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  • richardmitnick 4:14 pm on December 6, 2019 Permalink | Reply
    Tags: "More Than Just Astronomy: Radio Telescopes for Geophysics", , European Space Agency’s Sentinel-1 satellite constellation, InSAR, , International VLBI Service for Geodesy and Astrometry, VLBI   

    From Eos: “More Than Just Astronomy: Radio Telescopes for Geophysics” 

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    From Eos

    12.6.19
    Katherine Kornei

    Linking an existing network of radio telescopes with satellite radar would make it possible to measure ground displacements in a globally consistent way, scientists propose.

    1
    A radio telescope, part of the Goldstone Deep Space Communications Complex, looms over California’s Mojave Desert. Credit: NASA/JPL-Caltech

    Radio telescopes reveal distant solar systems and bubbles of gas near our galaxy’s center. But they’re useful for more than just astronomy—a subset of the world’s radio telescopes could also play an important role in geophysics research. A team of scientists has now demonstrated how radio telescopes could be linked to satellites that measure ground deformation, the first step toward studying changes on Earth’s surface on a global scale.

    Wanted: A Global View

    “The height of Earth’s surface is changing all of the time,” said Amy Parker, a satellite radar specialist at Curtin University in Perth, Australia. These displacements occur for a myriad of reasons, some natural and some anthropogenic: earthquakes, mining, and groundwater extraction, for example.

    But accurately monitoring these changes on intercontinental scales—important for determining how land movements affect calculations of sea level rise and fall, for instance—is currently impossible: Interferometric synthetic aperture radar (InSAR), which involves bouncing microwaves off Earth’s surface and measuring their travel time and phase to trace ground deformation, works only over contiguous swaths of land. (That’s because water scatters microwaves inconsistently.) InSAR is “pretty amazing,” said Parker, but it measures ground displacement only relative to an arbitrary reference like the mean value in an image. It doesn’t measure changes relative to an absolute reference frame, and it can’t be used to study global-scale processes, said Parker. “We need to tie measurements on different continents into a consistent reference frame.”

    One way of doing so, Parker and her colleagues suggest, is to connect two existing networks: InSAR satellites and radio telescopes capable of very long baseline interferometry (VLBI).

    Global mm-VLBI Array

    Here Come the Telescopes

    Astronomical observations often involve resolving fine details, like separating two objects that appear close together in the sky. Physically larger telescopes have better angular resolution, but there’s a practical limit to how large a single telescope can be.

    That’s where interferometry comes in. By carefully combining the light gathered by multiple telescopes linked together by precise timing, astronomers can, in a sense, build a much larger telescope: They can achieve an angular resolution equal to that of a telescope with a diameter that’s the distance between the linked telescopes. Very long baseline interferometry refers to interferometry done over very large distances (“baselines”), even across continents. (Astronomers used VLBI to create the Event Horizon Telescope, a network of telescopes that obtained the first image of a black hole.)

    The first image of a black hole, Messier 87 Credit Event Horizon Telescope Collaboration, via NSF and ERC 4.10.19

    When a network of VLBI telescopes accurately measures the arrival of light from a distant galaxy, researchers can compare the time stamps of the observations to determine the telescopes’ positions relative to one another. Thanks to precise timing, the distances between telescopes can be measured to within a few millimeters.

    Because telescopes don’t move relative to Earth’s surface, these measurements reflect changes in the planet’s crust and can be used to trace the motion of tectonic plates, for instance. The International VLBI Service for Geodesy and Astrometry coordinates these geodetic measurements from NASA’s Goddard Space Flight Center in Greenbelt, Md. Currently, there are about 40 VLBI telescopes worldwide that can do this sort of geodetic monitoring.

    Tests on Two Continents

    Connecting the capabilities of InSAR satellites and geodetic VLBI telescopes would open up new observing opportunities, Parker said. “We get a connection between what the satellite is measuring and the reference frame that the telescope is measuring.”

    To test the feasibility of this idea, the researchers focused on four geodetic VLBI telescopes, three in Australia and one in Sweden. They showed that the telescopes could be tied to the European Space Agency’s Sentinel-1 satellite constellation used for InSAR by simply pointing the telescopes statically toward the location of an overpassing satellite.

    ESA Sentinel-1B

    Microwaves emitted by the satellites were readily picked up by the telescopes and reflected back, even when the telescopes didn’t track a satellite’s overpass. “It’s the easiest solution for an operator to implement, and it’s as good as steering the telescope,” said Parker.

