But what will matter to most users is the relative accuracy of the two systems.

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"The two metres is what we call the relative accuracy, " explained Dr Manfred Zink, the project manager for the TanDEM-X ground segment at DLR.

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Manual docking requires astronauts to have a precise judgement on the relative distance between Shenzhou IX and the Tiangong-1 module, a challenge to their capability of coordination, accuracy and psychological stability.

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