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| Precise navigation |
In space the only way to measure distance is by measuring time.
Without accurate clocks, spacecraft can’t measure their speed relative to
anything else and ground control can’t compensate for the time it takes to
send radio messages across the huge distances between planets.
Atomic clocks have long provided the accuracy that space missions require,
but until now the best ones have all been on the ground. This means the
timing signals need to be beamed up to each spacecraft, which introduces
errors and reduces the number of spacecraft that can be controlled at
once. The Deep Space Atomic Clock (DSAC) is a miniaturised version of the
most accurate clock available on Earth. It’s packaged in a housing that’s rugged
enough to withstand the trip to space, while still only weighing 17.5 kilograms
(39 pounds). When it launches in 2015, it will be in orbit for a year to calibrate
and verify its own performance against the most accurate clocks currently
in space – those on GPS satellites. However, DSAC is over ten times more
accurate than GPS and will eventually enable spacecraft to determine their
position and speed, completely independently of ground control.
This is important, because the further we venture from Earth, the
greater the communication delay. Depending on the relative position of
the planets, it can take anything from 3 to 22 minutes for radio signals to
travel from Earth to Mars. That’s far too long for real-time control of orbital
manoeuvres. Even worse, spacecraft are cut off from Earth every time they
orbit around the far side of another planet. Whether a mission is manned
or robotic, it simply isn’t realistic for spacecraft to depend on timely
instructions from Earth. The DSAC is a key technology that will enable precise
and autonomous navigation.
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