\nAir & Space Magazine<\/strong>
\nSeptember 2019<\/strong><\/p>\n
![\"Potsdam](\"https:\/\/www.csr.utexas.edu\/wordpress\/wp-content\/uploads\/2019\/08\/opener-geodesic.jpg\")
<\/a>In the Davis Mountains of far west Texas, at the University of Texas McDonald Observatory, astronomers spend their nights peering at the stars through some of the world\u2019s most powerful telescopes. Soon they\u2019ll be adding a more down-to-Earth job. Within sight of the giant domes, NASA is installing a sprawling network of equipment to help researchers study planetary change.<\/p>\n
Last July, engineers achieved first light on a towering radio antenna, 12 meters across, that watches for signals flashing from cosmic beacons called quasars located in the distant universe. Nearby, scientists have set up new global-positioning-system stations\u2014tripods topped with bulbous heads that communicate with GPS satellites to determine their precise location on Earth\u2019s surface. On a neighboring mountain, technicians plan to build a powerful laser system that can zap a beam of light up to a satellite, then clock how quickly it reflects back to Earth. Together, all this high-tech equipment will allow scientists to pinpoint individual spots of ground at McDonald Observatory to within millimeters.<\/p>\n
Similar activities are going on all over the world. NASA is helping to upgrade Earth-measuring observatories from Tahiti to South Africa. By linking the McDonald measurements with the others, researchers aim to better understand how Earth\u2019s shape, rotation, and gravity change over time.<\/p>\n
Called geodesy, this field of science underlies almost every aspect of modern life, whether it\u2019s using Google Maps to find the nearest coffee shop or determining how sea level is rising as the planet warms. \u201cYou would never think that navigating your car is dependent on our measurements of distant quasars,\u201d says Stephen Merkowitz, an astrophysicist at NASA\u2019s Goddard Space Flight Center in Greenbelt, Maryland, who manages the agency\u2019s Space Geodesy Project. \u201cBut it is.\u201d<\/p>\n
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Earth may look like a perfect blue marble in photos taken from space, but it is not precisely round. It\u2019s squashed at the poles, and it bulges at the equator, making it about 43 kilometers (27 miles) wider than it is tall. Parts of the globe, such as Scandinavia and northern Canada, are rising into the air, still rebounding after the weight of the great northern ice sheets disappeared thousands of years ago at the end of the last ice age. The planet also shapeshifts throughout the year as mass moves around on its surface\u2014as when ice melts in the summer or water cycles between the ocean and atmosphere during an El Ni\u00f1o.<\/p>\n
All this information, and much more, feeds into something called the International Terrestrial Reference Frame (ITRF), the system scientists use to locate terrestrial observations. Think of it as a global web of coordinates that defines where any particular spot on Earth is at any given time. \u201cIf you want to do any kind of positioning or navigating on Earth or in space, it\u2019s all done within a reference frame,\u201d says Richard Gross, a research scientist and geodesy specialist at NASA\u2019s Jet Propulsion Laboratory in Pasadena, California.<\/p>\n
There are thousands of local reference frames in use today, but the ITRF is widely accepted internationally. Without global standards, engineers would have to constantly adjust for local differences, which is exactly what happened during the 2004 construction of the Laufenburg bridge on the Swiss-German border. Swiss engineers used the Mediterranean as their sea-level reference. German engineers used the North Sea. Both groups knew they would have to adjust, since the two seas\u2019 reference levels were 27 centimeters apart. But the correction was made with the wrong sign, resulting in a 54-centimeter error between the two ends of the bridge. It\u2019s the classic example of reference-frame screw-up. \u201cThat\u2019s why we are trying our best to provide an accurate global reference frame and promote its use,\u201d says Gross.<\/p>\n
Researchers generate the ITRF using data from four different Earth-measuring techniques, including GPS. Last updated in 2014, the system is accurate to within a few centimeters relative to the center of the Earth. But in an increasingly wired world, that level of precision is no longer good enough. \u201cYou want the driverless cars to stay in their lane,\u201d says Srinivas Bettadpur, director of the Center for Space Research at the University of Texas at Austin. Geodesy experts are now pushing to improve the accuracy of the ITRF down to one millimeter. That would allow scientists to better measure tiny signals such as sea level rise, which is happening globally at the rate of just over three millimeters a year.<\/p>\n
That\u2019s where NASA\u2019s equipment upgrades come in. It takes big facilities to measure tiny changes\u2014that plus the integration of data gathered using different geodetic techniques. \u201cThey help each other out because they are completely different types of measurements,\u201d says Jan McGarry, a mathematician at Goddard.<\/p>\n
The first and most familiar is GPS. The United States, Europe, Russia, and China each operates its own satellite constellations, which let GPS receivers like the one in your cellphone determine your location to within a meter or so. Scientists can pull additional information from the signals to improve accuracy, but only with a lot of effort. \u201cWe can take a system that was really only meant to measure things to a meter or 10 meters, but get it down to a millimeter or two,\u201d says Kristine Larson, a geophysicist and emerita professor at the University of Colorado at Boulder. \u201cOf course, you spend about 1,000 times more money.\u201d<\/p>\n
With millimeter precision, researchers can put GPS receivers on opposite sides of a geological fault like California\u2019s San Andreas and watch how they move relative to one another, perhaps hinting at a future earthquake. Or they can put receivers around volcanoes, such as Kilauea in Hawaii, to track how the ground moves up and down as magma shifts beneath it.<\/p>\n
