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Progress in Maintaining and Improving the Geodetic Infrastructure

The committee’s first task was to summarize progress in maintaining and improving the geodetic infrastructure, as detailed in the recommendations in Precise Geodetic Infrastructure: National Requirements for a Shared Resource (NRC, 2010), and aspirations for future improvements through, for example, new technology and analysis. A large number of U.S. federal agencies have a role in developing and maintaining the geodetic infrastructure, and the committee heard from six whose contributions are particularly relevant for achieving the Decadal Survey objectives laid out in Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space (NASEM, 2018). These agencies were the National Oceanic and Atmospheric Administration’s National Geodetic Survey (NOAA NGS), the National Geospatial-Intelligence Agency (NGA), the National Aeronautics and Space Administration’s Goddard Space Flight Center (NASA Goddard) and Jet Propulsion Laboratory (NASA JPL), the National Science Foundation (NSF), the U.S. Geological Survey (USGS), and the U.S. Naval Observatory (USNO). The agency responses to the NRC (2010) recommendations and their aspirations for future improvements are summarized below.

MAINTAINING AND IMPROVING THE GEODETIC INFRASTRUCTURE

NRC (2010) Recommendation 1. The United States, to maintain leadership in industry and science, and as a matter of national security, should invest in maintaining and improving the geodetic infrastructure through upgradesin network design and construction, modernization of current observing systems, deployment of improved multi-technique observing capabilities, and funding opportunities for research, analysis, and education in global geodesy.

The progress in maintaining and improving the geodetic infrastructure reported by each agency is summarized below. Funding for research and education is discussed in the response to Recommendation 8 below.

Progress and Aspirations

Since 2010, several agencies have made upgrades to their networks (e.g., by replacing datums and upgrading Global Navigation Satellite Systems [GNSS] sites). Progress has been slower on modernizing observing systems (e.g., Very Long Baseline Interferometry [VLBI] and Satellite Laser Ranging [SLR]).

Upgrade of Networks

NOAA NGS is modernizing the current U.S. National Spatial Reference System (NSRS) in two key ways: (1) by replacing the horizontal datum (NAD 83) with a set of plate-fixed frames more closely tied to the International Terrestrial Reference Frame (ITRF), and (2) by updating the current vertical datum (NAVD 88) with a gravimetric geoid-based version. These changes will enable GNSS-based ellipsoidal heights to be related to orthometric heights used for local vertical control. Although widely used by surveyors, the NSRS is not sufficiently precise to meet the science requirements of the Decadal Survey. However, the GNSS tracking data from the Continuously Operating

Reference Stations (CORS) in the NSRS are used for scientific applications. Some of these data are processed by the International GNSS Service (IGS) and so are included in the ITRF.

With the end of the EarthScope Plate Boundary Observatory (PBO) project, NSF has combined PBO stations and GNSS networks built by NSF investigators in Central America and the Caribbean to form the Network of the Americas (NOTA). The receivers are gradually being upgraded to multi-GNSS tracking and real-time streaming. This network of almost 1,300 GNSS stations extends from the Aleutians to northern South America. However, NSF recently announced that this network will be reduced by 10 percent, to 1,100 stations.¹

USGS has upgraded many of its GNSS sites to include real-time telemetry, and some sites have been upgraded to multi-GNSS receivers. The real-time data support the USGS shake-alert system, which uses instrumentation in the near-field of major earthquakes to send an accurate earthquake early warning to civilian populations (see the review by Allen and Melgar, 2019).

NASA JPL maintains a global GNSS network to support precise orbit determination and the ITRF. It has upgraded this network with multi-GNSS-capable receivers. In addition, its Global Positioning System (GPS) analysis software (GipsyX) has been modernized and is being extended to include multi-GNSS capability.

Modernization of Current Observing Systems

NGA has been working with NASA and the Department of Defense to deploy laser reflector arrays on the next-generation GPS-IIIF satellites, which will be launched after 2025. These arrays will allow SLR data to be used to evaluate the accuracy of GPS-III orbits.

