Andrew Yee[_1_]
May 27th 07, 01:35 AM
ESA News
http://www.esa.int
10 May 2007
Hyper-accurate clocks -- the beating heart of Galileo
Travellers have relied on accurate timekeeping for navigation since the
development of the marine chronometer in the eighteenth century. Galileo,
Europe's twenty-first century navigation system, also relies on clocks --
but they are millions of times more accurate than those earlier timepieces.
The operational Galileo satellites will carry two types of clocks -- passive
hydrogen masers and rubidium atomic frequency standards. Each satellite will
be equipped with two hydrogen masers, one of which will be the primary
reference for generating the navigation signals, with the other as a cold
(non-operating) spare.
Every operational satellite will also carry two rubidium clocks, one of
which will be a hot (permanently running) backup for the operational
hydrogen maser, instantly taking over should the maser fail and allowing
signal generation to continue uninterrupted. The second rubidium clock will
act as a cold spare.
GIOVE-A, the Galileo in-orbit verification satellite that is currently in
service, carries two rubidium clocks -- one operational and one cold spare.
GIOVE-B, which is projected to enter service later this year, will carry one
hydrogen maser and two rubidium clocks, one hot and one cold spare. The
GIOVE-A2 satellite, which will be ready for launch in the second half of
2008, will carry a similar timekeeping payload to GIOVE-A, but will transmit
additional navigation signals.
The Galileo passive hydrogen masers will keep time with an accuracy of
around one nanosecond (one one-thousand-millionth of a second) in 24 hours
-- equivalent to losing or gaining a second in 2.7 million years. The
rubidium clocks are accurate to 10 nanoseconds per day. In comparison, an
ordinary digital wris****ch has an accuracy of about one second per day.
Galileo's passive hydrogen maser clocks will be around one thousand million
times more accurate than a digital wris****ch.
The need for accuracy
Conceptually, Galileo users will determine their position by measuring how
much time radio waves transmitted by satellites in the Galileo constellation
take to reach them. Radio waves travel at about 300 million metres per
second, so they cover a distance of around 0.3 metres in one nanosecond. In
order to offer navigation accuracies of the order of a metre, Galileo time
measurements must therefore be performed with a precision in the nanosecond
range.
As a by-product of satellite navigation's need for accurate timekeeping,
Galileo will also be able to offer precision time services to be used, for
example, in the time stamping of financial transactions.
Galileo is a joint initiative between ESA and the European Commission. When
fully deployed in the early years of the next decade, it will be the first
civilian positioning system to offer global coverage.
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMDZUU681F_index_1.html ]
http://www.esa.int
10 May 2007
Hyper-accurate clocks -- the beating heart of Galileo
Travellers have relied on accurate timekeeping for navigation since the
development of the marine chronometer in the eighteenth century. Galileo,
Europe's twenty-first century navigation system, also relies on clocks --
but they are millions of times more accurate than those earlier timepieces.
The operational Galileo satellites will carry two types of clocks -- passive
hydrogen masers and rubidium atomic frequency standards. Each satellite will
be equipped with two hydrogen masers, one of which will be the primary
reference for generating the navigation signals, with the other as a cold
(non-operating) spare.
Every operational satellite will also carry two rubidium clocks, one of
which will be a hot (permanently running) backup for the operational
hydrogen maser, instantly taking over should the maser fail and allowing
signal generation to continue uninterrupted. The second rubidium clock will
act as a cold spare.
GIOVE-A, the Galileo in-orbit verification satellite that is currently in
service, carries two rubidium clocks -- one operational and one cold spare.
GIOVE-B, which is projected to enter service later this year, will carry one
hydrogen maser and two rubidium clocks, one hot and one cold spare. The
GIOVE-A2 satellite, which will be ready for launch in the second half of
2008, will carry a similar timekeeping payload to GIOVE-A, but will transmit
additional navigation signals.
The Galileo passive hydrogen masers will keep time with an accuracy of
around one nanosecond (one one-thousand-millionth of a second) in 24 hours
-- equivalent to losing or gaining a second in 2.7 million years. The
rubidium clocks are accurate to 10 nanoseconds per day. In comparison, an
ordinary digital wris****ch has an accuracy of about one second per day.
Galileo's passive hydrogen maser clocks will be around one thousand million
times more accurate than a digital wris****ch.
The need for accuracy
Conceptually, Galileo users will determine their position by measuring how
much time radio waves transmitted by satellites in the Galileo constellation
take to reach them. Radio waves travel at about 300 million metres per
second, so they cover a distance of around 0.3 metres in one nanosecond. In
order to offer navigation accuracies of the order of a metre, Galileo time
measurements must therefore be performed with a precision in the nanosecond
range.
As a by-product of satellite navigation's need for accurate timekeeping,
Galileo will also be able to offer precision time services to be used, for
example, in the time stamping of financial transactions.
Galileo is a joint initiative between ESA and the European Commission. When
fully deployed in the early years of the next decade, it will be the first
civilian positioning system to offer global coverage.
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMDZUU681F_index_1.html ]