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Satellites in space

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Introduction

  1. Satellites in space

1.1 A space engineer

Jeremy Curtis is an engineer and business development manager for space science at the Rutherford Appleton laboratory (RAL) in Oxfordshire. His job includes on the joint European telescope for X-ray astronomy (JET-X), due to have been launched in 1999 on the Russian Spectrum-X spacecraft. He says “I trained as a mechanical engineer, but I find space engineering exciting because I have to work with all kinds of experts such as astronomers, physicists, designers, programmers and technicians working around the world”. He was sponsored by RAL during his university degree and then spent several years on designs for a large proton synchrotron (a machine for accelerating protons to very high energies) before moving over to space instrument design. In the following passage he describes some of the aspects of space engineering.

Why satellites?

Getting spacecraft into orbit is a very expensive activity with typical launch costs generally measures in tens of thousands per kilogram. So what makes it worth the bother? There are three key reasons.

First, a satellite is a good vantage point for studying the earth’s surface and atmosphere – just think how many aircrafts would be needed to photograph the whole of the earth, or how many ships to monitor the temperature of the oceans.

Second, if we want to study most of the radiation coming for distant parts of the universe we have to get above the atmosphere.

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Middle

1.2 Studying with satellites

The UoSAT satellites are very small, relatively low-cost, spacecraft whose purpose is to test and evaluate new systems and space technology and to enable students and amateur scientists to study the near-earth environment. They are designed and built by the university of Surrey spacecraft engineering research unit. UoSAT, also known as Oscar 11 has sensors to record the local magnetic field, providing information about solar and geomagnetic disturbances and there affects on radio communications at various frequencies. Instruments on board also measure some 60 items relating to the satellites operation. These include; the temperature of its faces, its batteries and other electronic devices; the current provided by its solar arrays; and the battery voltages. It can also receive store and transmit messages to simple radio receivers anywhere in the world. UoSAT’s orbit takes it over both poles at a height of about 650km above the earth’s surface, and the spinning of the earth allows it to receive data about six times a day. Each UoSAT spacecraft is designed to last about 7 years. Even small spacecrafts such as these need electricity to run all onboard systems, form the computer that controls it all, to the radio transmitters and receivers that send and receive all data to and from ground stations on the earths surface. UoSAT’s are small, each with a mass of typically 50kg and about 0.5m across. For comparison, JET-X is about 540kg in mass and about 4.5m long.

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Conclusion

  1. 10kw for 10 years.

Solar and nuclear dynamic systems.

The most common primary source of energy used in satellites is the photovoltaic cell or solar cell. Hundreds of thousands of such cells are connected together to make up solar arrays. UoSAT 2 and the ISS have many arrays of solar arrays attached to them. Solar cells have one important characteristic; they only generate electricity when illuminated. Orbiting satellites undergo between 90 and 5500 eclipses, moving into the shadow of the earth, each year.

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The former is typical of a geostationary telecommunications satellite, the latter of a satellite is in a low orbit like UoSAT 2. The ISS will have sixteen thirty minute periods of shadow each day. The secondary power supply is therefore vital, because during eclipse electrical power has to be supplied by batteries. There are also occasions when batteries are needed to provide power in addition to that of the solar panels.

The spacecraft’s solar panels are used to recharge its batteries when it emerges into sunlight. To do this they must provide a high enough voltage – higher than the batteries own voltage. (A charger for a 12v car battery provides about 30v.) The power system must therefore be carefully designed to ensure that the solar panels can charge the batteries and that the batteries can operate the electrical equipment on-board.

So what voltage does a solar cell provide? How does this voltage vary with the brightness of the light? How can we connect up solar cells in order to charge batteries and operate equipment? These are questions I will explore in part two of this unit.

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