Innovation Insights with Richard Langley

IN THE BEGINNING of the space age, there was only one space-based positioning technique: satellite Doppler. Shortly after the launch of the first satellite, Sputnik 1, on Oct. 4, 1957, it was realized that by using a receiver to measure the Doppler frequency shift of a satellite’s transmitted signals combined with knowledge of the satellite’s orbit, the position of the receiver could be determined.

The United States Navy used this concept to develop the Navy Navigation Satellite System, commonly known as Transit. Although its initial use was for positioning Polaris submarines, it was released for commercial use in July 1967. Transit was used worldwide for positioning and navigation until it was decommissioned at the end of 1996. We talked about Transit in the introduction to the article “Easy Peasy, Lemon Squeezy: Satellite Navigation Using Doppler and Partial Pseudorange Measurements” in this column’s October 2012 edition.

Next on the scene was very long baseline interferometry (VLBI). This was, and still is, a technique for high-resolution mapping of galactic and extragalactic radio sources such as quasars. It was invented by Canadian and American radio astronomers with the Canadians getting the first interference “fringes” on a transcontinental baseline on May 21, 1967. VLBI uses radio telescopes, separated by 100s or 1,000s of kilometers, to record signals on storage media (previously magnetic tape and subsequently disk-based systems) synchronized by atomic clocks, typically hydrogen masers. The recordings are played back and cross-correlated at a central facility to produce the observation data – essentially the difference in arrival times of the radio signals at the radio telescopes. It was apparent that VLBI measurements could also be used to precisely determine the vector baselines between pairs of radio telescopes eventually down to a few millimeters, so VLBI became an important geodetic technique, even measuring the drift of the continents in essentially real time. We featured an article on VLBI in this column in February 1996, “The Synergy of VLBI and GPS.”

Around the same time that VLBI was being developed, satellite laser ranging (SLR) made its debut. SLR works by precisely measuring the two-way travel time of laser pulses sent from telescopes on Earth to arrays of corner-cube reflectors on specially equipped satellites. The first experiments were conducted with Beacon Explorer A in 1964. Initial results had a range accuracy of about three meters. Since then, more than 100 satellites have been launched with SLR reflectors, including the GLONASS, Galileo, BeiDou and Quasi-Zenith navigation satellites, the Indian regional satellites and a couple of GPS satellites with more to come. Ranging precisions are now as good as a few millimeters. Laser ranging is also conducted using reflector arrays on the surface of the moon. Back in September 1994, we had an SLR article in this column, “Laser Ranging to GPS Satellites with Centimeter Accuracy.”

Skipping over GNSS, with which most of us are very familiar, then came Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS). DORIS was developed in France by a group of institutions led by the Centre National d’Études Spatiales. Rather than transmitting signals from satellites and measuring the Doppler shift at receivers on the ground, the system transmits signals from a global network of ground-based beacons, which are picked up by receivers on specially equipped satellites and the data is subsequently downloaded to Earth. The first such equipped satellite was SPOT-2, launched in January 1990. Since then, 18 more satellites with DORIS receivers on board have been launched to date. DORIS, along with the other techniques, was discussed in the online GPS World article, “NASA Helps Maintain International Terrestrial Frame with GNSS,” published in February 2016.

Like the global navigation satellite systems with the International GNSS Service, the other techniques have their coordinated services, too: the International VLBI Service for Geodesy and Astrometry (IVS), the International Laser Ranging Service (ILRS), and the International DORIS Service (IDS).

All of these techniques and services contribute to the refinement of the International Terrestrial Reference Frame (ITRF), on which all positioning activities on Earth eventually depend. Tying the contributions from the different services together involves accounting for any systematic differences, which are reduced in part by using positional data at collocated sites where two or more techniques are sited with the vector ties between the instruments carefully measured. The September 1996 edition of “Innovation” was on the IERS and was aptly titled “International Terrestrial Reference Frame.”

The ITRF will enter a new era with the European Space Agency’s Genesis mission. The mission’s satellite will carry instruments for all four space-geodetic techniques: GNSS, VLBI, SLR and DORIS. In this quarter’s “Innovation” column, a team of Genesis mission engineers and scientists introduce the mission, describe its components and outline its benefits. My well-thumbed copy of the Concise Oxford Dictionary of Current English has two definitions for the word “genesis.” The first, with a capital “G,” is the title of the first book of the Old Testament with its well-known first verse. The second is “Origin, mode of formation or generation” and comes from the Greek word genēs, meaning birth, born or produced. It is clearly a fitting name for ESA’s new mission.

<p>The post Innovation Insights: A history of techniques and services that contributed to the refinement of the ITRF first appeared on GPS World.</p>

An artist’s impression of a Moon exploration scenario. (Image: ESA)

As there are several missions to the moon planned within the next decade, space agencies have started to consider how to keep time on the moon. To address time concerns, the LunaNet architecture, designed for lunar communications and navigation services, was introduced at the ESTEC technology center of the European Space Agency (ESA) in the Netherlands in November 2022.

“LunaNet is a framework of mutually agreed-upon standards, protocols and interface requirements allowing future lunar missions to work together, conceptually similar to what we did on Earth for joint use of GPS and Galileo,” said Javier Ventura-Trav

eset, ESA’s Moonlight navigation manager, coordinating ESA contributions to LunaNet. “Now, in the lunar context, we have the opportunity to agree on our interoperability approach from the very beginning, before the systems are actually implemented.”

“During this meeting at ESTEC, we agreed on the importance and urgency of defining a common lunar reference time, which is internationally accepted and towards which all lunar system and users may refer,” said Pietro Giordano, ESA navigation system engineer. “A joint international effort is now being launched towards achieving this.”

Each mission to the moon has operated on its own timescale from Earth. Deep space antennas have been used to keep onboard chronometers synchronized with terrestrial time at the same time to facilitate two-way communications. ESA stated that this way of working will not be sustainable in the coming lunar environment.

Time to think about time

Should a single organization be responsible for setting and maintaining lunar time? Also, should lunar time be set on an independent basis on the moon or kept synchronized with Earth? And what about time on other planets?

“Of course, the agreed time system will also have to be practical for astronauts,” said Bernhard Hufenbach, a member of the Moonlight Management Team from ESA’s Directorate of Human and Robotic Exploration. “This will be quite a challenge on a planetary surface where in the equatorial region each day is 29.5 Earth days long, including freezing fortnight-long lunar nights, with the whole of Earth just a small blue circle in the dark sky. However, having established a working time system for the moon, we can go on to do the same for other planetary destinations.”

