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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Summary

Satellite remote sensing is the primary tool for measuring global changes in the land, ocean, biosphere, and atmosphere. Over the past three decades, active remote sensing technologies have enabled increasingly precise measurements of Earth processes, allowing new science questions to be asked and answered. As this measurement precision increases, so does the need for a precise geodetic infrastructure.

The connections between the geodetic infrastructure and science applications are illustrated in Figure S.1. The geodetic infrastructure (level 1) comprises four measurement techniques used to accurately determine the Earth’s orientation in space, its gravitational field, the trajectories of satellites in orbit around the Earth, and the positions of reference points on the Earth. Data from these reference points are used to define the terrestrial reference frame (level 2), a set of coordinates and velocities of stable reference points on the surface of the Earth, which are used to define the locations of all other sites. Other geodetic products (e.g., orbit determination; level 3) are used to generate and interpret high-precision data from Earth-orbiting missions (level 4). These missions provide the connection between the terrestrial reference frame and the geophysical observables (level 5), which are needed to help answer science questions (level 6).

Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space (NASEM, 2018; referred to hereafter as the Decadal Survey) identified high priority questions and associated space observational requirements to support Earth system science and applications for 2017–2027. Many of the science questions in the Decadal Survey can be supported by the existing geodetic infrastructure, as long as it is maintained. However, other science questions require enhancements to the infrastructure. For example, active remote sensing systems at the core of the National Aeronautics and Space Administration (NASA) program—such as Jason-3, NASA-Indian Space Research Organisation Synthetic Aperture Radar, Ice, Cloud, and land Elevation Satellite 2, Gravity Recovery Climate Experiment Follow On (GRACE-FO), and Surface Water Ocean Topography (SWOT)—often require more accurate timing and orbit information to achieve their threshold science requirements. Understanding and implementing improvements to the geodetic infrastructure and terrestrial reference frame is urgent because high-precision data needed for Decadal Survey science questions are already flowing from satellites in orbit.

At the request of NASA managers, the National Academies of Sciences, Engineering, and Medicine established a committee to summarize progress in maintaining and improving the geodetic infrastructure and to identify improvements to the geodetic infrastructure to meet new science needs laid out in the Decadal Survey. The committee tasks are given in Box S.1 and the responses to these tasks are summarized below.

TASK 1: PROGRESS IN MAINTAINING AND IMPROVING THE GEODETIC INFRASTRUCTURE

The committee’s first task was to summarize progress and future aspirations for maintaining and improving the geodetic infrastructure, as detailed

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Image
FIGURE S.1 Illustration of how the geodetic infrastructure is connected to enabled scientific applications. NOTE: DORIS = Doppler Orbitography and Radiopositioning Integrated by Satellite; GNSS = Global Navigation Satellite System; InSAR = Interferometric Synthetic Aperture Radar; SLR = Satellite Laser Ranging; VLBI = Very Long Baseline Interferometry.

in the recommendations in Precise Geodetic Infrastructure: National Requirements for a Shared Resource (NRC, 2010). The geodetic infrastructure includes the measurement systems and facilities that allow continuous collection of data at the reference points that define the terrestrial reference frame, as well as international geodetic services that play a role in the measurement systems or produce enabling data sets or models. Four complementary measurement techniques are used to define the reference frame parameters (origin, orientation, and scale), with each technique bringing specific strength to the reference frame definition:

  1. Very Long Baseline Interferometry (VLBI), which provides information on Earth orientation angles and scale.
  2. Satellite Laser Ranging (SLR), which provides information on the location of the center of mass of the Earth and scale. SLR is also a passive backup tracking method that can be used for orbit determination when other instruments (e.g., GNSS) fail.
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
×
  1. A network of Global Navigation Satellite System (GNSS) stations, installed much more densely over the globe than the small number of VLBI and SLR sites. The density of this network allows tens of thousands of GNSS receivers on spacecraft, aircraft, ships, and buoys, and in local geodetic arrays to access or connect to the International Terrestrial Reference Frame (ITRF). The GNSS network also makes a vital contribution to the measurement of polar motion.
  2. Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), which is mainly used to compute accurate orbits of altimetric spacecraft and to enhance the global distribution of ITRF positions and velocities.