NASA Goddard continues work on modernizing the SLR and VLBI systems with new VLBI Global Observing System (VGOS) hardware, a 12-meter dish with broadband tracking capabilities, and Space Geodesy Satellite Laser Ranging (SGSLR) hardware (see also NRC [2010] Recommendation 2). This work is proceeding in concert with USNO, which is supporting the operation and upgrade of U.S. VLBI stations (including NSF’s Very Long Baseline Array) and international partners. USNO also collaborates with NASA Goddard to provide Earth orientation parameters and Celestial Reference Frame products to defense and civil communities.

Concerns

Long-standing efforts by NASA to design, build, and test the VGOS and SGSLR systems are not complete. The information provided to the committee was insufficient to assess the precision of these new prototype observing systems. Moreover, few peer-reviewed papers on VLBI and SLR error sources have been published by U.S. research groups in the past decade.

Modernization of the global VLBI network faces two challenges. The first is installing, testing, and commissioning the new telescopes and associated hardware and software. The second is the transitioning operations of the legacy systems to VGOS. This will involve new schedules and coordination, higher data rates and demand on correlators, and transition of the product base. For the global SLR network, only about a dozen global SLR stations (four supported by NASA) provide sufficient tracking data to contribute substantially to the ITRF, and those stations are geographically unbalanced, with too few in the southern hemisphere.

Although many U.S. agencies have supported the deployment of new equipment that is enabled to track multi-GNSS systems, the necessary software support for multi-GNSS users is not available. For example, multi-GNSS orbit and clock products are not currently provided by any of the U.S. analysis centers. Furthermore, none of the U.S. geodesy groups have an operational GNSS antenna calibration system, and some GNSS bias corrections are only provided by foreign partners.

ENHANCING SPECIFIC SLR AND VLBI SITES

NRC (2010) Recommendation 2. In the near term, the United States should construct and deploy the next generation of automated high-repetition-rate SLR tracking systems at the four current U.S. tracking sites: Haleakala, Hawaii; Monument Peak, California; Fort Davis, Texas; and Greenbelt, Maryland. It also should install the next-generation VLBI systems at the four U.S. VLBI sites: Greenbelt, Maryland; Fairbanks, Alaska; Kokee Park, Hawaii; and Fort Davis, Texas.

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¹ See https://www.nsf.gov/pubs/2019/nsf19072/nsf19072.jsp.

Progress and Aspirations

This recommendation was aimed at near-term enhancements of four SLR and four VLBI sites maintained by NASA (with USNO support at Kokee Park). In the decade since the NRC (2010) report, NASA has completed all of its site assessment studies. Next-generation VGOS systems have been operating in Greenbelt, Maryland, and NASA continues to operate the legacy broadband system at Westford, Massachusetts. One new VGOS system has been recently commissioned at Kokee Park (in concert with USNO and with a co-located legacy antenna there), and (as of this writing) the signal chain for a second VGOS system is being installed at the McDonald Observatory site. In addition, NASA supports the legacy station in Fortaleza, Brazil. The need for an Alaskan VLBI location is being reevaluated.

Achieving the full capabilities of the VGOS system will require equipping more stations with ultrawide bandwidth (multi-GHz) data acquisition backends and transporting up to 40 TB of data per station per day to central correlator facilities. There are now enough stations around the world to produce large-scale geodetic measurements.

The Goddard Geophysical and Astronomical Observatory hosts the prototype for the SGSLR. It will have a higher repetition rate, lower energy lasers, single photon detectors, additional laser wavelengths, shorter acquisition times and faster slewing, real-time data evaluation for quality control, and autonomous operations. NASA has stated its plan to deploy this new instrumentation at the four stations named above in 2019–2020 and at a new SLR station in Ny-Ålesund, Svalbard, in 2022. NASA also operates four legacy SLR stations (in Australia, Peru, South Africa, and Tahiti) and has begun discussions with local partners to upgrade or replace each of them (with the Peru station possibly moving to Brazil).

Concerns

NASA has upgraded VLBI instrumentation at three U.S. sites. None of these three sites have operating SGSLR systems. As noted in the previous section, the accuracy and long-term stability of the prototype SLR and VLBI systems have not been demonstrated.