    These observations can be completed in only a minute or two, Parker and her colleagues showed, and they don’t require any new instruments or infrastructure. However, it might be necessary to protect telescopes’ sensitive electronics from the satellites’ relatively strong signals, the researchers found. One option is to install metallic foil—impervious to radar frequencies—around a telescope’s low-noise amplifier. Another possibility, which Parker and her team tested, was to simply point the telescope slightly away from a satellite’s position.

    “The international network of Very Long Baseline Interferometry telescopes provides an existing, yet unexploited, link to unify satellite-radar measurements on a global scale,” the researchers concluded in their study, which was published in Geophysical Research Letters in November.

    “It’s a really nice piece of work,” said John Gipson, a physicist at NASA Goddard Space Flight Center and an International VLBI Service for Geodesy and Astrometry team member not involved in this research. “It’s very practical.”

    Parker and her colleagues are optimistic that the scientific community will see the advantages of using radio telescopes for geophysics applications. They hope to see a sizeable number of telescopes and InSAR satellites linked within the next year or two.

    See the full article here .

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  • richardmitnick 2:52 pm on April 4, 2017 Permalink | Reply
    Tags: , , , VLBI   

    From MIT: “Seeing black holes and beyond” 

    MIT News

    MIT Widget

    MIT News

    April 4, 2017
    Haystack Observatory

    A powerful new array of radio telescopes is being deployed for the first time this week, as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile joins a global network of antennas poised to make some of the highest resolution images that astronomers have ever obtained.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    The improved level of detail is equivalent to being able to count the stitches on a baseball from 8,000 miles away.

    Scientists at MIT and other institutions are using a method called VLBI (Very Long Baseline Interferometry) to link a group of radio telescopes spread across the globe into what is, in effect, a telescope the size of our planet. Although the technique of VLBI is not new, scientists have just recently begun extending it to millimeter wavelengths to achieve a further boost in resolving power. And now, the addition of ALMA to global VLBI arrays is providing an unprecedented leap in VLBI capabilities.

    European VLBI

    The inclusion of ALMA was recently made possible through the ALMA Phasing Project (APP), an international effort led by the MIT Haystack Observatory in Westford, Massachusetts, and principal investigator Sheperd Doeleman, now at the Harvard–Smithsonian Center for Astrophysics.

    Before this project, the ALMA dishes worked with each other to make observations as a single array; now, the APP has achieved the synchronizing, or “phasing,” of up to 61 ALMA antennas to function as a single, highly sensitive radio antenna — the most antennas ever phased together. To achieve this, the APP team developed custom software and installed several new hardware components at ALMA, including a hydrogen maser (a type of ultraprecise atomic clock), a set of very-high-speed data reformatters, and a fiber optic system for transporting an 8 gigabyte-per-second data stream to four ultrafast data recorders (the Haystack-designed Mark6). The culmination of these efforts is an order-of-magnitude increase in the sensitivity of the world’s millimeter VLBI networks, and a dramatic boost in their ability to create detailed images of sources that previously appeared as mere points of light.

    “A great many people have worked very hard over the past several years to make this dream a reality,” says Geoff Crew, software lead for the APP. “ALMA VLBI is truly going to be transformative for our science.”

    One of the goals of these new technological innovations is to image a black hole. This month, two international organizations are making observations that will allow scientists to construct such an image for the very first time. And the portrait they’re attempting to capture is close to home: Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way.

    Sag A* NASA Chandra X-Ray Observatory 23 July 2014, the supermassive black hole at the center of the Milky Way

    So much data will be collected during the two observation periods that it’s faster to fly them to Haystack than it would be to transmit them electronically. Petabytes of data will be flown from telescopes around the world to Haystack for correlation and processing before images of the black hole can be created. Correlation, which registers the data from all participating telescopes to account for the different arrival times of the radio waves at each site, is done using a specialized bank of powerful computers. MIT Haystack is one of the few radio science facilities worldwide with the necessary technology and expertise to correlate this amount of data. Additional correlation for these sessions is being done at the Max Planck Institute for Radio Astronomy in Bonn, Germany.

