The GPS on Bench Marks campaign of the National Geodetic Survey is a triumph of constituency participation to improve the nation’s geodetic infrastructure. The initiative is seeing success, but also faces challenges. Multi-constellation support, and acceptance of short RTK/RTN observations, will add a productivity boost.

The “GPS on Bench Marks” program of the U.S. National Geodetic Survey is a brilliant example of “crowd sourced” data collection to improve the national geodetic reference framework. Through this program, local surveyors perform GPS observations on Bench Marks, and these are submitted for processing by the NGS to improve the transformation models that are becoming vital to surveyors’ day to day operations. Therefore, surveyors have a direct stake in the quality of geodetic resources available in their own region.

The standard method for submission is two sets of four-hour-plus observations, with the start time of the repeat sessions separated by at least three hours, submitting these to the NGS OPUS (Online Positioning User Service), choosing the “Share” option, providing some photos and information about the field procedures, and any updates to the current benchmark info. Under normal conditions, this process will yield success, valuable data for the NGS, and ultimately this benefits all of us.

While it was announced recently that the submission deadline for GPS on Bench Marks was extended until the end of 2022, this deadline is for inclusion in the initial modeling—there will be an ongoing need for such data moving forward.

Crowdsourcing

There are challenges though, and this includes inconsistencies that can befall any form of “crowdsourcing”:

• Some states and localities make funds available to perform these observations, but many do not.
• Desire to participate can vary
• Resources available can vary
• Discretionary time available to participate can vary
• Bench Marks that the NGS has identified for observations can be far apart, difficult to access, or have sky view and multipath issues that make GPS observations difficult or impractical

The Sky’s the Limit… and it can be a Limiting Factor

The other key challenge lies in the name: “GPS on Bench Marks.” Presently the standard process for doing observations, and processing by the NGS, is limited to the GPS constellation. The standard process uses the NGS OPUS that presently only uses observations from the U.S. Navstar satellites (more commonly known as “GPS”). Additionally:

• Many legacy Bench Marks were set decades ago, prior to any anticipation that they would someday be observed with GPS
• The “Suitable for GPS” notation on many NGS datasheets turns out to not be suitable after all. Sometimes this notation was made somewhat over-optimistically. Or there have been changes in the sky view conditions due to vegetation or nearby construction.
• GPS submissions have seen a significant number of rejected sessions—mostly due to incomplete observation sets—often due to the above factors.

There are though, bright prospects on a near horizon, with at least one way to take advantage of multi-constellation (GNSS) processing right now.

Evolving NGS Services

“My job is to work with our stakeholders and users of our services, and in particular to incorporate their feedback into the product development process as we build the new tools for Modernized National Spatial Reference System (NSRS),” said Galen Scott, the constituent resources manager and GPS on Bench Marks team lead at NOAA‘s National Geodetic Survey. Scott outlined several key initiatives that will modernize multiple NGS resources and services, including the GPS on Bench Marks program, and ultimately the NSRS:

• An “alignment” service for operators of continuously operating reference stations, which that are not currently included in the NOAA CORS Network (NCN). For example, GNSS reference stations in state, local, and regional real-time networks (RTN). With this new service, that may commence in coming years, operators of such third-tier reference stations or “community CORS” will provide the NGS with access to station observation files for some or all of its stations. The NGS will process this data to provide up-to-date coordinates and position time-series, just as it does for NCN stations in the first and second tiers (also referred to at the moment as “foundation” and “definitional” CORS respectively), though this data will not be used as control in OPUS solutions. This provides the NGS with a greater density of position data for NSRS modelling, and the community CORS operators with positions that are aligned directly to the NSRS. This provides a greater density of CORS that can be utilized with alternate methods for GPS on Bench Marks (more on that later). “Community CORS” could be elevated to “definitional CORS” if deemed to be very stable and of high quality, or necessary to fill in a geographic gap. 
• Many of these community CORS are tracking multiple constellations that could be leveraged for shorter observation times.
• OPUS Projects 5.0 (Beta) can accept vector data from RTN (single-base, VRS, MAC, etc.) observation sessions that can be dramatically shorter than the standard long static sessions for GPS on Bench Marks (more on this later as well).
• Multi-constellation processing support for OPUS (i.e., M-Pages, slated for launch in the near future).
• A new format for submitting GNSS vector data (GVX), and additional formats for submitting levelling and classical angle and distance measurements.

