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A Year Of In-Orbit Experience For Galileo Rubidium Clocks
Paris, France (ESA) May 22, 2007 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 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 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 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. Email This Article
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Huntsville Al (SPX) May 22, 2007Standing on the Moon in 1971, Apollo 15 astronaut Dave Scott held his hands out at shoulder height, a hammer in one hand and a feather in the other. And as the world looked on via live television, he let go. It was an odd sight: the feather didn't drift to the ground, it plummeted, falling just as fast as the hammer. Without air resistance to slow the feather, the two objects hit moondust at the same instant. |
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