,. NOAA Technical Report NOS NGS 69 A Preliminary Investigation of the NGS's Geoid Monitoring Service (GeMS) Silver Spring, MD 2019 National Oceanic and Atmospheric Administration National Geodetic Survey Acknowledgements: A document like this requires the tireless work of many individuals at NGS. Recognition and thanks goes to the following individuals (in alphabetical order) for their contribution: Kevin Ahlgren, Vicki Childers, Theresa Damiani, Ryan Hardy, Jeff Kanney, Nicole Kinsman, Jordan Krcmaric, Daniel Roman, Dru Smith, Derek van Westrum, Daniel Winester, and Monica Youngman 1 Executive Summary For more than 200 years, the National Geodetic Survey (NGS) has defined, maintained, and provided access to the positioning framework for the United States of America and its Territories. For the majority of this 200 years, the datums that defined this positioning framework have been held fixed with respect to time. However, the Earth is not a static object, and we now have the scientific and observing capability to understand a vast number of physical changes occurring on Earth and incorporate them into a time-dependent datum. The NGS is modernizing the National Spatial Reference System (NSRS) with new reference frames and a geopotential datum in 2022. The modernized geopotential datum will be known as the North American- Pacific Geopotential Datum of 2022 (NAPGD2022). This datum will incorporate static and dynamic components to define—as accurately as possible—the nation’s elevations and other geopotential related quantities, such as the geoid undulation, acceleration of gravity, geopotential number, and deflection of the vertical. NGS is pioneering this type of science as it relates to regional reference frames and datums in alignment with NOAA’s Mission of Science, Service, and Stewardship. The time-dependent nature of NAPGD2022 is specified in the NGS Strategic Plan 2019-2023 (NGS, 2019a) under Objective 2-2: “Replace NAVD 88”, Define and provide access to a geocentric, time- dependent, geopotential datum by year 2022. This document describes the current state of knowledge and outlines next steps required to define a time-dependent geopotential datum for the Nation. This long-term goal has collectively been incorporated into an NGS project called “The Geoid Monitoring Service” or simply GeMS. Here, we present a foundational introduction to the various types of geophysical phenomena that are causing both size and shape change to the geoid, geodetic observing techniques that are presently available to monitor geoid change, an objective evaluation of NGS’s current ability to incorporate these techniques into a long term monitoring service like GeMS, known barriers to accomplishing such a project, and potential observing techniques that might become available in the next 10-20 years, but are not currently mature enough for operational use. The manner in which GeMS relates to existing and future NSRS components is also explained for additional context. Ultimately, this document presents a roadmap of options for how NGS could realize a time-dependent geopotential datum, and how NGS can support the dynamic datum into the future with independent validation surveys and datasets. Three different options are presented for constructing a GeMS model with the NGS Executive Steering Committee (ESC) recommending Option 2: GRACE/GRACE-FO Model + (Present day) Ice-Mass Model. Additionally, four separate and independent validation options are presented to determine the accuracy and quality of any GeMS model over many years. The ESC supports Validation Schemes 1, 2, and 3 pending resource availability in the future. The primary intent of this document is to assist NGS with initial development and support of GeMS, so feedback from external readers is encouraged in email form to [email protected]. Additionally, this document is not meant to be a comprehensive plan for building and supporting GeMS, but serves as a launch pad and overarching guide to what is possible now and into the future, as such, content within this document subject to change. 2 Table of Contents Acknowledgements:...................................................................................................................................... 1 Executive Summary ....................................................................................................................................... 2 1 Background ........................................................................................................................................... 6 1.1 Introduction .................................................................................................................................. 6 1.2 Goals of the Geoid Monitoring Service (GeMS) ............................................................................ 6 1.3 The importance of a static geoid model ....................................................................................... 8 1.4 GeMS and the National Spatial Reference System ....................................................................... 9 1.5 Spatial and Temporal Changes to the Geoid in North America and the U.S. Territories ........... 10 1.6 Methodology and Geodetic Constraints ..................................................................................... 13 1.7 Geoid Monitoring around the world ........................................................................................... 20 2 Available Geoid Monitoring Techniques ............................................................................................. 21 2.1 NGS’ Gravity Program ................................................................................................................. 22 2.1.1 Terrestrial Gravity ............................................................................................................... 22 2.1.2 U.S. Gravity Networks and Available Repeat Measurements ............................................. 28 2.1.3 Airborne Gravity .................................................................................................................. 35 2.2 The NOAA CORS Network (NCN) ................................................................................................. 40 2.2.1 NOAA CORS Network Background and Current Status ....................................................... 40 2.2.2 GGOS, IHRS, and IHRF ......................................................................................................... 44 2.3 NGS’s GPS and Leveling Campaign Capabilities .......................................................................... 44 2.3.1 International Great Lakes Datum (IGLD) ............................................................................. 44 2.3.2 Geoid Slope Validation Surveys (GSVS) ............................................................................... 45 3 Geoid Monitoring Techniques External to NGS .................................................................................. 46 3.1 Temporal Geopotential Models from Satellite Gravimetry ........................................................ 48 3.1.1 Global Spherical Harmonic Models from GRACE ................................................................ 48 3.1.2 Global Mass Concentration (Mascon) Models from GRACE ............................................... 52 3.2 Satellite InSAR ............................................................................................................................. 63 3.3 Geophysical Models and their Combinations ............................................................................. 66 3.3.1 Glacial Isostatic Adjustment Models for North America and Greenland ........................... 66 3.3.2 Ice Mass Models for Alaska, Northwest Canada, and Greenland ....................................... 69 3.3.3 Earthquakes ........................................................................................................................ 71 3.3.4 Volcanic Events ................................................................................................................... 72 3.3.5 Uplift and Subsidence of the Pacific and Caribbean Islands ............................................... 73 3 3.3.6 Global Hydrology Models .................................................................................................... 73 3.4 Other Terrestrial Gravity Programs in North America ................................................................ 75 3.4.1 NRCan Gravity Program ...................................................................................................... 75 3.4.2 Other Gravity Programs ...................................................................................................... 76 3.5 Other GNSS Networks (Non-NOAA CORS Networks) .................................................................. 77 3.6 Deflection of the Vertical (DoV) Measurements ........................................................................ 79 3.7 Tiltmeters .................................................................................................................................... 80 3.8 Satellite and Airborne Altimetry ................................................................................................. 81 4 Anticipated Future Techniques for Geoid Monitoring ....................................................................... 83 4.1 Future Satellite Gravity Missions ...............................................................................................