To efficiently collaborate, the international community will have to settle on a common “selenocentric reference frame,” similar to the role played on Earth by the International Terrestrial Reference Frame, allowing the consistent measurement of precise distances between points across the planet.

<p>The post Be there at noon moon time: ESA is researching how to tell time on the moon first appeared on GPS World.</p>

On Jan. 12, NGS held a webinar titled “Using RTN Data in OPUS Projects 5 for GPSonBM.” Users can download the video and PowerPoint slides here.

I’ve been highlighting NGS’s GPS on Bench Mark program that supports the 2022 Transformation Tool in my columns since 2018. NGS delayed the completion date for the new modernized NSRS until 2025, so they have extended the cut-off date for submitting GPS on Bench Mark data for use in the 2022 Transformation Tool until Sept. 30.

NGS GPS on BenchMarks Program (Image: NGS website)

NGS has been developing tools that facilitate submitting data to the NGS GPS on BM campaign such as OPUS Share. The latest tool is the OPUS Project 5.1 routine that allows the use of RTN vectors. OPUS Projects 5.1 is a beta product, but NGS is now allowing users to use the routine to submit data for the GPS on BM campaign. My October 2021 column highlighted NGS’s Beta OPUS Projects 5.1.

The 2023 requirements for using OPUS Projects in the GPS on BM program (Image: NGS website)

I’d like to note that OPUS has been updated to support the newly released ITRF2020 (IGS20) orbits. My October 2022column discussed the latest International Terrestrial Reference Frame of 2020 (ITRF2020) released by the International Earth Rotation and Reference System Service (IERS). A previous NGS news bulletin provided a statement about the new reference system and products.

Excerpt from NGS News Bulletin (Image: NGS website)

Clicking on the link titled “NEW: 2023 Requirements for Use in the GPSonBM Campaign” on the OPUS Projects 5.1 webpage provides the requirements for using OPUS Projects 5.1 and Real-Time Network (RTN) data to support the 2022 Transformation Tool; that is the 2023 GPS on BM campaign. There are five sections in the writeup: Introduction, Project Planning, Equipment and Configuration, Field Requirements and Office Requirements. The Introduction section states that the requirements are limited to the GPS on BM Campaign and will be replaced, or superseded, when NGS finishes its new GNSS surveying specifications.

Introduction Section from Requirement Write Up (Image: NGS website)

The project planning section of the announcement states that RTN vectors of 5-minute occupations can be used instead of the 4-hour occupations required for OPUS Share.

Project Planning Section from Requirement Write Up (Image: NGS website)

However, the Field Requirement section states that the mark must be occupied three different times.

“During the RTN survey, measure each mark in your project (including the RTN Validation Station) for a minimum of 5 minutes for three independent occupations. All three measurements must agree by 3 cm horizontal and 5 cm ellipsoid height. They also must be separated by at least 3 hours (even if occupied on different days). Plan to occupy a mark, go occupy a few more in the area, then circle back. Or rotate day-by-day,” the section states.

Field requirements Section from Requirement Write Up (Image: NGS website)

As stated in the section on office requirements for using OPUS-Projects 5 in the 2023 GPS on BM Campaign writeup,“The OPUS-Projects User Guide provides instructions on how to run the software and submit a project to NGS. The User Guide states to follow the steps in the order listed below, and it explains steps 1 – 7 and 9 – 11 in detail. For step 8 and when including GVX data in OPUS-Projects 5, refer to those portions of the User Guide’s Quick Start which are highlighted in yellow. NGS is working on fully updating the User Guide to include more details; for now, use the Quick Start Guide for assistance with GVX.”

OPUS Projects User Guide (Image: NGS website)

Quick start guide. (Image: NGS website)

I recently used OPUS Projects to analyze some GNSS results using Harris-Galveston Subsidence District CORS and PAMS GNSS data. I want to emphasize that it may seem like a lot of work the first time you use the routine, but NGS makes it fairly simple to complete each task. The manual is very complete and does a good job of describing every step. The manual can be downloaded here. In my experience, the most time-consuming task is creating the descriptions. There are several items that must be correctly entered because the answer to some entries affect the answers to other entries. That said, NGS supports a description entry software called WinDesc that facilitates entering the appropriate information. The OPUS Projects User Guide provides an appendix that describes using the WinDesc module to enter description metadata.

For marks that are in the NGS database, known as the NGS Integrated Data Base (NGSIDB), WinDesc will import information from NGSIDB, thereby decreasing the number of entries users need to address. In other words, if the mark has a PID then it should be in the NGSIDB. If you are occupying a mark that is part of NGS GPS on Bench Marks website then it probably has a PID and a description in NGSIDB.

Example of PID from Mark Priority List (Image: NGS website)

I’ve included three slides from the Jan. 12 webinar that summarize the basic requirements.

This slide is a depiction of how a CORS station must be connected to the RTN vectors. (Image: NGS website)

This slide provides the occupation and precision requirements. (Image: NGS website)

This slide provides a list of the required metadata for the project. (Image: NGS website)

As for the requirement of at least three independent RTN occupations on different times, in my opinion at least one occupation should be on a different day. My October 2021 column addressed a study that reported on using RTN solutions to estimate accurate horizontal and vertical coordinates.

The report stated, “When differenced with coordinates from a static GNSS survey campaign, the horizontal and vertical RMSE of the NRTK-derived coordinates was 2.3 cm horizontally and 4.5 cm vertically at 95% confidence. Repetitive NRTK vectors on each baseline differed between ± 2.4 cm horizontally and ± 3.4 cm vertically at 95% confidence.”

The report also stated, “Adjustment of hybrid survey networks with four repeat NRTK vectors per bench mark produced network accuracies at 95% confidence for the adjusted coordinates at all bench marks less than 1 cm horizontally and 2 cm vertically (ellipsoid height).”

The requirements are limited to the GPS on BM Campaign and will be replaced, or superseded, when NGS finishes its new GNSS surveying specifications.

(Image: Screenshot of Accuracy of GNSS Observation from Tree Real-Time Networks in Maryland, USA)

The paper by Gillins, et. al was presented at the 2019 FIG Working Week held in Hanoi, Vietnam, on April 22–26, 2019. The International Federation of Surveyors (FIG), involves a wide range of professional fields within the international surveying community; this includes surveying, cadastre, valuation, mapping, geodesy, hydrography, and geospatial and provides an international forum for discussion and development to promote professional practice and standards. FIG meetings are held all over the world. I’d like to highlight that the 2023 FIG Working Week is going to be held in Orlando, Florida, on May 28 – June 1, 2023.