A large number of U.S. federal agencies contribute to the development and maintenance of the geodetic infrastructure. NASA operates a set of VLBI and SLR sites and hosts a few DORIS sites. The U.S. Naval Observatory supports the operation and upgrade of U.S. VLBI stations and provides Earth orientation parameters that describe irregularities in the rotation of the Earth. NASA, the National Science Foundation (NSF), the National Oceanic and Atmospheric Administration’s National Geodetic Survey, and the U.S. Geological Survey operate about one-quarter of the GNSS sites that form the core of the International GNSS Service. The National Geospatial-Intelligence Agency maintains a Global Positioning System (GPS) tracking network. In addition, U.S. federal agencies make substantial contributions to the international geodetic infrastructure through participation and leadership in international geodetic services.

The committee asked the above U.S. agencies to present their progress in and aspirations for maintaining and enhancing the geodetic infrastructure. Since the NRC (2010) report Precise Geodetic Infrastructure: National Requirements for a Shared Resource was published, several agencies have upgraded their networks (e.g., by replacing datums and upgrading GNSS sites to allow real-time streaming). Progress has been slower in modernizing VLBI and SLR systems. The committee found three areas of concern. First, the precision of the next-generation VLBI and SLR systems has not been validated with long-term data-driven studies (as opposed to simulation) in the refereed literature. Second, few VLBI or SLR stations have been added to complement and increase the density of the international geodetic network, especially in the southern hemisphere, leading to greater errors in the north-south location of the center of mass of the Earth. Third, a unified, highly accurate, national GNSS observing system has not been developed that could both (a) serve as the U.S. realization of and connection to the ITRF and (b) support the Decadal Survey science questions. Most of the geodetic networks operated by U.S. agencies have upgraded their GPS systems to receive signals from multiple satellite systems (multi-GNSS) or have clear plans to do so. However, plans to support the software and associated products (orbits and clocks) and models (e.g., location of antenna phase centers) needed for multi-GNSS data streams are not clear.

A broader concern is that, with an aging workforce and declining number of graduates trained in geodetic techniques and models, the United States risks losing its leadership role in geodesy or even its ability to meet the needs of U.S. geodesy programs. It is also at risk of losing redundancy (and hence validation capability) in the highest-grade geodetic data analysis software, independently written and maintained by more than one research group.

TASKS 2 AND 3: DECADAL SURVEY SCIENCE QUESTIONS THAT DEPEND ON THE GEODETIC INFRASTRUCTURE

Task 2 was to identify science questions in the Decadal Survey that depend on geodesy and to describe the connections between these questions, associated measurement requirements, and geodetic data. The committee selected a range of science questions that depend primarily on maintaining the current geodetic infrastructure (weather and climate and ecosystems) or on improving its capabilities (sea-level change, terrestrial water cycle, and geological hazards). Those science questions were discussed at a 2-day workshop in February 2019 attended by geodesists working to maintain and improve the geodetic infrastructure and discipline scientists seeking to answer questions that require an accurate terrestrial reference frame. Together, they identified what specific aspects of the geodetic infrastructure need to be maintained or improved to help answer the science questions being considered (Task 3). The science questions and their geodetic needs are summarized below.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
×

Sea-Level Change

Sea level is a leading indicator of climate change because its long-term change is driven mainly by the amount of heat being absorbed by the oceans and the amount of land ice being melted by a warmer atmosphere and oceans. Monitoring sea-level changes at global to regional scales, understanding the causes of these changes, and projecting how sea level might change in the future are critical for mitigating adverse impacts on coastal infrastructure, ecosystems, and human society. A precise geodetic infrastructure is essential for studies of (1) absolute sea-level change (sea level measured with respect to the Earth’s center of mass or other suitable reference surface), which is important for understanding climate change; and (2) relative sea level (sea level measured with respect to the possibly moving land surface), which is important for assessing the impacts along the coasts.