    Two observing sessions are taking place. The GMVA (Global mm-VLBI Array) session will observe a variety of sources at a wavelength of 3 millimeters, including Sgr A* and other active galactic nuclei, and the EHT (Event Horizon Telescope) session will observe Sgr A* as well as the supermassive black hole at the center of a nearby galaxy, M87, at a wavelength of 1.3 millimeters. The EHT team includes researchers from MIT’s Haystack Observatory and MIT Computer Science and Artificial Intelligence Laboratory (CSAIL), working with the Harvard-Smithsonian Center for Astrophysics and many other organizations.

    Global mm-VLBI Array

    _______________________________________________________________________________________________________________________________________

    Event Horizon Telescope Array

    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    __________________________________________________________________________________________________________________________________

    “Several factors make 1.3 mm the ideal observing wavelength for Sgr A*,” according to APP Project Scientist Vincent Fish. “At longer observing wavelengths, the source would be blurred by free electrons between us and the galactic center, and we wouldn’t have enough resolution to see the predicted black hole shadow. At shorter wavelengths, the Earth’s atmosphere absorbs most of the signal.”

    The current observations are the first in a series of groundbreaking studies in VLBI and radio interferometry that will enable dramatic new scientific discoveries. Data from the newly phased ALMA array will also allow better imaging of other distant radio sources via improved data sampling, increased angular resolution, and eventually spectral-line VLBI — observations of emissions from specific elements and molecules.

    “Phasing ALMA has opened whole new possibilities for ultra high-resolution science that will go far beyond the study of black holes,” says Lynn Matthews, commissioning scientist for the APP. “For example, we expect to be able to make movies of the gas motions around stars that are still in the process of forming and map the outflows that occur from dying stars, both at a level of detail that has never been possible before.”

    The black hole images from the data gathered this month will take months to prepare; researchers expect to publish the first results in 2018.

    The MIT Haystack Observatory team of scientists includes Geoff Crew, Vincent Fish, Michael Hecht, Lynn Matthews, Colin Lonsdale, and Sheperd Doeleman (now at the Harvard-Smithsonian Center for Astrophysics).

    The organizations of the APP are: MIT Haystack Observatory (lead organization), Harvard–Smithsonian Center for Astrophysics, Joint ALMA Observatory (Chile), National Radio Astronomy Observatory (NRAO), Max Planck Institute for Radio Astronomy (Germany), University of Concepción (Chile), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan), National Astronomical Observatory of Japan (NAOJ), and Onsala Observatory (Sweden).

    See the full article here .

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  • richardmitnick 11:42 am on July 25, 2016 Permalink | Reply
    Tags: , , VLBI   

    From blueshift: “Thirty Years of Space VLBI” 

    NASA Blueshift

    NASA Blueshift

    July 25, 2016
    Koji Mukai

    As I write this in July 2016, it has been 30 years since the first successful space very long baseline interferometry (VLBI) observations were made. VLBI is the radio astronomy technique to use widely separated radio dishes to produce exquisite images of celestial radio sources – and space VLBI allows separation between dishes larger than the diameter of the earth, potentially producing higher resolution images.

    But astute readers might be questioning my sanity. Many sources, including a page on our Imagine the Universe site will tell you that the first space VLBI satellite was Japan’s HALCA, which was launched in 1997. And 1997 was less than 20 years ago. Both these statements cannot be true – can they? Actually, yes, they can be, and they are. The actual sentence on the linked page reads: “The first mission dedicated to space interferometry was the Japanese HALCA mission which ran from 1997 to 2005.” The key phrase is “dedicated to” – you see, we sometimes use somewhat awkward phrasing in communicating with the general public when we don’t want to bother you with all the details, at least not initially. The hidden detail behind the sentence above is that well before HALCA, there was an earlier satellite which was used to demonstrate that space VLBI is possible, even though it was not specially designed for that purpose.

    1
    Top: This radio image of the galaxy M87, taken with the Very Large Array (VLA) radio telescope in February 1989, shows giant bubble-like structures where radio emission is thought to be powered by the jets of subatomic particles coming from the the galaxy’s central black hole. The false color corresponds to the intensity of the radio energy being emitted by the jet. M87 is located 50 million light-years away in the constellation Virgo. Bottom: A Very Long Baseline Array (VLBA) radio image of the region close to the black hole, where an extragalactic jet is formed into a narrow beam by magnetic fields. The false color corresponds to the intensity of the radio energy being emitted by the jet. The red region is about 1/10 light-year across. The image was taken in March 1999. Credit: NASA, National Radio Astronomy Observatory/National Science Foundation, John Biretta (STScI/JHU), and Associated Universities, Inc.