NGS will be presenting a full-day worth of content on NSRS Modernization during the FIG Working Week 2023. For the first time in more than 20 years, this annual FIG gathering will take place in the United States, hosted by the National Society of Professional Surveyors (NSPS).

I’ve participated in several FIG meetings. I’ve learned a lot from presentations as well as holding hallway meetings with experts from the international surveying and mapping community. All geospatial users should plan on attending this event. I have provided information about the FIG commissions in my August 2021 newsletter. I would encourage everyone to visit the FIG website and review the information about the 2023 FIG Working Week. The a list of the FIG Commissions can be found here. More information can be obtained on each commission by clicking on its title.

Future columns will highlight the FIG Working Week as the agenda is developed. I would encourage everyone to check NGS’s Website for updates on Beta products and new surveying specifications. Geospatial users should also subscribe to NGS’s News Services at the following here. Check out the NGS News Services site for what’s available.

<p>The post New feature in OPUS Projects: Using RTN vectors to support 2022 Transformation tool first appeared on GPS World.</p>

I first mentioned this in my July 2020 article for the “First Fix” column of GPS World, where I stated that the shortage of American trained geodesists poses a significant economic risk for the United States. In that column, I mentioned how geodetic science and technology now underpin many sciences, large areas of engineering (such as driverless vehicles and drones), navigation, precision agriculture, smart cities and location-based services. That is why I believe understanding geodesy is more critical today than ever. In January 2022, Mike Bevis, collaborating with others, prepared a white paper titled “The Geodesy Crisis,” documenting the concern about the lack of trained geodesists in the United States.

Image: Dana Caccamise II

“The inverted geospatial pyramid” graphic depicts how the entire $1 trillion geospatial economy is supported and dependent on geodesy, and how it’s close to collapsing without an increase of support for geodesy. A lack of geodetic expertise in the United States presents a significant challenge, with future impacts on positioning, navigation, mapping and dependent geospatial technologies.

In my opinion, without investment in geodesy, the United States will not have the available skills and knowledge to develop new geodetic technologies and improve models to address challenges to society, such as

• how the Earth’s surface is changing as sea level rises and the Earth’s glaciers and ice sheets change on timescales of months
• how the tectonic plates are deforming and what physical processes control earthquakes, and
• the ability to monitor the temporal changes in Earth’s water reservoirs by measuring changes in Earth’s gravitational field as it responds to the moving water mass and the deformation of the solid Earth caused by moving water.

These challenges need a well-maintained, stable terrestrial reference frame (TRF) with sub-1 mm/year vertical accuracy. Errors in TRF heights can propagate systematically into estimates of atmospheric water vapor, sea level, satellite orbits and other parameters. An accurate TRF can lead to important observations and discoveries because it enables revelations from coherent global motions. (My previous column described the latest International Reference Frame of 2020 [ITRF2020].)

Geodesy has been a significant part of my life for 50 years. I’ve seen a lot, and unless we address the Geodesy Crisis, the innovations in geodetic science of the past will not continue in the future. At least not in the United States.

The Geodesy Crisis paper was mentioned in the Fall 2022 ION Quarterly Newsletter by Everett Hinkley (see the box below). Hinkley noted, “The geospatial community relies on geodesists, though few in the community are fully aware of this connection nor understand the importance of geodesy to their work.” I encourage everyone to download the white paper and the ION Quarterly Newsletter to understand the importance of the need for more trained geodesists.

Excerpt from Everett Hinkley’s Article

“In January 2022, a white paper entitled America’s loss of capacity and international competitiveness in geodesy, the economic and military implications, and some modes of corrective action was released (Bevis et al.). This collaborative paper paints an alarming picture of the dwindling pool of trained geodesists within the United States. The report highlights America’s loss of capacity and international competitiveness in geodesy and states: ‘The U.S. is on the verge of being permanently eclipsed in geodesy and the downstream geospatial technologies. This decline in capability threatens our national security and poses major risks to an economy strongly tied to the geospatial revolution, on Earth and, eventually, in space.’ Though the word crisis correctly describes the dire predicament well, it didn’t occur overnight. Due to several converging trends, the geodesy crisis has been decades in the making. A national lack of geodetic expertise presents a significant challenge with downstream impacts on positioning, navigation, mapping, and dependent geospatial technologies. The Department of Defense, intelligence community, and federal civil agencies’ mapping entities rely on accurate and precise maps for a broad range of purposes, and reliable maps depend on an accurate geodetic underpinning. The geospatial community relies on geodesists, though few in the community are fully aware of this connection nor understand the importance of geodesy to their work.” (Reproduced with permission from ION.)

In my “First Fix” column, I mentioned that I attended The Ohio State University (OSU) to obtain my graduate degree in Geodetic Science in 1979. Therefore, I admitted that I am a little biased — once a geodesist, always a geodesist. That said, in OSU’s geodesy heyday (1960–1990s), many Americans trained were sent there by federal agencies: National Geospatial-Intelligence Agency (NGA), NOAA/National Geodetic Survey (NGS), USGS, Army, Navy and Air Force. During the 1970s, NGS sent two employees back to school every year. These agencies needed geodesists because they were undertaking significant projects, such as the NGS projects to readjust the U.S. national horizontal (NAD83) and vertical geodetic (NAVD88) networks. I was one of the employees NGS sent to OSU to be trained to support the NAD83 and NAVD88.

Today, the environment is different. U.S. federal agencies still need geodesists to develop enhanced and refined geodetic models and tools. However, major U.S. companies, such as Google and FedEx, the automobile industry, the construction industry (automated machine guidance), precision farming companies and mining companies also need more accurate geodetic models, tools and algorithms. Therefore, these companies also need trained geodesists to perform essential research on topics that address their geodetic requirements. As indicated in “the inverted geospatial pyramid” graphic, the entire $1 trillion geospatial economy is supported by geodesy.

As implied in Hinkley’s article, geodesy has played a role in developing geospatial products but most users didn’t realize that it was their foundation. Since it’s been in the background, everyone assumes it will always be there. A participant at one of my workshops stated that “GPS has made geodesists out of all of us.” In my opinion, the advancements in GNSS equipment and processing software provided some users with a “false sense of knowledge or security” that they understood what was happening within the “black box.” One of my colleagues at NGS said that the new equipment and software programs were creating a field force of “buttonologists.”

