NGS beta version of a new NOAA CORS Network station web page

My past GPS World newsletters (February 2024, March 2024, April 2024 and May 2024)  highlighted the NGS Geospatial Modeling grantees which included creating a CORS Dashboard that will be very useful to NGS employees monitoring the CORSs and evaluating the Intra-Frame Deformation Model (IFDM).

I mentioned in the May 2024 newsletter that NGS announced the release of a beta version of a new NOAA CORS Network (NCN) Station Web Page. Each CORS station in the NCN will have its own page with data, metadata, maps and photos displayed in a modular layout so information is easily found in one location. This newsletter will describe some features of the new beta site.

The beta site is located here.

I will highlight some of the information provided by the routine, but I would encourage others to access the beta site and provide feedback to NGS. NGS states on the site that “This is a Beta product. We are interested in your feedback. Please email us at: ngs.feedback@noaa.gov and indicate the subject as “NCN Station Pages Feedback.”

When you access the website, it defaults to the CORS station GODE. The user has the option to enter their own CORS station in the box located on the right-hand side of the webpage.

Texas CORS Station TXLV.

A nice feature of the site is that the CORS data availability for the last seven days is provided under the Station Information section. For those interested in downloading data, there is a button titled “Quick Data Download,” on the top left corner. The site allows users to download daily data from the past 30, seven or two days.

In my example, I downloaded the last seven days of data for CORS TXLV. It only took a few seconds to download and provide the data in a zipped file. If a user includes this process in their standard operating procedure, they can easily download all the CORS data required for their project.

Downloading TXLV GNSS data.

Another planning tool available is the weather information for a week. Today, most users can get the weather information on their phone. However, this is a convenient option to have when you are looking at available CORS on the day of occupying marks. It can help in managing schedule changes.

There is an option to show the five nearest CORS relative to your selected CORS by clicking on the button titled “Show Closest 5 on Map.”

CORS Located near TXLV.

Clicking on the button labeled “Show Legend” provides information about the CORS depicted on the map. This is a useful feature especially if selecting CORS that provide GNSS data other than GPS and/or data at different sampling rates.

If a user clicks on the button “Open NGS Map,” the site will access the NGS Map website and provide information about the selected CORS. This allows users to get information about the CORS. I found that the beta site provided most of the same information using the various options on the NGS Map website.

NGS Map depicting CORS TXLV.

The site provides photos and equipment history that may help in troubleshooting an issue associated with processing sessions or during the analysis of the adjustment results. I have highlighted that a new antenna was installed at the TXLV CORS on August 5, 2021. I will explain later in this newsletter how this information helped me during my analysis of a GNSS project.

Photos and equipment history of TXLV.

Under the Coordinates and Velocities section, the site provides information about the latest coordinates and velocities along with superseded values for the selected CORS. The superseded values may not be of interest to most users, but I am always looking at the changes in CORS coordinates. It is my nature to try to understand the reason why something has changed; especially for CORS that I am including in a GNSS project.

Coordinates and velocities.

Clicking on the link titled “Position and Velocity” under the Coordinates and Velocities section provides the coordinate and velocity information for your selected CORS. I have highlighted the ITRF2014 velocities, the NAD 83 (2011) velocities, the latest antenna type, installation date and the dates the positions and velocities were revised.

As shown in the image above, the position and velocity sheet provide the dates that the position was revised. Clicking on the link titled “Datasheet with GRP/MON included (if available)” in the Coordinates and Velocities section provides the datasheet that lists the NAD 83 (2011) superseded survey control values. The superseded ellipsoid heights from the datasheet are provided in the box titled “Excerpt from TXLV Datasheet.”

When you are trying to estimate heights to the 2 cm level, changes in published NAD 83 (2011) CORS heights at the 2 cm level are significant and should be investigated and understood. This beta CORS website offers useful information that can help understand some of these changes. I will explain later in the newsletter how this information and other data from the beta site helped me in the analysis of my GNSS project.

Excerpt from TXLV data sheet.

The beta site provides plots that depict the daily positions and residuals for a CORS. In my May 2024 newsletter, I stated that NGS has developed a Beta CORS Time Series Tool that provides information that assists users in selecting appropriate CORS for a project. The Beta CORS Time Series Tool provides the residual differences from the daily NGS OPUS-NET solutions with the coordinates from the official CORS’ coordinate functions. The excerpt below explains the plots and residuals:

NCN Residual Time Series Comparison Tool (NCN PloTS)

This tool computes and displays the residuals for up to 50 CORS stations within the NCN. The mean, standard deviation, and root-mean-square error of the residuals are also provided in a summary table that is available for download. This tool is informational, not authoritative.

