Galileo rubidium clocks -- a year of in-orbit experience (Forwarded)

ESA News
http://www.esa.int

21 May 2007

Galileo rubidium clocks -- a year of in-orbit experience

Europe's first satellite-borne rubidium clocks have been in orbit for over a
year. There is good news for the building of the Galileo system: the results
obtained from GIOVE-A's first year of operations show performance that is
largely in line with the specifications.

GIOVE-A, the first Galileo in-orbit validation element, was launched on 28
December 2005. One of its two rubidium clocks was switched on for the first
time on 10 January 2006 and Galileo signals were transmitted two days later.

The timekeeping of the clocks on the Galileo spacecraft will play an
important role in determining the overall accuracy of the system, so
evaluation of their performance is a crucial part of the Galileo in-orbit
verification process.

Indirect measurement

The navigation signals broadcast from GIOVE-A, and from the GPS spacecraft
constellation, are received by the world-wide network of 13 Galileo
experimental sensor stations belonging to the GIOVE Mission Segment. In
addition, the orbit of GIOVE-A is precisely measured by a network of 10
ground-based laser ranging stations of ILRS (International Laser Ranging
Service), to provide orbital data independent of the navigation data.

The technique used to characterise clock performance is known as Orbit
Determination and Time Synchronisation (ODTS). ODTS is a statistical method
which takes the Galileo and GPS data, optionally together with the laser
ranging data, and calculates spacecraft orbits, clock times, the effects of
the Earth's atmosphere on the radio signals and the delays in the receiving
systems. The precision of the calculations is so great that even the tiny
orbit disturbances caused by the pressure of sunlight shining on the
satellites are taken into account.

The ground systems cannot measure the 'pure' clock performance on-board
GIOVE-A. The 'apparent' clock performance observed on the ground is seen
through the satellite signal generation chain, the radio transmission path
through space, the receiver network and the algorithm used to perform the
performance estimation.

Performance to date

Comparisons between the on-board clocks and identical units undergoing
on-ground life testing show that no unexpected ageing or performance
degradation is occurring due to the space environment. Extrapolation of
performance measurements for limited-life components such as the rubidium
lamps shows that they will meet the required 12-year operational lifetime.

The measured performance of the clocks meets the specification over short
and medium timescales. A few 'jumps' in clock frequency have been observed,
which impact the long term accuracy. Such frequency changes are a well known
phenomenon in rubidium clock technology but their cause is not yet well
understood. Their effect on GPS performance has already been analysed and
corrective measures proposed. The Galileo team are ground testing a number
of improvements to the clock design which are intended to minimise both the
occurrence and size of the jumps.

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.

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