These statements concerned me at the time and concern me today. With the last generation of trained geodesists either retired or getting ready to retire, we are at a critical stage of not being able to meet the geospatial needs of the future. As indicated in the white paper, there are significant challenges in rebuilding programs that support the training of geodesists.

Hinkley’s article summarized several action items that could help improve the lack of trained geodesists in the United States. I’ve provided his list in the box below. I’ve highlighted several items the surveying and mapping community can help achieve.

So how do we build and educate the next generation of geodesists?

• Make the White House and Congress aware of this crisis, particularly its national security implications; seek direct support in the federal budget to correct this issue. It has become clear that, without engagement at the highest echelons of the U.S. government, averting this current crisis and its eventual outcome is unlikely.
• Teach rigorous math in our public schools; follow the scholastic math approach used in many Asian and European countries.
• Encourage creative thinking!
• Actively market geodesy in high schools as a rewarding career for the math stars before college entry.
• Build back, support and sponsor geodesy programs at select universities. This support needs to be strategic, with backing from the highest levels of the U.S. government.
• Break our cultural trend of reactions to crises and seize the opportunity to be proactive and prevent the foreseen consequences of this crisis.
• Encourage U.S. government support in the form of grants, professional development of staff, and research collaborations/affiliations. There are early efforts underway to bring new talent into the pipeline:
  • the National Geospatial-Intelligence Agency (NGA) is forming an emerging scientist consortium (ESCON) with partnerships that exist with Ohio State, UT-Austin, and other industry/academic/government partners
  • a pilot Ph.D. geodesy educational program with three NGA and one NGS employee is in place; the NGA expects to continue growing this program.
  • the NGA’s new western headquarters in St. Louis will bring 350 companies and organizations into the regional GEOINT ecosystem.

If we answer this call to action collectively, there is hope that a new cadre of U.S. geodesists can be cultivated before it’s too late to recover.

(Reproduced with permission from ION.)

With all that said about the need for more geodesists, one thing that this ASCE publication may do is make some readers realize how much they don’t know about the roots of the technology that they’re using to create geospatial products and services. This knowledge gap is not just correctly using GNSS and other geospatial technology to perform a survey, but also integrating various instruments to create an accurate mapping system, such as mobile mapping and terrestrial laser systems. My intent is not to criticize the expertise or knowledge of anyone, and I only mean to point out that in today’s use of computers and programs, many technical concepts are hidden in “black boxes.” I learned many things about some topics by reviewing this book.

The book is 556 pages and has 15 chapters. As part of my responsibilities as a Blue-Ribbon Panel member, I read every word in the book, and not many people will read the entire book. Still, I would encourage surveyors, engineers, geodesists, photogrammetrists and GIS and remote-sensing practitioners to obtain a copy of the book for reference and to understand the limitations of geospatial technology.

Surveying and Geomatics Engineering: Principles, Technologies, and Applications
Edited by Daniel T. Gillins, Ph.D., P.L.S.; Michael L. Dennis, Ph.D., P.E., P.L.S.; and Allan Y. Ng, P.L.S.

Now to the book’s content. I want to highlight that the forward is written by Juliana Blackwell, director of the National Geodetic Survey (NGS). She states that “A common thread running through the manual is the importance of the National Spatial Reference System (NSRS) to modern geospatial applications.”

Most of my columns highlight something relevant to the NSRS. That’s because the NSRS is the foundation layer for United States federal geospatial products, and geodesy provides the foundation for all geospatial products and services as indicated in the “The inverted geospatial pyramid” figure.

I would also like to highlight a statement by Gene Roe in the preface. He states, “Because entire books could be devoted to each of these topics, this manual only provides a summary, and it points the readers to important references where they can find more details. The manual is meant to provide a comprehensive but general overview to help support education and inform practicing engineers on the important role of the surveying engineer. It is too important for this not to occur.”

I agree with Roe’s statement that the book is important for surveying engineers. Still, I would add that this book is important to anyone working with GNSS and other geospatial data, especially geodesists, surveyors and GIS practitioners.

This publication is edited by three individuals that are licensed surveyors; two of them are geodesists who work for NGS. These individuals have performed a fantastic job of ensuring that all chapters have been reviewed for correctness and that the information provided is current and essential for users of geospatial data.

Readers can download copies of the book and specific chapters here. You can buy it as an e-book or in print. The “Abstract” box summarizes the book from the ASCE Library website.

Abstract

Sponsored by the Surveying Committee of the Surveying and Geomatics Division of the Utility Engineering and Surveying Institute of ASCE and the National Geodetic Survey of the US National Oceanic and Atmospheric Administration

Surveying and Geomatics Engineering: Principles, Technologies, and Applications, MOP 152, is a comprehensive yet general overview to help support education and inform practicing engineers on the important role of the surveying engineer. It provides a much-needed update on the modern practice of surveying and geomatics engineering.

Topics include:

• geodesy
• coordinate systems and transformations
• least squares adjustments and error propagation
• modern surveying and remote sensing technology
• analysis and establishment of control
• geographic and building information systems
• construction surveying, and
• best practices.

MOP 152 can be used as a summary and a reference for practicing engineers, surveying and otherwise, to help provide a solid understanding of the state of the surveying and geomatics engineering field.

Below is a list of the chapters and their authors. This column cannot highlight everything important in this book, but I will select a few items to which I believe users of geospatial data should pay attention.

Chapter Titles

As a geodesist, I usually focus on topics relevant to geodetic science. This book has a lot of topics that use geodesy concepts to create an engineering product or service. For example, chapter 2, “Geodesy and Geodetic Computation” by Earl Burkholder, provides a good summary of geodetic concepts that anyone using or generating geospatial products should know and understand. It gives basic equations without lengthy derivations of how they were developed.

