If you want to know precisely where you are, get an accurate clock. This
was true in the seventeenth century, when sailors struggled to find a way
of measuring longitude at sea. And it’s equally true now, even though
satellites have taken over from sextants, the Sun and fixed stars as the
navigation aids of choice.

Seventeenth century navigators could tell local time from the position of
the Sun, but to determine their longitude, they needed also to know the
time at a fixed reference point, i.e. Greenwich, the location of the prime
meridian. This is because local time moves forward or backward by one hour
for every 15 degrees longitude one travels. The clocks available at the
time were notoriously inaccurate at sea and the problem was solved only
when John Harrison, a joiner from England, built a seaworthy clock that
kept time to within about 1 second per day, equivalent to a positioning
accuracy of about 500 metres. His clocks, which took him all his life to
build, can still be seen at the Royal Observatory, Greenwich.

The clocks that will mark time for the next generation of navigation
satellites will look very different from Harrison’s elegant timepieces.
Based on atomic frequency standards, they are under development for
Europe’s Galileo satellite navigation system at the Observatoire de
Neuchatel and Temex Neuchatel Time, both located in the centre of the
Swiss clock industry. These clocks are being designed to a standard that
seventeenth century sailors would find impossible to comprehend. They
will keep time to within a few hundred millionths of a second per day.

Why, you may ask, would anyone want to keep time so closely? The answer
has to do with the speed of light. Nowadays, you can determine your
position on the Earth’s surface by measuring the time taken for a signal
broadcast by a navigation satellite to reach you. As signals travel at
the speed of light, this means measuring tiny fractions of a second very
accurately. And to do that, you need to know precisely when the signal
left the satellite and precisely when it arrived at your receiver. “In
navigation, clocks are the driving factor for determining positions
accurately. With an accuracy of better than one billionth of a second
in one hour, the clocks on the Galileo satellites will allow you to
resolve your position anywhere on the Earth’s surface to within 45 cm,”
says Franco Emma, the clock expert and navigation engineer at ESTEC,
ESA’s technical centre in the Netherlands.

Each of the 30 satellites in the Galileo system will have two clocks on
board; one based on the rubidium atomic frequency standard and the other
using a passive hydrogen maser. Both clocks use different technologies,
but make use of the same principle — if you force an atom to jump from
one particular energy state to another, it will radiate a microwave
signal at an extremely stable characteristic frequency.

This frequency is around 6GHz for the rubidium clock and around 1.4GHz
for the hydrogen clock. “We will use the clock frequency as a very stable
reference by which other units can generate the accurate signals that the
satellites will broadcast,” says Emma. The broadcast signals will also
provide a reference by which the less stable clocks in user’s receivers
can continuously re-set their time.

ESA chose the rubidium and hydrogen maser clocks because they are very
stable over a few hours and their technology can fly onboard the Galileo
satellites. If they were left to run indefinitely, though, their accuracy
would drift, so they need to be synchronised regularly with a network of
even more stable ground-based reference clocks.

These will include clocks based on the caesium frequency standard, which
show far better long-term stability than either rubidium or hydrogen maser
clocks. The caesium frequency standard is based on atomic transitions in
the caesium atom and is the standard on which universal time is based.
“The clocks on the ground will also generate what we are calling Galileo
System Time,” says Emma.

The clocks that will fly on the satellites are the first of their type to
be developed and built in Europe. “Similar clocks are available in the US
and in Russia (e.g. those flown on the GPS and GLONASS satellites), but we
believe that we need to have an independent capability,” states Emma. The
passive hydrogen maser clock will actually be the first one of its type
ever to fly. It is being built by the Observatoire de Neuchatel in
co-operation with Officine Galileo of Italy, the former being responsible
for the overall development and in particular for the so-called physics
package, and the latter being in charge of the electronics. A similar
arrangement applies for the rubidium clock, with Temex Neuchatel Time
assuming overall responsibility and Astrium, Germany contributing the

The rubidium clock should be ready for qualification by the end of 2001,
by which time an engineering model of the hydrogen maser should be
available. Both clocks will really show their capabilities in 2004, when
the first Galileo satellites go on trial.

Related News

* What is Galileo?

* ESA continues work on Galileo pending EU’s deferred decision


* 1st slice of GalileoSat 2001 workplan approved


Related Links

* ESA’s navigation homepage


* The Royal Greenwich Observatory


* Temex Neuchatel Time


* Observatoire de Neuchatel



[Image 1:

John Harrison’s 1st Marine Timekeeper, H1 (Courtesy National Maritime
Museum, London)

[Image 2:

Rubidium clock

[Image 3:

Hydrogen maser clock