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  • richardmitnick 1:28 pm on February 8, 2019 Permalink | Reply
    Tags: ESA Galileo navigation system, ESA Galileo satellites prove Einstein's Relativity Theory to highest accuracy yet, ESA's answer to USA GPS,   

    From Scientific American: “Wayward Satellites Test Einstein’s Theory of General Relativity” 

    Scientific American

    From Scientific American

    February 8, 2019
    Megan Gannon

    The botched launch of two Galileo navigation probes made for an unexpected experiment.

    1
    Galileo satellite. Credit: P. Carril and ESA

    ESA Galileo’s navigagtion constellation

    In August 2014 a rocket launched the fifth and sixth satellites of the Galileo global navigation system, the European Union’s $11-billion answer to the U.S.’s GPS. But celebration turned to disappointment when it became clear that the satellites had been dropped off at the wrong cosmic “bus stops.” Instead of being placed in circular orbits at stable altitudes, they were stranded in elliptical orbits useless for navigation.

    The mishap, however, offered a rare opportunity for a fundamental physics experiment. Two independent research teams—one led by Pacôme Delva of the Paris Observatory in France, the other by Sven Herrmann of the University of Bremen in Germany—monitored the wayward satellites to look for holes in Einstein’s general theory of relativity.

    “General relativity continues to be the most accurate description of gravity, and so far it has withstood a huge number of experimental and observational tests,” says Eric Poisson, a physicist at the University of Guelph in Ontario, who was not involved in the new research. Nevertheless, physicists have not been able to merge general relativity with the laws of quantum mechanics, which explain the behavior of energy and matter at a very small scale. “That’s one reason to suspect that gravity is not what Einstein gave us,” Poisson says. “It’s probably a good approximation, but there’s more to the story.”

    Einstein’s theory predicts time will pass more slowly close to a massive object, which means that a clock on Earth’s surface should tick at a more sluggish rate relative to one on a satellite in orbit. This time dilation is known as gravitational redshift. Any subtle deviation from this pattern might give physicists clues for a new theory that unifies gravity and quantum physics.

    Even after the Galileo satellites were nudged closer to circular orbits, they were still climbing and falling about 8,500 kilometers twice a day. Over the course of three years Delva’s and Herrmann’s teams watched how the resulting shifts in gravity altered the frequency of the satellites’ superaccurate atomic clocks. In a previous gravitational redshift test, conducted in 1976, when the Gravity Probe-A suborbital rocket was launched into space with an atomic clock onboard, researchers observed that general relativity predicted the clock’s frequency shift with an uncertainty of 1.4 × 10–4.

    The new studies, published last December in Physical Review Letters, again verified Einstein’s prediction—and increased that precision by a factor of 5.6. So, for now, the century-old theory still reigns.

    See the full article here .


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    Scientific American, the oldest continuously published magazine in the U.S., has been bringing its readers unique insights about developments in science and technology for more than 160 years.

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  • richardmitnick 10:41 am on December 5, 2018 Permalink | Reply
    Tags: , , , , , ESA Galileo satellites prove Einstein's Relativity Theory to highest accuracy yet   

    From European Space Agency: “Galileo satellites prove Einstein’s Relativity Theory to highest accuracy yet” 

    ESA Space For Europe Banner

    From European Space Agency

    4 December 2018

    Dr Pacôme Delva
    SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, LNE
    Email: pacome.delva@obspm.fr

    Dr Sven Hermann
    ZARM Center of Applied Space Technology and Microgravity
    Email: sven.herrmann@zarm.uni-bremen.de

    Dr Javier Ventura-Traveset
    Head of Galileo Navigation Science Office, ESA
    Email: javier.ventura-traveset@esa.int

    Dr Erik Schoenemann
    ESOC Navigation Support Office, ESA
    Email: erik.schoenemann@esa.int

    Europe’s Galileo satellite navigation system – already serving users globally – has now provided a historic service to the physics community worldwide, enabling the most accurate measurement ever made of how shifts in gravity alter the passing of time, a key element of Einstein’s Theory of General Relativity.

    Two European fundamental physics teams working in parallel have independently achieved about a fivefold improvement in measuring accuracy of the gravity-driven time dilation effect known as ‘gravitational redshift’.

    The prestigious Physical Review Letters journal has just published the independent results obtained from both consortiums here and here , gathered from more than a thousand days of data obtained from the pair of Galileo satellites in elongated orbits.