In my opinion, chapter 3, “Map Projections and Local Coordinates Systems” by Michael Dennis, does the best job of explaining the concepts of map projections that are relevant to the surveying and mapping community. Many GIS practitioners use map projections in their software but don’t have a working knowledge of what’s happening to their original data. This chapter describes the current United States State Plane Coordinate System of 1983 (SPCS83) and the future State Plane Coordinate System of 2022 (SPCS2022) that is scheduled to be adopted in 2025. Dennis uses figures and diagrams to describe map projections, angular and linear distortion, and methods for reducing map projection distortion to make it easier for readers to understand the concepts. One section of interest to many surveyors after SPCS2022 is adopted is the Low-Distortion Projection (LDP) Coordinate Systems section. This is useful because, in SPCS2022, many states have designed LDP systems for their state’s SPCS2022. The box below provides a diagram with the number of zones for each state.

Image: NGS Presentations Webpage “Grids for the Future: A New Approach for Designing State Plane Coordinate System Zones” by Michael Dennis.

One purpose of an LDP is to reduce linear distortion; it is not a new concept. Many surveyors have performed a simplified form of it for decades. It’s known by many as a “modified” or “scaled” State Plane. The American Congress on Surveying and Mapping (ACSM) taught a workshop for decades describing how to compute a “modified” State Plane Coordinate. I was an instructor of this class in the 1980s and 1990s. “Modified” State Plane Coordinates had several issues, but they worked reasonably well in small areal extents. Today, with the advancements in computers and computer software, there are better ways to accomplish an LDP. Dennis’ section does a great job explaining the new SPCS2022 and the design of LDPs in the SPCS2022. The use-case examples provide a simplified description of understanding the linear distortion behavior in an area.

Chapter 4, “Local, Regional, and Global Coordinate Transformation” by Michael Dennis, is one that every surveyor and GIS practitioner should read. Dennis highlighted the differences between “equation-based” transformations and “grid-based” transformations, as well as combined equation-based transformations with grid-based transformations. Understanding the information provided in chapter 4 will be important when NGS replaces the NAD 83 (2011) and NAVD 88 datums with the new, modernized NSRS in 2025. NGS will provide models and tools for users to perform coordinate transformations, but hopefully, some users will want to understand what’s happening behind the scenes.

Chapters 8 and 9 discuss laser scanning systems. In chapter 8, “Terrestrial Laser Scanning,” the “Data Quality Considerations” section highlights common artifacts or limitations encountered with terrestrial lidar system data. The authors provide many examples of these artifacts, making the concept easy to understand. At the end of this chapter, there are 14 pages of references that will be very helpful to users involved with terrestrial laser scanning systems.

Chapter 9, “Mobile Terrestrial Laser Scanning and Mapping,” is very informative, especially the section on georeferencing. This section is not just the description of properly using GNSS to perform a survey, but also the integration of various instruments to create an accurate mobile mapping system. I like how the authors discussed the error sources in georeferencing the system, listed the source, and provided an explanation of the error.

Anyone performing a GNSS survey project that meets NGS’s requirements needs to read chapter 11. I like the section describing how users should evaluate CORSs before using them as control. Evaluating CORS is something all users should do before using any CORS in their project, because not all CORS are created equal. See the excerpt from chapter 11 below for the recommended steps from the author.

Excerpt from Chapter 11 – Steps for Evaluation of CORS

The author recommends the following steps:
1. Choose stations that are within 100-300 km of a project site. It is well known that errors in GNSS baseline processing are directly correlated with baseline length (Chapter 6). Tropospheric delay is reduced when baselines are shorter and atmospheric conditions at each end of the line are similar. In addition, mutual satellite visibility at each end of the line for differencing diminishes as baselines grow longer. That said, errors in GNSS processing are more occupation time-dependent than baseline length-dependent (Eckl et al. 2001). Therefore, for short GNSS sessions (i.e., < 2 hours), choose CORS within approximately 100 km as control; for moderate GNSS sessions (i.e., 2 to 8 h), choose CORS within approximately 300 km. Note that even longer baselines can be successfully processed when GNSS sessions are very long in duration (e.g., up to 2,000 km for 24 h sessions).

2. Determine if GNSS data are available at a given CORS during the time of your survey. Of course, if data are unavailable, then the station simply cannot be used as control. NGS provides a tool known as “User Friendly CORS (UFCORS)” for entering a date and time range to view available data at a given station (NGS 2021c). This tool can also be used to download the raw GNSS data for processing and adding a station to the survey network.

3. As discussed previously and when possible, choose a CORS with computed velocities rather than modeled velocities from HTDP. NGS provides tables of official coordinates with “computed” versus “htdp” coordinates and velocities on the website for CORS.

4. Review the aforementioned short-term time-series plot for the station, ideally at the time of the project. Stations with large spikes, data gaps, bias from the published “red” line, or large standard deviations should be avoided. A good rule-of-thumb is for the RMS in the short-term time-series plot (Figure 11-2) to be less than 1.0 cm in north and east and 2.0 cm in the up direction in a local geodetic horizon frame at the station.

5. Examine the formal uncertainties for the official coordinates of the CORS. Standard deviations in north, east, and up are provided on the station’s datasheet, accessible from the webpage for the CORS (more on datasheets are discussed in the following under Passive Control). Stations with unusually large standard deviations (> 3 cm) should be avoided. Note that standard deviations are not available for CORSs with modeled velocities.

I believe that the evaluation of NOAA CORS is critical, so I’ve described Dan Gillins’ “Steps for Evaluation of CORS” below. First, users can access the NOAA CORS using the NGS CORS Map utility. After the map appears, users can move the cursor over the center of the project area, where it provides the location of the cursor and the three closest CORS. Users can click on a CORS icon and get coordinates and other information about the CORS. Also, they can place an X on the map, and the utility will draw a 250-km circle around the point. The box in the lower left-hand side of the map provides a list of the sites within 250 km of the marked location.

Using CORS Map to Identify CORS

Image: https://geodesy.noaa.gov/CORS_Map/

Users can download the NOAA CORS coordinates and velocities (computed and modeled). I downloaded the files and plotted three circles (with radii of 100, 200, and 300 km) around CORS NC77 in Charlotte, North Carolina. I only plotted CORS that are operational and have computed velocities. North Carolina has a lot of CORS to select from. In contrast, I’ve plotted three circles (also with radii of 100, 200 and 300 km) around CORS WYRF in Casper, Wyoming.

Buffer Zones around Charlotte, NC

Image: Dave Zilkoski

The plot depicting the buffer zones around Casper indicates that there are no CORS within the 100-km circle and only a few between 100 and 200 km.

Buffer Zones around Casper

Image: Dave Zilkoski

The data availability of the CORS site can be obtained by clicking on the CORS icon, selecting “Get Site Information,” and then selecting “Data Availability.”

