Observation, either for defence or navigation, has been an objective for man down the centuries. Prehistoric man and medieval lord built settlements and castles on high ground for a clear view of the approaches. At sea, crowsnests were added to masts to aid navigation. Towards the end of the last century armies used manned balloons for battlefield observation. In the First World War aircraft were used for this purpose. With the development of space rockets came satellites, providing man with the ultimate in navigation aid, military observation, telecommunications and meteorlogical observation.
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