October 5, 2022 – By David B. Zilkoski
On Aug. 5, the National Geodetic Survey (NGS) stated it will be updating the NOAA CORS to be aligned with the latest International Terrestrial Reference frame, ITRF2020 (see below). As stated in the announcement, NGS will soon compute a third multi-year continuously operating reference station (CORS) solution, MYCS3.
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 technique: 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
The following plots depict the locations using each space geodetic techniques: GNSS sites, DORIS sites, SLR sites and VLBI sites.
Plot of GNSS Sites
Plot of DORIS Sites
Plot of SLR Sites
Plot of VLBI Sites
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
The plot below depicts the sites in the state of Alaska.
Plot of the Four Different Space Geodetic Techniques in the Alaska
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
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.
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
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
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
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
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
Transformation parameters at epoch 2015.0 and their rates from ITRF2020 to ITRF2014 (ITRF2014 minus ITRF2020)
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
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
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.
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