of 12dBi. Aero-I is a widely used aviation terminal. It has 1 to 4 voice/data channels. It can make phone calls (4.8kb/s) within the spot beam of the third-generation satellite, but can only transmit within the global beam coverage. Low speed data (rate below 2.4kb/s).