ÜDS-Fen 2012-1 Soru=64

We understand from the passage that the sites of the Deep Space Network were chosen because ----.

NASA’s Deep Space Network is responsible for
tracking, commanding and receiving data from space
probes throughout the solar system. Some of the
spacecraft are so far away and they use such small
transmitters that radio signals received from them are
about 20 million times weaker than a watch battery. To
receive such weak signals, the Deep Space Network
uses huge dish-shaped antennae up to 70 meters
across. To dispatch commands to the most remote
spacecraft, the same giant dishes are used to send
radio signals from immensely powerful, 400-kilowatt
transmitters. The Deep Space Network’s antennae are
located at three sites – Madrid (Spain), Canberra
(Australia), and California (the US). The sites were
chosen because they are roughly 120 degrees and a
third of the way round the world from each other. As the
Earth turns, at least one of the stations is always in
contact with a space probe. This ensures a 24-hour
observation, with overlapping time to transfer the radio
link to the next station. The sites are also surrounded by
mountains, which protect them from radio interference.