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ESA's Atmospheric re-entry Demonstrator (ARD) is an unmanned, 3-axis stabilised automatic capsule that was launched aboard Ariane 5 flight 503 on 21 October 1998. ARD was a major step towards developing and operating space transportation vehicles that can return to Earth, whether carrying payloads or people. For the first time, Europe accomplished a complete space mission - launching a vehicle into space and recovering it safely. One of ARD's objectives was to validate the flight control algorithms that were developed as part of the former Hermes spaceplane programme. The guidance algorithm is similar to that used by NASA's Space Shuttle, based on a reference deceleration profile and also used by Apollo. This approach allows a good final guidance accuracy with limited real-time calculation complexity and storage requirements. The ARD's automatic navigation, guidance and control system consists of a Global Positioning System (GPS) receiver, an inertial navigation system, a computer, the associated data bus and power supply and distribution system, and a Reaction Control System. The Reaction Control System was derived from the Ariane 5 Attitude and Orbital Control System, designed, developed and produced at Astrium Lampoldshausen. ARD's Reaction Control System comprised seven CHT 400 N hydrazine thrusters and two 58 litre BT/01 bladder tanks pressurised with nitrogen. The thrusters are positioned so that three provide control in pitch, two in roll and two in yaw. From its maximum height of 830 km, ARD commenced Earth re-entry at an altitude of 120 km and a velocity of around 27,000 km/h. Using the 400 N steering control, ARD achieved a precision landing within the 5 km target range, entering the Pacific Ocean beneath parachutes. Such accuracy is equivalent to scoring a goal from a distance of 25 km. The buoyant ARD was later recovered by a French Naval vessel guided by Sarsat radio beacon.
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