All of the measurements of sea-level change and its components (ocean thermal expansion, ice sheet and glacier mass change, land water hydrology, vertical land motion, and the effects of melting ancient and modern land ice) require a terrestrial reference frame that is accurately defined as a function of time. The terrestrial reference frame needs to have an accurately defined origin and be free of drifts and other errors, lest they create errors in the satellite measurements that could be misinterpreted as climate signals. This will become particularly challenging as the Earth’s shape and gravity field change due to climate change. Of particular concern is the movement of the Earth’s center of mass relative to the reference frame origin as the ice sheets melt, which could amount to several centimeters over the course of a century. In addition, geodetic sites near areas of ice mass loss may show anomalous motion and should be treated carefully if used to define the reference frame. It is also important to be able to reconstruct the terrestrial reference frame back in time, so that sea level measurements made a century from now can be compared to sea-level measurements made today or 25 years ago.

Terrestrial Water Cycle

Observing and understanding the water cycle and changes in the water cycle are essential for protecting this life-enabling resource both now and in the future. High-precision geodesy has become an important tool for hydrologists, climate scientists, and water managers, enabling a range of studies, including (1) elastic loading caused by changes in terrestrial water storage; (2) aquifer-system compaction and land subsidence caused by groundwater overdraft; (3) surface-water monitoring to support science, water management, and flood forecasting; and (4) water-cycle monitoring to track changes in total water storage and measure water cycle components (soil moisture, snow water equivalent, and vegetation water content).

The main geodetic focus of terrestrial water cycle applications is the ability to monitor absolute vertical deformation at local, regional, and continental scales. In the United States, this monitoring ability requires a backbone of core GNSS sites having a spacing of ~40 km and weekly Interferometry Synthetic Aperture Radar (InSAR) and altimetry acquisitions. Swath altimetry (e.g., SWOT) is needed to frequently measure surface water level (lakes and rivers), and is calibrated using tide gauges tied to the terrestrial reference frame by GNSS. The orbits of the InSAR and altimetry satellites rely on well-distributed GNSS stations at the surface of the Earth, as well as a stable and accurate terrestrial reference frame. Monitoring the water mass changes in the larger basins requires monthly time-variable gravity measurements from GRACE-type missions with support from the SLR network. Timely production and distribution of water cycle products relies on open data, accurate/open software, and a skilled workforce.

Geological Hazards

Earthquakes and volcanic eruptions open a window on processes operating within the Earth. They are also capable of great destruction, which has led to substantial efforts to forecast their occurrence and mitigate their impacts (e.g., reinforcing buildings to withstand expected shaking). Because earthquake and volcanic cycles occur on hundred- to thousand-year time scales, global and long-duration observations are needed to capture enough partial cycles to understand and model the underlying physical processes and so advance forecasting. The required measurements include surface deformation, time-variable gravity, surface topography, sea surface tsunami waves, and surface cover and atmospheric changes. All of these measurements depend on maintenance and moderate improvements of the geodetic infrastructure.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
×

The surface deformation measurements depend on a global backbone of GNSS sites that is augmented with higher spatial resolution, but less frequent (weekly) InSAR measurements. The combined system should be able to monitor global plate motions at mm/yr accuracy with local strain rate measurements at sub 50 nanostrain/yr precision, which requires a slight enhancement in the GNSS network. Approximately 40 km or better spacing of geodetic-quality GNSS stations is needed for monitoring tectonically and volcanically active sites in North America. Accurate and near-real-time satellite orbits and clocks are needed for both long-term monitoring and disaster mitigation. A time-dependent terrestrial reference frame combined with time-dependent gravity will be needed to track deformations from major tectonic events, especially in ocean areas not monitored by GNSS and InSAR. Ocean GNSS sites, with real-time data delivery, can increase the accuracy of tsunami forecasts as well as provide platforms for seafloor geodesy. All of these applications rely on open data as well as accurate/open software and a skilled workforce to deliver reliable products in a timely manner.

