Advanced communications technology to support the operation of the space station transfer vehicle HTV
Source: Japan Aerospace Exploration Agency (JAXA)
The International Space Station (ISS) orbits the earth at a height of approximately 400km and a speed of approximately 8km/sec. Goods are transported to the station by means of the H-II Transfer Vehicle, or HTV. Mitsubishi Electric Corporation is responsible for the development of communications technologies that support the safety and proper operation of the HTV. We have now succeeded in manufacturing an original small, high-performance four-wire helical antenna and a transponder. Transponders are core components of communications devices, and are made up of a modulation and demodulation section and a high-frequency section. The modulation and demodulation section converts the received radio waves into digital signals, and then converts these digital signals into radio waves. The high-frequency section serves to convert the high-frequency band radio waves that the HTV uses for communications to low-frequency waves of several tens of MHz, which can be used by the modulation and demodulation section. We are proud that the advanced communications technologies that Mitsubishi Electric Corporation has fostered over the course of many years are able to support Japan's space exploration.
In addition to communicating with a ground station via a relay satellite, the HTV also communicates with the ISS for the purpose of taking position measurements via GPS signals. High-precision communication is essential to safe space flight. To ensure this, a wide-directivity antenna is required that can pick up radio waves from the relay satellite or other source no matter what the heading of the HTV is. Our newly developed four-wire helical antenna is not only compact, but also realizes wide directivity and high sensitivity. The key to this achievement was a "rewinding" technology that applies the principle of radio wave superposition. This was process an original concept developed by Mitsubishi Electric. Rather than simply winding four helical elements together in a spiral, this method employs a "rewound" section, effectively combining two antennas with different characteristics into a single antenna, ensuring wide directivity. The concept of rewinding wire-shaped helical elements is a simple one, but precisely because of this simplicity, the antenna is small, with no need for complex circuits, and it also achieves high reliability. Mitsubishi Electric has been involved in a wide range of antenna developments, spanning the fields of terrestrial and satellite communications. This valuable experience and the many associated technologies that it has produced was the foundation that enabled us to successfully develop the new four-wire helical antenna.
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A high-frequency device called a mixer is fitted in the high-frequency section of a transponder. Until now, whenever mixers have converted frequencies, for example radio waves in the 2GHz frequency band, it has been necessary to convert the waves to a frequency of around several hundred MHz, and then to once again convert them to several dozen MHz. Mitsubishi Electric's newly developed planar mixer, however, is able to convert 2GHz radio waves to several dozen MHz in one step. Reducing the number of steps involved in converting the frequency from two to one has made it possible to simplify the circuitry, and this in turn has allowed us to reduce the circuit area of the high-frequency section to approximately two-thirds of the standard size. Determining how to reduce the size of circuitry is one important aspect of fitting devices in satellites. The newly developed planar mixer also does not use cross-wiring. Cross-wiring is a method of connecting intersecting wires over other wires, as in an overpass on a road. Components going into space require high durability and high reliability. Eliminating cross-wiring simplifies the wiring configuration, and thus reduces the chance of errors occurring. The circuits used in the newly developed planar mixer are formed on a ceramic circuit board.
The newly developed modulation and demodulation ASIC is fully digital. Conventionally, modulation and demodulation sections have necessitated the inclusion of some analog parts, but the new ASIC is entirely realized on one chip. The performance of analog parts can decline in high temperatures and as a result of age-related degradation, so the digitalization of parts results in a significant increase in reliability. The integration of necessary components into a single chip has also led to reductions in size and weight. The main difference between the newly developed modulation and demodulation device and the devices used in terrestrial communications, such as in base stations for mobile telephones, is that it has to be capable of simultaneously dealing with radio waves used for communications and radio waves used to measure the distance between the HTV and the ISS. The question was how could we measure distance with a high degree of precision while at the same time realizing high-sensitivity communications, and a great deal of effort was required in order to determine a fundamental solution. Complex processing would make it possible to deal simultaneously with two radio signals, but this would increase the size of the circuit. We conducted countless simulations in order to enable both communications and distance measurement using a simpler method and with a smaller chip. As a result of this process of trial and error, we have led the industry in realizing fully digital operation. Communications performance is in no way inferior, and we have achieved a high sensitivity with a degradation value of less than 1dB. The unit is also capable of distance measurements that are accurate within 1m.
This development project commenced around seven years ago. However, our engineers had long been eager to create communications equipment for the HTV using original Mitsubishi Electric components. The HTV will continue to be launched at intervals of one to one and a half years. At Mitsubishi Electric, we are immensely proud to have been involved in a project of such great importance in the history of Japan’s space exploration. It has also been decided that the newly developed transponder will be employed in NASA’s Cygnus 9 cargo craft, which will be used to transport goods to the ISS in place of the space shuttle. The fact that a technology we developed in Japan will be used outside the country not only gives us tremendous confidence as engineers, but also hints at great possibilities for the future of Japan’s space industry.