Dr. Thomas A. Herring, Professor of Geophysics; Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA
Research interests: Applications and development of geodetic systems, with a current focus on using the Global Navigation Satellite System (GNSS) to study surface deformation processes on Earth and the media through which satellite microwave signals propagate.
Research highlights: One of the great achievements of the space age is the Global Navigation Satellite System (GNSS). Haven’t heard of GNSS? Chances are you might be more familiar with the name used for the U.S. version of the GNSS constellation of satellites: the Global Positioning System (GPS). The European version of the GNSS constellation is called Galileo, while the Russian constellation is called the GLObal NAvigation Satellite System (GLONASS). China has their BeiDou satellites, Japan the Quasi-Zenith Satellite System (QZSS), and India has the Indian Regional Navigation Satellite System (IRNSS). Through these constellations of Earth-orbiting satellites, the precise location of any point on the planet and, more importantly, the determination of precise changes in location of any point on the planet can be established.
While the national names of GNSS constellations may differ, the technique is the same – clusters of satellites constantly transmit location and timing data that are detected by ground-based receivers. By using precisely timed signals from at least four satellites in known orbits, the exact position of a receiving station can be determined. As of November 13, 2020, 30 operational satellites comprise the U.S. GPS constellation, according to GPS.gov.
GNSS is one technique in the field of geodesy, which is the science of measuring Earth’s geometric shape, orientation in space, and gravity field. GNSS is also a key element of geodesist Dr. Thomas A. Herring’s research into surface deformation processes and how microwave signals from satellites propagate through various media. The deformation processes Herring studies can be steady (such as the motions of tectonic plates, which move at a rate of three to five centimeters per year, according to National Geographic), transient (such as rapid deformations from earthquakes and processes that follow earthquakes), or intermediate (such as the subsidence of land through water pumping or oil extraction that occurs on timescales of days to months). He observes that the media (soil, rock, etc.) through which the satellite signals propagate can affect the location information of sites on and near Earth’s surface and must be accounted for to determine precise location change over time.
The geodetic data used by Herring come from a number of sources, including data freely and openly available through NASA’s Crustal Dynamics Data Information System (CDDIS). CDDIS archives and distributes geodetic data in NASA’s Earth Observing System Data and Information System (EOSDIS) collection. The data and derived products available through CDDIS come from a global network of observing stations equipped with one or more of the following geodetic techniques:
- Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)
- Global Navigation Satellite System (GNSS)
- Satellite Laser Ranging (SLR; including Lunar Laser Ranging (LLR))
- Very Long Baseline Interferometry (VLBI)
CDDIS has served as a global data center for the International GNSS Service (IGS) since 1992, and actively supports the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), the International DORIS Service (IDS), and the International Earth Rotation and Reference Systems Service (IERS) as a global data center.
Along with the GNSS data he acquires from CDDIS, Herring and his colleagues at the Massachusetts Institute of Technology (MIT) also create their own GNSS data products. These include the GNSS At MIT and Global Kalman filter (GAMIT/GLOBK) software, which is a comprehensive suite of programs for analyzing GNSS measurements primarily to study crustal deformation. Freely available from MIT, GAMIT/GLOBK was developed by MIT, the Harvard-Smithsonian Center for Astrophysics, the Scripps Institution of Oceanography, and the Australian National University, with support from the U.S. National Science Foundation and NASA’s Earth’s Surface and Interior (ESI) focus area.
One of Herring’s recent research projects involves analyzing large datasets collected for studying the North America Pacific plate boundary and the deformations happening in North America and the region around it. A well-known part of this region is along the San Andreas Fault, where a section of western California, as part of the Pacific Plate, slides north-northwestward past the rest of North America. As noted by the USGS, Point Reyes National Seashore, Golden Gate National Recreation Area, and Pinnacles National Park are landscapes shaped by movement of the San Andreas Fault, and Cabrillo National Monument south of San Diego lies within the broad zone of deformation created as the Pacific Plate slides past the North American Plate. GNSS data enable the precise mapping of this deformation as well as the rate of plate movement.