The residuals are calculated as the difference between the daily observation at a station and the official daily coordinates for a station. The daily observation is processed from the GPS L1 and L2 signals only, using a network adjustment program. There must be a minimum of 8 hours of data present in a 24 hour file for a solution to be generated. The network adjustment program is an internal application developed by NGS for monitoring the position of the CORS stations in the NCN (Gillins et al., 2019). The official daily coordinates for a station are calculated using the reference epoch (2010.0) position and velocity published as the station coordinate function in the Position and Velocity File. An example of a Position and Velocity File for NCN station GODE can be found here. To obtain Position and Velocity Files for NCN stations please visit the NCN Station Pages and navigate to the Coordinates and Velocities section.

This tool is optimized for plotting data extending between 30 to 90 days in length but can be customized to other time frames. The earliest start date currently available is October 27, 2018, which is the completion date of the MYCS2 and the end date can be as recent as 3 days before the present day. This three-day time lag is so that the final orbits can be used in the network adjustment to create the daily solutions. Then, please enter the 4-character station ID for at least one and up to 50 CORS stations in the NCN and submit this request to obtain a map, summary table of comparative statistics, and residual plots during the date range.

The beta NGS NCN station pages show similar plots to the Beta CORS Time Series Tool. the station pages also allow users to create position and residual plots at different periods. I find these plots very useful when selecting CORS to be included in a GNSS project. The latest plots are of interest to users when selecting CORS to be included in their GNSS project but there are reasons to look at plots depicting older time periods.

Position and residual plots for TXLV.

I previously mentioned that the antenna of CORS TXLV was changed on August 5, 2021, so I used the option to plot the last five years to include data before and after the date the antenna was changed. I highlighted August 7, 2021, on both plots. This was two days after the antenna was changed on CORS TXLV.

There appears to be a 2 cm upward shift in the up component after the new antenna was installed. There was also a change of about 1 cm in the north component. Something else to notice in the position plot is that the east component has a significant tilt during the five years. The below provides the ITRF2014 velocities — the eastward component velocity is —1.21 cm/year. In 5 years, one could expect to see about a 6 cm change.

Position and residual plots for TXLV.

Five-year position plot of TXLV.

Five-year residual plot of TXLV.

Position plot of TXLV for selected time interval.

These small changes affected my analysis and network adjustment results. During the past several years, I have participated in several Harris-Galveston Subsidence District (HGSD) GNSS projects performed in the Houston-Galveston, Texas, region. I have been involved with estimating subsidence in the Houston-Galveston, Texas, region for about 40 years so when I see changes in height values indicating an apparent uplift it makes me question my results. Therefore, I started investigating the CORS involved in the GNSS project. I looked at the Texas CORS surrounding the GNSS project: WHARTON CORS, COLUMBUS CORS, HEMPSTEAD CORS, LIVINGSTON CORS, and LIBERTY CORS.

The table below provides the differences between the published ellipsoid height and the previous superseded height for the five CORS. As the table indicates, the published ellipsoid height of the CORS increased by about 2 cm from the superseded height. This led me to use the NGS NCN Station Pages to investigate these CORSs. I found that all five of these CORSs had new antennas installed in 2021 and their position plots depicted a similar shift.

I want to emphasis that I am not saying that anyone did anything wrong or incorrect.  The CORS manager of these sites provided the appropriate metadata to the NGS CORS team so the site information could be updated and correctly reported. What this indicates to me is that the installation of the new antenna and setup may have affected the height component of these CORS, that is, it may have changed the official position of the monument’s reference point. Again, I want to emphasize that I am not saying that anyone did anything wrong or incorrect.  NGS’s process includes monitoring all CORS that are part of the NOAA CORS Network (NCN). The NGS CORS Team noticed the significant change in the up component comparing it to its expected value, so they computed a new coordinate and published the new coordinate in 2022. In my opinion, anyone using these CORSs as constraints in their GNSS projects after the date that the new antenna was installed and before the new coordinate was published could have generated adjusted heights that are in error by 2 cm. As previously stated, when you are estimating heights to the 2 cm level, changes in published NAD 83 (2011) CORS heights at the 2 cm level are significant. In my opinion, this type of analysis should be performed by all users that are incorporating CORS in their GNSS processing.

Keep checking NGS beta site because NGS makes changes based on user feedback. As I previously stated, I would encourage everyone to access the beta site and provide your feedback to NGS. NGS states on the site that “This is a Beta product. We are interested in your feedback. Please email us at: ngs.feedback@noaa.gov and indicate the subject as “NCN Station Pages Feedback.”  I have talked to the CORS team and they really would like feedback. The team will make changes to the website based on feedback from users.