INTERNATIONAL GEODETIC NETWORK

NRC (2010) Recommendation 3. In the long term, the United States should deploy additional stations to complement and increase the density of the international geodetic network, in a cooperative effort with its international partners, with a goal of reaching a global geodetic network of at least 24 fundamental stations.

Progress, Aspirations, and Concerns

Little progress has been made on implementing this recommendation. Only a handful of fundamental stations—defined as including the three techniques of VLBI, SLR, and GNSS—exist and they are poorly distributed globally. NASA Goddard has established one of these three-system fundamental stations and also operates a Doppler Orbitography and Radiopositioning Integrated by Satellite beacon.

NASA Goddard plans to deploy next-generation VLBI and SLR stations in Hawaii, Maryland, and Texas, and is currently in discussions with Australia, Brazil, South Africa, and Tahiti to replace NASA legacy stations. At current funding levels, subsequent deployments in Columbia, Kenya, and Nigeria would begin in 2028.

GNSS/GPS NATIONAL NETWORK

NRC (2010) Recommendation 4. The United States should establish and maintain a high-precision GNSS/GPS national network constructed to scientific specifications, capable of streaming high-rate data in real time.

Progress and Aspirations

The United States has not established a high-precision GNSS national network to scientific specifications. The PBO project (and now NOTA) installed many high-quality GNSS sites and many are currently being upgraded to track multi-GNSS and to stream high-rate data in real-time. However, because almost all PBO sites were installed to study plate boundary deformation, the sites are mostly concentrated along the west coast. More than 500 of these sites are in California and only a handful are east of the Rocky Mountains.

State departments of transportation have installed many GNSS sites in the eastern United States, but these sites were not built to scientific standards.

Although the data are sometimes freely available, their quality is highly variable and they are not archived according to scientific standards. For example, access to raw GNSS observations is generally not allowed and data streams are decimated to save disk space after 30 days.

NASA’s Global GNSS Network provides high-rate, real-time data from more than 70 stations worldwide. NASA also works with NOAA NGS to align their efforts by augmenting the existing national GNSS array with foundation CORS to improve geometric coverage and linkage with the ITRF.

Concerns

Although GNSS data are available in the United States, the lack of coordination and disparate sources of funding mean that users, particularly real-time and scientific users, cannot rely on high-quality observations or support for their continued operation. This adversely affects both scientific and hazard applications. For example, despite the large number of continuously operating GNSS stations in the United States, observations of ground-based atmospheric water vapor for operational weather forecasting, for instance, lags far behind many other countries. In addition, the value of adding GNSS real-time positioning streams for tsunami warning has been demonstrated (Melgar et al., 2016), but it is unlikely that these data will be included unless the tsunami warning centers can rely on the continued support for GNSS networks in the United States. The recent decision by NSF to eliminate a large number of GNSS sites in North America is a further reminder that long-term support for this critical GNSS infrastructure is at risk.

INTERNATIONAL GEODETIC SERVICES AND THE ITRF

NRC (2010) Recommendation 5. The United States should continue to participate in and support the activities of the international geodetic services (IGS, ILRS, IVS, IDS, IGFS, and IERS).

NRC (2010) Recommendation 6. The United States, through the relevant federal agencies, should make a long-term commitment to maintain the International Terrestrial Reference Frame (ITRF) to ensure its continuity and stability.

Progress and Aspirations for Recommendations 5 and 6

An essential requirement for maintaining the global geodetic infrastructure is international collaboration, which is facilitated by the free and open exchange of raw data as well as synchronous observing schedules for VLBI and coordinated schedules for satellite tracking using SLR. This collaboration is achieved in part through the international geodetic services, which also enable the creation and maintenance of the ITRF. Both objectives were strongly endorsed by the NRC (2010) report.

All of the agencies that presented to the committee have firm commitments to the international geodetic services of the International Association of Geodesy (IAG), and many provide leadership and substantial institutional support. For example, all agencies with significant GNSS assets contribute raw GPS tracking data to public archives. Many also host web services and provide products. NASA JPL leads the IGS Central Bureau. NASA Goddard leads the Central Bureau of the International Laser Ranging Service (ILRS), contributes an analysis center, and operates eight legacy SLR stations (out of 39 ILRS stations). It also provides the coordinating center and an analysis center for the International VLBI Service for Geodesy and Astrometry (IVS), operates three legacy and two next-generation VLBI stations, and provides support for two partner legacy stations (out of 47 IVS stations). NOAA NGS provides the only current U.S. surveying team that measures high-accuracy local tie vectors at multi-technique co-location sites needed for ITRF.