Herring points out that the steady motions at the plate boundaries indicate how strain and stress are accumulating. Transient phenomena, often referred to as slow slip events (SSE), affect the interaction between the stress building at the plate boundary and the release of this stress, which is experienced as an earthquake. The precise relationship between the steady accumulation of stress and the resulting transient phenomena is still being determined, and the ultimate aim of this research is to better understand the processes leading to earthquakes. The GNSS data used in these studies are available through CDDIS along with products generated by the IGS.
GNSS also is useful in studies of how architectural structures, such as tall buildings, radio towers, and similar objects, react to transient phenomena. This is what took Herring to the top of the tallest building in Kuwait – the 1,355-foot Al Hamra Tower. A GPS system was installed near the top of this sinuously curving skyscraper to study the response of the building to earthquakes and environmental changes. In November 2017, the research team used GPS data to analyze the motions at the top of the building due to a distant magnitude 7.3 earthquake. While motion at ground level was approximately 40 mm, motion at the top of the building was measured at approximately 160 mm. According to the research team, the building responded as expected, although it continued shaking for an extended period of time after the first waves from the earthquake arrived. This research has direct applications for architectural design in earthquake-prone regions and was a collaboration with the Kuwait Institute for Scientific Research (KISR) and the Kuwait Federation for the Advancement of Science (KFAS).
Whether you’re trying to measure the movement of a continental plate moving mere centimeters a year or the rapid deformation to land and structures following an earthquake, a constellation of GNSS satellites traveling tens of thousands of miles per hour around Earth makes this possible. The research of Herring – using data openly available through CDDIS and other sources – is helping make these data even more precise.
Representative data products used or created:
Available through CDDIS:
- Daily GNSS tracking data; Receiver INdependent EXchange (RINEX) format
- IGS and Multi-GNSS experiment (MGEX) orbits and broadcast ephemeris information (doi:10.5067/GNSS/GNSS_IGSMGEXP_001)
- Global ionospheric delay gridded models; IONosphere EXchange (IONEX) format
Products generated by Herring and his colleagues for distribution through CDDIS:
- GPS orbit, clock, and Solution (Software/technique) INdependent EXchange Format (SINEX) files for IGS final GPS orbits from the MIT IGS Analysis Center (AC) (doi:10.5067/GNSS/gnss_igsorb_001)
- SINEX files with combined IGS analysis centers (Associate Analysis Center, AAC) and regional combined SINEX files (Regional Network Associate Analysis Center, RNAAC); available through the CDDIS GNSS Station Position Products page
Other data products used:
- Interferometric Synthetic Aperture Radar (InSAR) deformation maps; available through NASA’s Alaska Satellite Facility DAAC (ASF DAAC)
- Imagery created from data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra and Aqua satellites; available for interactive exploration using the NASA Worldview data visualization application
- Meteorological products from the Technical University of Vienna; Vienna Mapping Functions (VMF) products (doi:10.17616/R3RD2H)
- GPS/GNSS RINEX data from multiple archives around the world
Read about the research:
Herring, T.A., Gu, C., Toksöz, M.N., Parol, J., Al-Enezi, A., Al-Jeri, F., Al-Qazweeni, J., Kamal, H. & Büyüköztürk, O. (2018). GPS Measured Response of a Tall Building due to a Distant Mw 7.3 Earthquake. Seismological Research Letters, 90(1): 149-159. doi:10.1785/0220180147
Herring, T.A., Melbourne, T.I, Murray, M.H., Floyd, M.A., Szeliga, W.M., King, R.W., Phillips, D.A., Puskas, C.M., Santillan, M. & Wang, L. (2016). Plate Boundary Observatory and Related Networks: GPS Data Analysis Methods and Geodetic Products. Reviews of Geophysics, 54(4): 759-808. doi:10.1002/2016RG000529
Ji, K.H. & Herring, T.A. (2013). A method for detecting transient signals in GPS position time-series: smoothing and principal component analysis. Geophysical Journal International, 193(1): 171-186. doi:10.1093/gji/ggt003
Ji, K.H. & Herring, T.A. (2012). Correlation between changes in groundwater levels and surface deformation from GPS measurements in the San Gabriel Valley, California. Geophysical Research Letters, 39(1). doi:10.1029/2011GL050195