Weather and Climate

The atmosphere is a complex system that varies spatially at length scales ranging from meters to the circumference of the Earth and time scales ranging from minutes and weeks (weather) to years and longer (climate). Understanding and predicting weather and climate requires high spatial and temporal sampling using a wide variety of sensitive terrestrial and space-based sensors combined with large numerical models that assimilate these data. Science applications that rely on maintenance or enhancement of the geodetic infrastructure include (1) improving weather models, and (2) monitoring climate and reducing uncertainty in climate projections.

These applications use ground-based GNSS to measure total column water vapor over land as well as space-based GNSS radio occultation to measure the vertical structure of the atmospheric water vapor and temperature over both land and ocean areas. The measurements rely on accurate clocks and orbits of the GNSS constellations, which in turn rely on the geodetic infrastructure. The sheer number of radio occultations per day requires a fully automated system with frequent updates of clocks and orbital information. Maintaining absolute accuracy over perhaps hundreds of years will require a stable terrestrial reference frame, accurate orbits for the GNSS satellites as well as the low-Earth orbiting satellites, and a consistent approach to antenna models and data processing.

Ecosystems

Ecosystems supply the services upon which all life depends. Understanding how ecosystems are changing and how these changes influence the Earth system are important for sustaining life on the Earth. Ecosystem science topics that use active remote sensing, and thus rely on the geodetic infrastructure, include (1) vegetation dynamics; (2) lateral transport of carbon, nutrients, soil, and water; (3) global soil moisture; and (4) permafrost and changes in the Arctic.

The main geodetic tools used to investigate ecosystems are (a) Synthetic Aperture Radar (SAR) and InSAR for estimating changes in vegetation land cover, lidar for measuring vertical biomass structure, bare-earth topography, and surface motion associated with erosional and depositional processes; and (b) GNSS-derived total column water vapor and radio occultation for measuring atmospheric water vapor and soil moisture. These tools rely on accurate satellite orbits and clocks and thus depend on maintaining the current accuracy of the geodetic infrastructure and terrestrial reference frame. The application of GNSS to ecosystem science is emerging, and so the signal-to-noise ratio from GNSS ground stations will need be archived to support future research. Sustained gravity measurements are also a priority. New geodetic needs include increasing the number of GNSS stations across environmental gradients and placing the stations at locations with tide gauges and soil moisture sensors. In addition, many more radio occultation measurements are needed to support water vapor observations.

TASK 4: IMPROVEMENTS TO THE GEODETIC INFRASTRUCTURE

Task 4 was to identify priority improvements to the geodetic infrastructure that would facilitate advances across the science questions summarized above. These improvements cover five main areas: (1) accuracy and stability of the terrestrial reference frame; (2) accuracy

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
×

and stability of satellite orbits; (3) accuracy of the global-scale gravity field; (4) augmentation of the GNSS station network; and (5) analytical support for an enhanced geodetic infrastructure.

Most of the passive satellite systems recommended in the Decadal Survey rely on moderately accurate (<1 m) and near-real-time satellite orbits that are enabled by the continued maintenance of the geodetic infrastructure. In contrast, all of the active sensors that measure height (radar and laser altimetry), surface deformation (SAR), or path delay (radio occultation) require three-dimensional orbit accuracies that are better than or equal to the accuracy of the geophysical observable. For all of the satellite systems, active or passive, the availability of accurate orbits has enabled fully automated processing and accurate geolocation, which increases the exploitation of the large data sets being collected by Decadal Survey missions.

The accuracy and stability of satellite orbits relies on the accuracy and stability of the terrestrial reference frame, which is derived from the geodetic infrastructure. The committee identified three areas of improvement in the geodetic infrastructure needed to help answer the Decadal Survey science questions:

  1. Finalize deployment and testing of next-generation VLBI and SLR systems and complete deployment of multi-GNSS receivers to achieve a balance of geodetic measurement techniques between the northern and southern hemispheres, document the errors in the systems, and improve the ability to estimate their positions accurately and automatically.
  2. Increase the capabilities for measuring the center of mass motions expected over the next 100 years, due to the melting of the Greenland and Antarctic ice sheets.
  3. Work with the international community to implement a fully time-dependent terrestrial reference frame that will accommodate sudden, annual, and long-term changes in the locations of the fundamental stations.