GPS isn\u2019t perfect. The satellites drift a tiny bit in their orbits, so errors can creep in, undetected, over time. So geodesists need other techniques to provide an independent check. \u201cThe way to improve the reference frame is to run these instruments side by side,\u201d says Larson.<\/p>\n The second-line method, called satellite laser ranging, bounces a laser beam off a satellite and measures how long it takes to return. This technique got its start at Goddard in 1964, just four years after the laser was invented. NASA scientists fired a laser at a satellite called Beacon Explorer-B, which was covered in reflective surfaces, and a telescope measured the beam\u2019s return. Calculations based on that measurement provided the distance to Earth\u2019s center of mass.<\/p>\n Today laser ranging is done off many different kinds of satellites, including NASA\u2019s ICESat-2 launched last year, and a shiny metal-and-glass ball called LAGEOS-2, launched in 1992. NASA operates eight satellite laser ranging stations around the globe, but most of them are outdated. \u201cWe buy parts on eBay sometimes to maintain them,\u201d says Merkowitz.<\/p>\n Running a satellite laser-ranging system also takes a lot of time and attention. Operators typically use a radar system mounted alongside the laser to detect airplanes that might stray into the area and automatically shut down the laser. McGarry remembers the old days when operators had to scan the sky with binoculars. \u201cI could only do that for about an hour at a time,\u201d she says.<\/p>\n The third tool available to geodesists is very long baseline interferometry, or VLBI. It uses large radio telescopes to clock when radio signals arrive on Earth from quasars, the energetic hearts of massive galaxies billions of light-years away. By noting the difference in arrival times at different VLBI stations around the globe, scientists can calculate small variations in the rotation rate of the planet\u2014due to factors such as the changing seasons, the tides, and weather patterns.<\/p>\n VLBI observations come in spurts, according to Frank Lemoine, a geodesy expert at Goddard. Typically, up to a dozen VLBI stations around the globe will track a set of quasars for up to 24 hours, once or twice a week. These observations are supplemented by one-hour daily sessions. Occasionally, longer campaigns lasting weeks help push the precision even further.<\/p>\n Existing VLBI telescopes have an unfortunate limitation: They can\u2019t move fast enough. One major source of error is the changing atmospheric conditions in the sky over the radio dish; older antennas literally can\u2019t slew fast enough to account for this, Lemoine says. Newer telescopes like one installed on Kauai, Hawaii, in 2016 and the one installed at McDonald this year are faster and therefore more precise.<\/p>\n The fourth and final tool used in geodesy\u2014a system called DORIS\u2014sends transmissions from Earth up to special satellites whose receivers calculate their distance from the transmitter based on Doppler shifting of the signal. The French space agency established DORIS in 1990 to help with sea-level measurements, which means DORIS has been observing for longer than any other geodetic network.<\/p>\n The distribution of 50 DORIS transmitters nearly evenly around the globe is an advantage in trying to understand planet-wide changes. Most of the satellite laser-ranging and VLBI stations are in the northern hemisphere. \u201cIf all your stations are in Europe, you kind of know what Europe is doing, but you don\u2019t know what South America or Africa is doing,\u201d says Gross.<\/p>\n As NASA upgrades its geodetic networks, it is also working to fix the imbalance. An ideal system would have 24 to 30 stations distributed evenly around the globe, with at least three of the four techniques used at each site, says Merkowitz. That kind of observing network would achieve the ITRF goal of millimeter accuracy. \u201cThe big barrier right now is getting that global network in place,\u201d he says.<\/p>\n NASA is spending around $18 million a year on its Space Geodesy Project to update old equipment and bring new instruments online.<\/p>\n International partners are doing their part in this global effort. The Norwegian Mapping Authority is cooperating with NASA to place a satellite laser-ranging system on the Arctic island chain of Svalbard. Situated just 1,000 kilometers from the North Pole, the system will have the best view of polar-orbiting satellites as they fly overhead. It should be up and running by 2024.<\/p>\n NASA is also working with the French space agency to upgrade equipment in Tahiti and has collaborators in Australia, South Africa, and Brazil. \u201cWe\u2019re looking continent by continent,\u201d Merkowitz told his audience at a meeting of the American Geophysical Union in December.<\/p>\n * * *<\/p>\n Twin satellites launched in 2002, then succeeded by a follow-on mission launched last year, have given geodesists another powerful yardstick for fine measurements of Earth-size change. Geophysicist Isabella Velicogna, at the University of California, Irvine, uses them to track how the Greenland and Antarctic ice sheets lose weight in the summer as ice melts and gain it back in the winter as snow piles up and freezes. The U.S.-German satellites, known as GRACE, for Gravity Recovery and Climate Experiment, measure the changing force of gravity at Earth\u2019s surface.<\/p>\n The twin GRACE satellites fly about 220 kilometers apart and use microwaves and lasers to measure the distance between them very precisely. When the leading satellite passes over an object with a lot of mass, like a mountain range or an ice sheet, it is tugged ever so slightly forward, and the distance between the two satellites increases. When the trailing satellite approaches that same mountain range or ice sheet, it too is tugged forward and the gap gets shorter. GRACE can measure changes in distance as small as the width of a human blood cell.<\/p>\n<\/a><\/a><\/a><\/a>