Once MetOp is in its polar orbit 800 kilometres above the surface of the Earth and the solar panels are in position, the Advanced Scatterometer (ASCAT) two three-metre long antenna arms are gently deployed into a fixed V-shaped position at +135° and -135° with respect to the spacecraft flight direction. Each of the antenna arms provides the fore-beam of one swath and the aft-beam of the other. ASCAT uses radar to measure electromagnetic backscatter from the wind-roughened ocean surface, from which data on wind speed and direction can be derived.