    “It is hugely satisfying for ESA to see that our original expectation that such results might be theoretically possible have now been borne out in practical terms, providing the first reported improvement of the gravitational redshift test for more than 40 years,” comments Javier Ventura-Traveset, Head of ESA’s Galileo Navigation Science Office.

    “These extraordinary results have been made possible thanks to the unique features of the Galileo satellites, notably the very high stabilities of their onboard atomic clocks, the accuracies attainable in their orbit determination and the presence of laser-retroreflectors, which allow for the performance of independent and very precise orbit measurements from the ground, key to disentangle clock and orbit errors.”

    These parallel research activities, known as GREAT (Galileo gravitational Redshift Experiment with eccentric sATellites), were led respectively by the SYRTE Observatoire de Paris in France and Germany’s ZARM Center of Applied Space Technology and Microgravity, coordinated by ESA’s Galileo Navigation Science Office and supported through its Basic Activities.

    Happy results from an unhappy accident

    These findings are the happy outcome of an unhappy accident: back in 2014 Galileo satellites 5 and 6 were stranded in incorrect orbits by a malfunctioning Soyuz upper stage, blocking their use for navigation. ESA flight controllers moved into action, performing a daring salvage in space to raise the low points of the satellites’ orbits and make them more circular.

    Once the satellites achieved views of the whole Earth disc their antennas could be locked on their homeworld and their navigation payloads could indeed be switched on. The satellites are today in use as part of Galileo search and rescue services while their integration as part of nominal Galileo operations is currently under final assessment by ESA and the European Commission.

    However, their orbits remain elliptical, with each satellite climbing and falling some 8500 km twice per day. It was these regular shifts in height, and therefore gravity levels, which made the satellites so valuable to the research teams.

    Reenacting Einstein’s prediction

    Albert Einstein predicted a century ago that time would pass more slowly close to a massive object, a finding that has since been verified experimentally several times – most significantly in 1976 when a hydrogen maser atomic clock on the Gravity Probe-A suborbital rocket was launched 10 000 km into space, confirming Einstein’s prediction to within 140 parts per million.

    In fact, atomic clocks aboard navigation satellites must already take into account the fact that they run faster up in orbit than down on the ground – amounting to a few tenths of a microsecond per day, which would result in navigation errors of around 10 km daily, if uncorrected.

    The two teams relied upon the stable timekeeping of the passive hydrogen maser (PHM) clocks aboard each Galileo – stable to one second in three million years – and kept from drifting by the worldwide Galileo ground segment.

    “The fact that the Galileo satellites carry passive hydrogen maser clocks, was essential for the attainable accuracy of these tests,” noted Sven Hermann at the University of Bremen’s ZARM Center of Applied Space Technology and Microgravity.

    “While every Galileo satellite carries two rubidium and two hydrogen maser clocks, only one of them is the active transmission clock. During our period of observation, we focus then on the periods of time when the satellites were transmitting with PHM clocks and assess the quality of these precious data very carefully. Ongoing improvements in the processing and in particular in the modelling of the clocks, might lead to tightened results in the future.”

    Refining the results

    3
    Passive hydrogen maser, ESA
    Passive hydrogen maser atomic clock of the type flown on Galileo, accurate to one second in three million years.

    A key challenge over three years of work was to refine the gravitational redshift measurements by eliminating systematic effects such as clock error and orbital drift due to factors such as Earth’s equatorial bulge, the influence of Earth’s magnetic field, temperature variations and even the subtle but persistent push of sunlight itself, known as ‘solar radiation pressure’.

    “Careful and conservative modelling and control of these systematic errors has been essential, with stabilities down to four picoseconds over the 13 hours orbital period of the satellites; this is four millionth of one millionth of a second,” Pacôme Delva of SYRTE Observatoire de Paris.

    “This required the support of many experts, with notably the expertise of ESA thanks to their knowledge of the Galileo system.”

    Precise satellite tracking was enabled by the International Laser Ranging Service, shining lasers up to the Galileos’ retro-reflectors for centimetre-scale orbital checks.

    4
    Laser ranging station

    Major support was also received from the Navigation Support Office based at ESA’s ESOC operations centre in Germany, whose experts generated the reference stable clock and orbit products for the two Galileo eccentric satellites and also determined the residual errors of the orbits after the laser measurements.

    See the full article here .


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    Please help promote STEM in your local schools.

    Stem Education Coalition

    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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