GPS receivers are found in satnavs, phones and handheld GPS devices (1). They use the Global Positioning System to measure position to within 5–10 metres. The European Galileo satnav system will be a network of satellites that orbit in three planes, so they sweep over the entire Earth’s surface (2, 3). At any time, about 30 satellites will be in operation, and anywhere on Earth’s surface at least four will be above the horizon.
The intensity of solar radiation on Earth is 1400 W m–2; this is the satellites’ energy supply. A Galileo satellite has four photovoltaic panels, each measuring 5m × 1m (4). Sensors keep the panels angled towards the Sun and together they provide a peak output power of 1900W.
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