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 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 easily exceed 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.
Dominique Detain | alfa
Cutting edge research for the industries of tomorrow – DFKI and NICT expand cooperation
21.03.2017 | Deutsches Forschungszentrum für Künstliche Intelligenz GmbH, DFKI
Molecular motor-powered biocomputers
20.03.2017 | Technische Universität Dresden
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
24.03.2017 | Materials Sciences
24.03.2017 | Physics and Astronomy
24.03.2017 | Physics and Astronomy