Data Availability at CORS NC77

Image: https://geodesy.noaa.gov/cgi-cors/corsage_2.prl?site=nc77

The position and velocity for the CORS can be obtained by clicking on the Coordinates button on that CORS webpage.

Position and Velocity Sheet for CORS NC77

Image: https://www.ngs.noaa.gov/cgi-cors/CorsSidebarSelect.prl?site=nc77&option=Coordinates14

The CORS Short- and Long-Term plots can be obtained by clicking on the Time Series button on that CORS webpage.

Short-Term Plot of CORS NC77

Image: https://www.ngs.noaa.gov/cgi-cors/CorsSidebarSelect.prl?site=nc77&option=Time%20Series%20(short-term)

The Datasheet for the CORS can be obtained by clicking on the Coordinates button and then on the Datasheet button on that CORS webpage.

Datasheet for CORS NC77

Image: https://www.ngs.noaa.gov/cgi-bin/ds_cors.prl?CorsSelected=|NC77&CorsTypeSelected=Arp

There are too many chapters to describe each one, but I encourage users to check each chapter’s abstract on the ASCE website and decide which ones would be the most beneficial to them (see the box titled “Abstract for Chapter 11 Survey Control”). The manual provides numerous references and can serve as a helpful resource for finding further details on the fields of geodesy and surveying.

Abstract for Chapter 11 Survey Control

Image: https://ascelibrary.org/doi/10.1061/9780784416037.ch11

A goal of mine is for some readers of this column to obtain enough knowledge to “whet their appetite” and encourage them to pursue an education in geodesy and surveying. Others who are influential in federal government programs and those responsible for geospatial research for industries will recognize the need for more trained geodesists in the United States and help by doing the following:

• actively market geodesy in high schools as a rewarding career for the math stars before college entry
• build back, support, and sponsor geodesy programs at select universities; this support needs to be strategic with backing from the highest levels of the U.S. government
• encourage U.S. government support in the form of grants, professional development of staff, and research collaborations/affiliations.

<p>The post The inverted geospatial pyramid shows our vulnerability first appeared on GPS World.</p>

The last multi-year CORS solution, MYCS2, was performed by NGS in 2019. I discussed the MYCS2 in my February 2019 and April 2019 columns. This new multi-year CORS solution will be important to the 2022 modernized National Spatial Reference System (NSRS), because NGS will establish a strict mathematical relationship between the 2022 NSRS frames and the ITRF2020 frame. This will allow direct access to the NSRS (NOAA Technical Report NOS NGS 67).

NGS Aligns National System to Global Reference Frame

August 5, 2022

The International Global Navigation Satellite System (GNSS) Service, which provides GNSS data products globally, recently released a new GNSS-only version of the International Terrestrial Reference Frame. This provides GNSS users access to the reference frame through coordinate functions for a global set of reference stations. In response, NGS will soon compute the multi-year Continuously Operating Reference Station (CORS) Solution 3, which will modernize the National Spatial Reference System. Aligning the National Spatial Reference System with the updated global reference frame will allow greater access for the global community of scientists, educators, and commercial users of location science.

For more information, contact: Phillip McFarland

As in the past, the multi-year CORS solution will mean that the NOAA CORS coordinates will be updated to be consistent with the latest International Terrestrial Reference Frame of 2020 (ITRF2020). The International GNSS Service provides information about its GNSS products and services. Readers can find information on the latest International Terrestrial Reference Frame 2020 here. This column will provide basic information on the ITRF2020. Please note: NGS stated that it will soon start computing the third multi-year CORS solution, but — as of October — all NOAA CORS coordinates are still based on MYCS2 and provide coordinates in ITRF2014 epoch 2010.00 and NAD 83 (2011, MA11, PA11) epoch 2010.00. As in the past, NGS will provide advance notice before publishing the results of its third multi-year CORS solution.

A document on the ITRF website stated the ITRF2020 is expected to be an improved solution compared to the previous solution, ITRF2014. It listed several innovations introduced in the ITRF2020 processing.

Description from ITRF2020 Document

ITRF2020 is the new realization of the International Terrestrial Reference System. Following the procedure already used for previous ITRF solutions, the ITRF2020 uses as input data time series of station positions and Earth Orientation Parameters (EOPs) provided by the Technique Centers of the four space geodetic techniques (VLBI, SLR, GNSS and DORIS), as well as local ties at colocation sites. Based on completely reprocessed solutions of the four techniques, the ITRF2020 is expected to be an improved solution compared to ITF2014. A number of innovations were introduced in the ITRF2020 processing, including:

• The time series of the four techniques were stacked all together, adding local ties and equating station velocities and seasonal signals at colocation sites;
• Annual and semi-annual terms were estimated for stations of the 4 techniques with sufficient time spans;
• Post-Seismic Deformation (PSD) models for stations subject to major earthquakes were determined by fitting GNSS/IGS data. The PSD models were then applied to the 3 other technique time series at earthquake colocation sites.

The box below provides a good summary of the International Reference Frame and why it’s important to the scientific community as well as the surveying and mapping community. Readers can download the article from the June 2022 International GNSS Service Issue 4 newsletter. Users also can sign up to receive notices and newsletters from the International GNSS Service.

ITRF2020: A new release of the International Terrestrial Reference Frame By Zuheir Altamimi

What is the current rate of sea level rise in different regions of the globe? How does our Earth deform under the effect of plate tectonics, seismic phenomena, or the melting of ice caps? How the Earth’s center of mass is varying? How to determine the position of a point on the surface of a constantly deforming Earth and compare it to positions estimated decades apart? The answers to these fundamental questions for understanding the dynamics of our planet require the availability of a global, long-term stable terrestrial reference frame, but preferably a standard reference so to ensure interoperability and consistency of various measurements collected by sensors on the ground, or via artificial satellites. The International Terrestrial Reference Frame (ITRF) is the standard reference recommended by a number of international scientific organizations, including the International Union of Geodesy and Geophysics (IUGG) and the International Association of Geodesy (IAG) for earth science, satellite navigation and operational geodesy applications. The ITRF is an international effort that is built on the investments of space and mapping agencies, universities and research groups in operating geodetic observatories, archiving and analyzing the collected geodetic observations to derive not only the ITRF, but also critical geodetic products for science and society.

