Rubidium Clock Marks Year In Orbit

Physorg reports on the rubidium clocks being tested for ESA’s Galileo satellite system, which, when fully deployed in the early years of the next decade, will be the first civilian positioning system to offer global coverage:

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.

The orbit of GIOVE-A is precisely measured by a network of 10 ground-based laser ranging stations, to provide orbital data independent of the navigation data. 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.

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, 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 is taken into account…. 

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. 

(It ain’t pretty, but it’s accurate!)

The ESA website offers further explanation about the accuracy of the rubidium clock:

The Galileo satellites will carry two types of clocks: Rubidium atomic clocks and Hydrogen atomic clocks. The stability of the Rubidium clock is so good that it would lose only 3 seconds in 1 million years, while the Hydrogen maser is even more stable and it would lose only 1 second in 3 million years. However this kind of stability is really needed since an error of only a few nanoseconds (billionths of a second) on the Galileo measurements would produce a positioning error of meters which would not be acceptable.

For those who really want to get into the complexities of such atomic clocks, check out this page from Harvard’s Department of Physics. In addition to details about the frequencies used by atomic masers (hey, no one said this wasn’t rocket science!) the page features a downloadable poster illustrating N-resonances and atomic clocks.

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