Much of the UK's space industry is centred on EADS Astrium, based in Stevenage and Portsmouth. The company builds the buses – the underlying structure onto which the payload and propulsion systems are built – for most of the European Space Agency's spacecraft, as well as commercial satellites. The world leader in compact satellite systems, Surrey Satellite Technology, is also part of Astrium. Reaction Engines Limited, the company planning to build Skylon, a single-stage-to-orbit spaceplane using their SABRE rocket engine, a combined-cycle, air-breathing rocket propulsion system is based Culham. The UK space industry was worth £9.1bn in 2011 and employed 29,000 people. It is growing at a rate of 7.5 per cent annually, according to its umbrella organisation, the UK Space Agency. In 2013, the British Government pledged £60 million to the Skylon project: this investment will provide support at a "crucial stage" to allow a full-scale prototype of the SABRE engine to be built.