The ITRF integrates and unifies technique-specific reference frames provided by the four IAG’s international services of space geodetic technique (DORIS/IDS, GNSS/IGS, SLR/ILRS, VLBI/ IVS). It is supplied to the users in the form of temporal coordinates of more than 1500 stations, Earth Orientation Parameters, as well as parametric functions describing nonlinear station motions: seasonal signals due to mainly loading effects and post-seismic deformations for sites subject to major earthquakes. It is necessary to regularly update the ITRF (approximately every 5 years) in order to benefit from continuous observations so to improve its accuracy, considering station position temporal variations due to geophysical phenomena.

The ITRF is maintained by a research group at IGN-France and IPGP (Institut de Physique de Globe de Paris), and whose new release called ITRF2020 was published on April 15 and accessible here: https://itrf.ign.fr/en/solutions/ITRF2020. The ITRF2020 brings significant improvements compared to previous achievements: it confirms the estimate of the position of the center of mass of the Earth as it was determined in 2016, but also provides its seasonal variations; it improves the accuracy of the scale of the frame at the millimeter level, which represents a gain in precision of a factor of 8 on the measurement of the size of the Earth (compared to that determined in 2016); it provides a precise quantification of co- and post-seismic displacements caused by devastating earthquakes, such as that of Sumatra in 2004, Chile in 2010 and Japan in 2011. The IAG Services rely on the ITRF to align their geodetic products to it, and therefore disseminate it widely among the various users. In particular, using the IGS products, such as the orbits, allows a universal access in space and time to the ITRF.

As stated in the article by Zuheir Altamimi, ITRF2020 involves IAG’s international services of four space geodetic techniques: DORIS/IDS, GNSS/IGS, SLR/ILRS, VLBI/ IVS. Computing an International Terrestrial Frame is very complex and requires analyses of difference types of geodetic and geophysical data. It is beyond the scope of this column, but online is more detailed technical information.

For this column, I downloaded the station lists from the four space geodetic techniques and provided a few plots that depict the location and velocities of these sites. The box below depicts the location of the space geodetic techniques around the world. As indicated in the plot, some locations have more than one technique collocated at the same site.

Plot of the Four Different Space Geodetic Techniques

Image: Dave Zilkoski

The following plots depict the locations using each space geodetic techniques: GNSS sites, DORIS sites, SLR sites and VLBI sites.

Plot of GNSS Sites

Image: Dave Zilkoski

Plot of DORIS Sites

Image: Dave Zilkoski

Plot of SLR Sites

Image: Dave Zilkoski

Plot of VLBI Sites

Image: Dave Zilkoski

The box below shows the location of the techniques in the conterminous United States.

Plot of the Four Different Space Geodetic Techniques in the CONUS

Image: Dave Zilkoski

The plot below depicts the sites in the state of Alaska.

Plot of the Four Different Space Geodetic Techniques in the Alaska

Image: Dave Zilkoski

The images below depict each of the four space geodetic techniques in the conterminous United States.

Plots of the Space Geodetic Techniques by Technique in the CONUS

Plot of GNSS Sites in CONUS Image: Dave Zilkoski

Plot of DORIS Sites in CONUS (Image: Dave Zilkoski)

Plot of SLR Sites in CONUS (Image: Dave Zilkoski)

Plot of VLBI Sites in CONUS (Image: Dave Zilkoski)

Altamimi’s article on the ITRF2020 stated it is “necessary to regularly update the ITRF (approximately every 5 years) to account for station position temporal variations due to geophysical phenomena.” My February 2022 column discussed the tectonic plates and why is it necessary to account for movement in a geodetic reference frame. As I stated then, coordinates basically change because the Earth’s surface is moving due to the movement of major tectonic plates. See the box titled “What is Tectonic Shift?” for information about why it is called plate movement or tectonic shift. The world’s geodesists understand this and are attempting to manage the changing coordinates by providing a time-dependent component of the international terrestrial reference frame.

Image: National Ocean Service website

Image: National Ocean Service website

The box below depicts the horizontal velocity based on the ITRF2020 velocities (downloaded on 08/12/2022).

Plot of the Horizontal Velocity Vectors based on the ITRF2020 Velocities

Image: Dave Zilkoski

The box below depicts the horizontal velocities in the North America. These vectors look very similar to the velocities reported in my February 2022 column.

Plot of the Horizontal Velocity Vectors in North America based on the ITRF2020 Velocities

Image: Dave Zilkoski

For a comparison to North America vectors, the box below depicts the velocity vectors in Europe.

Plot of the Horizontal Velocity Vectors in Europe based on the ITRF2020 Velocities

Image: Dave Zilkoski

They are similar in magnitude, but not in direction. Once again, looking at the map of tectonic plates, North America is located mostly on the North American plate and Europe is on the Eurasian plate.

Australia is on the Indo-Australian plate and has some fairly large horizontal velocities vectors. See the box below.

Plot of the Horizontal Velocity Vectors in Australia based on the ITRF2020 Velocities

Image: Dave Zilkoski

So, what’s the difference between ITRF2014 and the new ITRF2020? The box below provides the 14 transformation parameters from ITRF2020 to ITRF2014. These transformation parameters have been estimated using 131 stations located at 105 sites. See the box “Plot of the Stations used in the Transformation Parameters from ITRF2020 to ITRF2014” for the location of these stations. Notice that the translation values in X,Y,Z are very small (<1.5 mm) between the two reference frames.

Transformation Parameters from ITRF2020 to ITRF2014

(https://itrf.ign.fr/en/solutions/ITRF2020)

Transformation parameters at epoch 2015.0 and their rates from ITRF2020 to ITRF2014 (ITRF2014 minus ITRF2020)

(https://itrf.ign.fr/docs/solutions/itrf2020/Transfo-ITRF2020_TRFs.txt)

X,Y,Z are the coordinates in ITRF2020, and XS,YS,ZS are the coordinates in ITRF2014.

Plot of the Stations used in the Transformation Parameters from ITRF2020 to ITRF2014

Image: Dave Zilkoski

The transformation parameters from ITRF2020 and past ITRFs are provided in the table below. As indicated in the table, most of the changes in X,Y and Z are very small since ITRF2005.

Transformation Parameters from ITRF2020 to Past ITRFs

(https://itrf.ign.fr/docs/solutions/itrf2020/Transfo-ITRF2020_TRFs.txt)

As previously stated, the third multi-year CORS solution will be important to the new 2022 modernized National Spatial Reference System (NSRS) because NGS will establish a strict mathematical relationship between the 2022 NSRS frames and the ITRF2020 frame. This will allow direct access to the NSRS, according to NOAA Technical Report NOS NGS 67. Again, there will not be any changes to NGS’s NOAA CORS coordinates due to ITRF2020 until NGS completes its third multi-year CORS solution.

