Tom Herring on High-Precision Geodesy
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There are three main types of geodetic measurement systems — satellite-based systems such as GPS, very long baseline interferometry (VLBI), and interferometric synthetic-aperture radar (InSAR). While each type of systems has its particular strengths, the cost of satellite-based receivers has plummeted. Millimeter-level accuracy will soon be incorporated into phones. This has broadened the kinds of geological questions we can now address with such systems. In the podcast, Tom Herring describes how these systems are giving us new insight into plate motions, slow and fast deformation associated with faults and earthquakes, the Earth’s rotation, as well as applications in civil engineering, such as dams and tall buildings, and agriculture.
Herring is a pioneer in high-precision geodetic analytical methods and applications for satellite-based navigation systems to study the Earth’s surface. He is a Professor in the Earth, Atmospheric, and Planetary Sciences department at the Massachusetts Institute of Technology.
Podcast Illustrations
All images courtesy of Tom Herring unless otherwise noted.
Principle of satellite-based position measurement. The distances are referred to pseudo-distances as they include errors from clock differences and atmospheric delays.
Signal corrects for clock error, but the variability in atmospheric delay still needs to be accounted for in order to obtain an accurate measurement. This is done by using more than three satellites.
Atmospheric layers alter the GPS signals and can introduce significant errors. The most important effects arise as the radio waves pass through the Earth’s charged ionosphere and water-laden troposphere. As Herring describes in the podcast, when four or more satellite signals are detected, the receivers can correct for atmospheric effects.
Inexpensive Global Navigation Satellite System (GNSS) receiver. GNSS refers to a set of satellite-based systems such as GPS, the Russian GLONASS system, and European Galileo system.
Expensive GNSS receiver costing about $20,000. The disk shapes of the antennas are covers that partially block signals from ground reflections. The larger the ground plane, the better the signal blockage.
Permanent GNSS receiver in Oman where Herring has studied the subsidence of an oil and gas field during production.
GPS antenna being installed near the top of a tall building.
Testing a GPS system on the roof of a tower on the MIT campus.
GNSS receiver installed in the Southern Alps of New Zealand to measure uplift rates across the Alps.