    But let’s back up and start with a refresher on the basics. Professional astronomers and the general public alike like to have the sharpest, the most detailed images of astronomical objects. For UV and optical telescopes, we need bigger telescope mirrors for this, and to preferably launch them into space so the images are not blurred by the Earth’s atmosphere. With these telescopes, we can approach the diffraction limit – the fundamental limit on the sharpness of images set by the physics of light. You see, light is a wave, and there is an intrinsic fuzziness in how it goes through a slit, is reflected by a mirror, etc. The minimum angular size of an image – the diffraction limit – is proportional to the wavelength and inversely proportional to the diameter of the telescope mirror.

    Radio waves have wavelengths often measured in centimeters, much larger than the wavelength of visible light, by a factor of almost a million. While it is easier to build a bigger radio dish than a bigger optical telescope, there is a practical limit. The giant Arecibo radio telescope, famously featured in the film Contact, based on a book by Carl Sagan, used to be the biggest radio telescope in the world.

    NAIC/Arecibo Observatory, Puerto Rico, USA
    NAIC/Arecibo Observatory, Puerto Rico, USA

    Now China just completed what is considered to be the world’s biggest radio telescope.

    FAST Chinese Radio telescope under construction, Guizhou Province, China
    FAST Chinese Radio telescope

    Though the diameters of these big radio dishes are on the order of 100 times the diameters of the biggest visible light mirrors, the wavelengths of radio waves are still so large that the diffraction limited images from any of these single dish telescopes are not very sharp.

    4
    Primary mirror size comparisons. Note Arecibo is so big that it is only represented by a dark gray arc at the bottom of the image. [FAST is not represented at all.] Credit: Cmglee, creative commons.

    Interferometry to the rescue. If you have an array of radio dishes, they can be combined to increase the effective size of the telescope and obtain sharp images. In technical terms, a baseline is the separation between a pair of radio dishes; you want long (and short) baselines in a variety of directions to make a sharp image. For example, the Karl G. Jansky Very Large Array (VLA) has 27 movable dishes in a Y shaped configuration, each arm of which is 21 km (13 miles) long. Image-wise, its performance is similar to a single, 40 km diameter, telescope.

    NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA
    NRAO/VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    VLBI is when you combine signal from multiple radio telescopes on Earth. Space VLBI allows you to have baselines that are longer than the diameter of the Earth. With VLBI (Earth-bound or including a satellite), you tend to have fewer participating telescopes, and you may have to rely on the rotation of the Earth or the orbital motion of the satellite to give you a variety of baselines. HALCA allowed baselines up to about 30,000 km (3 times the diameter of the Earth), and the Russian RadioAstron satellite has an orbit that takes it up to distances equivalent to halfway to the distance to the Moon.

    6
    Active galaxy (PKS 1519-273) as imaged with HALCA satellite, along with the National Science Foundation’s VLBA and VLA ground-based radio telescopes. This is the first VLBI image ever made using an orbiting radio-astronomy satellite. Credit: NRAO

    But space VLBI started 30 years ago, before these purposefully built satellites. What they used prior to them was the tracking and data relay satellite system (TDRSS), which NASA started in the 1980s for communication between the Space Shuttles and other satellites and ground stations. The communication is via radio waves in some of the same frequency bands used for astronomical radio observations. Back in July and August of 1986, astronomers and engineers used the TDRSS satellite (there was only one in orbit back then) together with the 64-m antenna of the NASA Deep Space Network at Tidbinbilla, Australia and the 64-m antenna of the Institute for Space and Astronautical Science in Usuda, Japan. They demonstrated space VLBI was possible, and that the three quasars they observed were very compact and beaming radio sources. This success opened the way for HALCA and RadioAstron.

    So, here’s to the 30th anniversary of the first successful space VLBI observations!

    See the full article here .

    [This article suffers from no mention of the Event Horizon Telescope (EHT), a new adventure in VLBI. Aimed specifically at exploration of supermassive black hole Sagittarius A*, at the center of the Milky Way, this new adventure will surely take on other projects in a life of its own. That is how science works.

    So, here is the EHT

    Event Horizon Telescope Array

    Event Horizon Telescope map
    Event Horizon Telescope map

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    Future Array/Telescopes

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL ]

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