Users can receive emails about the latest NGS News by signing up for NGS’s newsletters. These notices will highlight the release of new products, updates to existing services, progress reports for major projects, information about upcoming NGS-sponsored events, and job opportunities at NGS.

<p>The post NGS will soon compute third multi-year CORS solution first appeared on GPS World.</p>

The surface of Earth is constantly being reshaped by earthquakes, volcanic eruptions, landslides, floods, changes in sea levels and ice sheets and other processes. Since some of these changes amount to only millimeters per year, scientists must make very precise measurements of the landscape and ocean in space and time in order to study their evolution and help mitigate their impacts.

The foundation for these precision measurements is the terrestrial reference frame, which serves the same purpose as landmarks along a trail. Earth-orbiting satellites and ground-based instruments make use of this reference system to pinpoint their own locations and, in turn, those of the features they are tracking. It is also the hidden framework relied upon by aircraft to determine their locations and by mobile phone apps that provide maps and driving directions. And it is a fundamental reference for interplanetary navigation of spacecraft.

NASA helps maintain the worldwide standard called the International Terrestrial Reference Frame, or ITRF, and recently contributed to an update issued by the International Earth Rotation and Reference Systems Service’s International Terrestrial Reference System Product Center at the Institut National de l’Information Géographique et Forestière (IGN) in Paris.

“The new release lays the groundwork for more detailed studies than ever before of global changes in Earth’s ocean, ice sheets, land and atmosphere,” said Stephen Merkowitz, manager of NASA’s Space Geodesy Project at the Goddard Space Flight Center in Greenbelt, Md.

Earth-observing satellites — such as the Jason 3 spacecraft, launched in January through a U.S.-European partnership, and the upcoming ICESat-2 mission — will be among the beneficiaries of the new standard.

Officially called ITRF2014, the update released in late January is the ninth ITRF issued since 1992. More than a thousand observing stations run by NASA and other scientific institutions worldwide contributed to it, collecting data through 2014.

Global in nearly every sense of the word, the ITRF is made up of specific geographic positions around the world, along with information about how each one drifts over time. This is important because the positions move relative to each other, with some drifting more rapidly than others. The reference frame includes details about how quickly and in which directions the positions are expected to move.

Some of the drift happens because of the motion of Earth’s tectonic plates, which is well understood. Drift motions may also include the gradual rebounding of land that was covered by ice sheets during the last ice age, as well as land subsiding due to climatic effects or human activity, such as withdrawal of groundwater. Less predictable are changes due to earthquakes. Large quakes will cause a sudden shift in position and also may alter the drift rate or direction at that location. Recent versions of the reference frame have started to include these effects.

“An important feature of the latest International Terrestrial Reference Frame is that the model has a more sophisticated way of incorporating the effects of earthquakes,” said Chopo Ma, a geophysicist at Goddard who was involved in producing and analyzing data for the latest reference frame.

Helping to improve the ITRF is one of the primary goals of NASA’s Space Geodesy Project. Four measurement techniques are used by stations worldwide to collect data for the reference frame.

In Satellite Laser Ranging, or SLR, precise measurements are made by sending short laser pulses from ground stations to Earth-orbiting satellites equipped with suitable reflectors. The distance is calculated from the time it takes for the pulse to complete the round trip back to the ground station.

The second method is called Very Long Baseline Interferometry, or VLBI. Ground stations spread across the globe observe dozens of quasars, which are distant enough to serve as stable reference points. By carefully timing when the signals from the quasars are recorded by each station, the precise geometry of the antenna network can be deduced, and Earth’s orientation in space and its rotation rate can be measured.

The technique known as Doppler Orbitography and Radiopositioning Integrated by Satellite, or DORIS, takes advantage of the Doppler effect, which is what we hear when an ambulance’s siren changes pitch as it drives toward or away from us. The frequency of a radio signal from a DORIS beacon experiences the same effect while traveling from Earth to an orbiting satellite. By measuring the frequency change, it’s possible to work backward to figure out the distance from the beacon to the satellite.

The final method makes use of the Global Navigation Satellite System, known as GNSS — a network that includes GPS and other navigation satellites. Radio signals are broadcast by GNSS satellites and received at many locations worldwide.

“The big advantage of GNSS is the dense network of stations distributed around the world,” said Richard Gross, who manages the Terrestrial Reference Frame combination center at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. “For the reference frame, on the order of a thousand GNSS stations contribute position measurements.”

Because there are GNSS receivers at the stations that perform the other three measurement techniques, GNSS also provides a method for tying together all four approaches. And when scientists worldwide want to measure how the ground is moving, they access the reference frame by using GNSS to determine their positions.

In preparation for the new reference frame, research teams worldwide carried out data analysis, looking at between 20 and 30 years of data for each method. Scientists at Goddard and the University of Maryland, Baltimore County, coordinated the data analysis for VLBI, SLR and DORIS, and JPL contributed GNSS data. All of the geodetic data for the reference frame have been archived at the NASA Crustal Dynamics Data Information System, located at Goddard, and distributed to users worldwide.

Looking forward, NASA is upgrading the stations in its Space Geodetic Network. The Space Geodesy Project at Goddard is managing these upgrades, and work is already under way at stations in Hawaii and Texas. The upgraded stations will help fill in geographic gaps in the global system, helping to improve future versions of the reference frame.

In addition, scientists are looking at other possible approaches for combining the four data types to produce an improved reference frame. Research on advancing the ITRF is conducted not only at IGN, but also at JPL’s Terrestrial Reference Frame combination center and at a similar center at the Deutsches Geodätisches Forschungsinstitut in Munich. Each center produces its own independent solution, which scientists will compare to see what they can learn from different approaches.

“We renew the International Terrestrial Reference Frame every few years because it’s more than a set of geographical positions,” said Frank Lemoine, a Goddard scientist involved in producing and analyzing data for the new standard. “It’s a projection about what will happen to those positions in the future, and our ability to extend the reference frame into the future gets better and better over time.”

— By Karen C. Fox, NASA Goddard Space Flight Center

<p>The post NASA helps maintain International Terrestrial Reference Frame with GNSS first appeared on GPS World.</p>