What is a transponder in communication satellites

Alphabus: a new generation of satellites

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With Alphasat I-XL, the Alphabus satellite bus, specially developed for a new performance class of communication satellites, will also experience its flight premiere. Alphabus was created in cooperation between ESA, the French space agency CNES and industrial partners.

The base platform is the new flagship of the European space industry in its product lines for communication satellites. It enables the construction of spacecraft that weigh up to 8.8 tons and can carry a maximum of two tons of payload. This corresponds to up to 230 transponders, the transmission devices of a communication satellite. A transponder receives the signals to be broadcast from the earth station, amplifies them and sends them back to earth on a different frequency. For this purpose, Alphabus provides up to 22 kilowatts of electrical power and the planned service life of the entire satellite should be 15 years.

The bus has a modular design so that it can be optimally tailored to the requirements of the payload. For example, the two solar cell booms each have four to six panels with gallium arsenide solar cells, depending on the required output. The antenna module enables the installation of various antennas according to the task at hand.

The main contractors for development and production are the two European space companies EADS Astrium and Thales Alenia Space. EADS Astrium is responsible for the chemical propulsion system, power supply, solar generators and the assembly, integration and testing of the entire spacecraft. Thales Alenia Space is building the mechanical and thermal subsystems, the avionics for controlling the satellite and the electrical propulsion systems.

The thrust comes from Lampoldshausen

The Unified Propulsion System (UPS) for Alphabus is manufactured in Lampoldshausen at EADS Astrium. The core element is the so-called apogee motor EAM (European Apogee Motor), which brings the satellite at the highest point of the geostationary transfer orbit into its circular orbit around the earth, the geostationary orbit above the earth's equator. The engine with a thrust of 500 Newtons was also developed in Lampoldshausen in cooperation with ESA and DLR and specially designed for extra-large communication satellite platforms. Combustion chamber and nozzle are made of a new, heat-resistant ceramic material.

The drive system is a two-fuel system, i.e. it is operated with a fuel (monomethylhydrazine) and an oxidizer (a mixture of nitrogen oxides). The two components are used for both the apogee thruster and the attitude control thrusters. The drive system has 16 of these.

The largest tanks from Augsburg

For the storage of fuel and oxidizer on board, MT Aerospace, a subsidiary of the European space and technology group OHB Technology, has developed a new generation of carbon fiber-wrapped liquid fuel tanks for satellites in Augsburg. They are the world's largest and yet lightest satellite tanks. They have a total volume of up to 1925 liters and a structural mass of less than 85 kilograms. The tanks consist of a thin titanium shell with a high-strength carbon fiber wrapping and are designed for a service life of 18 years. For this purpose, new production technologies had to be developed in order to safely accommodate the largest possible amount of fuel within the dimensions of the satellite platform.
MT Aerospace was also responsible for the development and integration of the tank subsystem. This also included an innovative, high-performance, bubble-free fuel supply manufactured by MT Satellite Products, a subsidiary of MT Aerospace, in Wolverhampton, England. It works on the principle of surface tension and enables extremely low fuel residues in the tank.

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