U.S. agencies have been active participants and leaders in the Global Geodetic Observing System (GGOS) of the IAG. The primary role of GGOS is to promote the work of the IAG and the geodetic products generated by the IAG services.²

Concerns

None.

FEDERAL GEODETIC SERVICE

NRC (2010) Recommendation 7. The United States should establish a federal geodetic service to coordinate and

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² See http://www.ggos.org.

facilitate the modernization and long-term operation of the national and global precise geodetic infrastructure.

Progress and Aspirations

A federal geodetic service has not been established, and none of the presenting agencies identified it as a future objective.

Concerns

Research performed by one government agency (e.g., USGS) depends on networks funded by another agency (e.g., NSF or NASA), with no mechanism to guarantee continued operations. This poses risks to scientific and societal applications of geodesy (e.g., geologic hazards) because, for example, one agency may change or decommission a network that another agency relies on. The same holds true for software assets. In some cases, U.S. investigators are relying on software provided by international partners because no U.S. agency has agreed to support it. While NASA makes a strong commitment to the international GNSS geodetic infrastructure and the terrestrial reference frame, it relies on other agencies to densify it in the United States. In the absence of a federal geodetic service, an interagency forum would help identify and mitigate the risks.

GEODESY WORKFORCE

NRC (2010) Recommendation 8. A quantitative assessment of the workforce required to support precise geodesy in the United States and the research and education programs in place at U.S. universities should be undertaken as part of a follow-up study focused on the long-term prospects of geodesy and its applications.

Progress and Aspirations

A formal labor analysis of the geodetic workforce was commissioned by NGA and carried out in 2012 (see Box 2.1). However, data on the number of graduates from geodesy programs or the number of people working in geodesy-related occupations are not tracked by the federal government, and so quantitative estimates of the current and future geodetic workforce cannot be made. Anecdotal evidence points to a current and growing shortage of experts in geodetic techniques. Several agencies noted that their geodesists are aging. Because they are unable to find replacements with the needed skills, they need to provide on-the-job training in geodesy. NGA used to send some employees to universities for advanced training in geodesy, but is now training staff in house (NRC, 2013). NASA has a student fellowship program, but it primarily funds students who study science applications of geodesy, rather than those who improve geodetic techniques or models.

Concerns

The small and declining number of geodesists in the workforce poses risks for data analysis. For example, there are currently only two GPS data analysis software systems of the highest geodetic caliber in the United States: GipsyX (NASA JPL) and GAMIT

(Massachusetts Institute of Technology). Although GipsyX was designed to enable full GNSS data processing, the lack of multi-GNSS orbit and clock production by NASA JPL limits the value of this software for many scientific investigators. While research is ongoing, GAMIT is not currently capable of simultaneous multi-GNSS data processing, and ongoing support for its maintenance is unclear. Similar risks exist for VLBI and SLR, with too few software systems to assure robust data analysis.

SUMMARY

The United States continues to make a strong contribution to the international geodetic infrastructure with significant participation and leadership in international geodetic services. However, there are three areas of concern. First, the accuracy of the next-generation VLBI and SLR systems developed with NASA funding have not been validated with long-term, data-driven studies (as opposed to simulation) in the refereed literature. Second, few core or SLR stations have been added to complement and increase the density of the international geodetic network, especially in the southern hemisphere. Third, a unified, highly accurate, national GNSS observing system has not been developed that could both serve as the U.S. realization of and connection to the ITRF and support the scientific studies described in the next chapters. Although most of the networks operated by U.S. geodetic agencies have upgraded their GPS systems with multi-GNSS capabilities (or have clear plans to do so), plans for the software and associated products (orbits and clocks) and models (e.g., phase centers) needed for multi-GNSS data streams are not in place.

With an aging workforce and declining number of graduates trained in geodetic techniques and models, the United States is at risk of not being able to maintain a leadership role in geodesy or even 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.

REFERENCES