The most stringent requirements for enhancements to the accuracy and stability of the terrestrial reference frame are driven by science questions related to sealevel change, ice-mass loss, and land-surface deformation associated with (a) the movement of water over the surface of the land, cryosphere, and oceans; and (b) the elastic and viscoelastic response of the solid Earth to water loading, earthquakes, and volcanic eruptions.

Ground-based GNSS receivers are essential for achieving the Decadal Survey science objectives related to sea level, cryosphere, weather, climate, geological hazards, and ecosystems. The density of core GNSS stations in the United States needs to be increased in high priority regions, including plate boundary zones to capture the earthquake cycle, coastlines to capture land motion that could affect sea-level impacts and coastal ecosystems, and regions with substantial terrestrial water storage. In addition, the United States will need to work with the International GNSS Service to deploy additional GNSS sites in remote, rapidly deforming areas, such as the perimeters of the ice sheets that deform by changes in mass loading. Such sites need good stability of geodetic monuments, long duration, and high data rate and availability. The U.S. stations should be considered part of the U.S. geodetic infrastructure, open to everyone, and thus have long-term financial support. Many of these stations already exist, but the infrastructure is aging and users cannot rely on their continued operation by NSF.

Maintaining and enhancing the geodetic infrastructure to compute the terrestrial reference frame, satellite orbits, and other products requires complex software systems developed over decades by teams of scientists and engineers. The software systems ingest both the raw measurements from the geodetic infrastructure and models for the deformation of the Earth and for propagation of the electromagnetic waves through the ionosphere and atmosphere. Support for software is critical for using GNSS data to calibrate and validate future satellite missions. The most important aspects of this activity are that all of the raw data are completely open and that cross-checking occurs by at least two independent groups using largely independent and open software.

An important component of both the GNSS and InSAR infrastructure is the development of new software delivery tools to make these data seamlessly available to more users. The dramatic improvement in satellite orbits and clocks over the past decade has enabled automated processing of very large sets of repeated observations (e.g., SAR, optical, radar altimetry, and

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
×

lidar) that was not possible just a few years ago. This advance is important because the data sets are too large for a human to be in the processing loop, and will require that the geodetic workforce work in close collaboration with the high performance computing community.

CONCLUDING REMARKS

The international geodetic infrastructure is the largely invisible foundation of Earth system science and applications. Most of the Decadal Survey science questions require maintenance of the geodetic infrastructure. However, key science questions—particularly those that need high-precision measurements from active remote sensing instruments—require enhancements to the geodetic infrastructure. Maintaining and in some cases enhancing the geodetic infrastructure will require collaboration among U.S. federal agencies and international partners as well as open data, accurate and open software, and a skilled geodetic workforce capable of developing and implementing improvements.

REFERENCES

NASEM (National Academies of Sciences, Engineering, and Medicine). 2018. Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space. Washington, DC: The National Academies Press.

NRC (National Research Council). 2010. Precise Geodetic Infrastructure: National Requirements for a Shared Resource. Washington, DC: The National Academies Press.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Evolving the Geodetic Infrastructure to Meet New Scientific Needs. Washington, DC: The National Academies Press. doi: 10.17226/25579.
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Satellite remote sensing is the primary tool for measuring global changes in the land, ocean, biosphere, and atmosphere. Over the past three decades, active remote sensing technologies have enabled increasingly precise measurements of Earth processes, allowing new science questions to be asked and answered. As this measurement precision increases, so does the need for a precise geodetic infrastructure.

Evolving the Geodetic Infrastructure to Meet New Scientific Needs summarizes progress in maintaining and improving the geodetic infrastructure and identifies improvements to meet new science needs that were laid out in the 2018 report Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space. Focusing on sea-level change, the terrestrial water cycle, geological hazards, weather and climate, and ecosystems, this study examines the specific aspects of the geodetic infrastructure that need to be maintained or improved to help answer the science questions being considered.

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