M-Pages and OPUS

“Among the motivations for GPS on Bench Marks is actually improving our modeling of deformations over time,” said Dan Gillins, Ph.D., NGS Geodesist. “We recognize that, especially on the West Coast, the CORS that are in the NCN may not be at a dense enough spacing for us to develop and maintain a high quality information model to provide time dependent transformations, like HTDP.”

A key software that the NGS uses for GPS vector reduction is PAGES. It has been around for many years and is the heart of OPUS. A new version that will support multi-constellation processing is in development.

“We might see it by this time next year” said Gillins. M-PAGES uses a single difference processing strategy. For example: satellite specific terms cancel, receiver terms (clock, biases) do not, and all single difference observables are on the same frequency. This type of approach has been widely recognized by those in academia and the GNSS development community as being well suited for multi-GNSS processing of this type. For more technical detail on how this approach works, view the NGS presentation here.

When implemented, M-PAGES will be utilized by the NGS for not just OPUS, but also in GNSS orbit determinations and NCN CORS monitoring.  While the initial development focused on GPS and Galileo, the NGS plans to eventually extend capabilities to include all viable constellations.

Gillins said that with the implementation of M-PAGES in OPUS, prescribed four-hour observation times could likely be reduced. “Another big goal of the M-PAGES team is to eventually be able to provide solutions in 15 minutes,” said Gillins.

“GNSS on Bench Marks” Today

Is it possible to leverage multiple constellations to submit for GPS on Bench Marks today? And to do so with observation times as short as five minutes each? Yes, and state, local, and regional RTN are the key. Plus, the NGS GVX format for GNSS vector data transfer.

Jeff Jalbrzikowski, PS, GISP, CFS, NGS Appalachian Regional Advisor is currently testing this method in conjunction with the Ohio Department of Transportation (ODOT) and the statewide RTN it operates. The workflow is two five-minute RTN observations on desired Bench Marks, with vector data transferred to Trimble Business Center (TBC), one of the software packages that several manufacturers have enabled with a GVX export option.

Jalbrzikowski then brings the resultant GVX file into an OPUS Projects 5.0 (beta) project. He notes that there are several other steps to submit additional data and photos. You will need to run NGS WinDesc to create updated mark descriptions for uploading, and upload the three required photos: close-up, from eye level, and a horizon photo. You do not need to create a new project each time, but if observing marks over a broad area, it could be a good idea.

“You get the classic green check marks as you proceed through the steps,” said Jalbrzikowski. “When they are all green, you can hit the submit button and it is submitted to the NGS for review and loading in the IDB; essentially the same as Blue Booking a mark.” This might be a little more work than the standard GPS on Bench Marks process, but Jalbrzikowski says it can be worth it to be able to submit more marks with less of a time burden in the field.

If RTN are to be used, there is a question of quality control as there may be CORS included in solutions that are not NCN CORS. Until the “alignment” service for “community CORS” has been implemented, there are interim steps that can be taken to ensure fidelity to the NSRS:

• If your observations are single-base to an NGS CORS, this ensures a tie in the project.
• If your observations are to a non-NCN CORS, you can add 24-hour observation files from that base for the day or days encompassing each of the five-minute observations.
• If your observations are network RTK (VRS, MAC, etc.) and the “physical base station” (PBS) is an NGS CORS, you are covered
• If the PBS of your network RTK observation is a non-NCN CORS, you can add the 24-hour static files to the project for post-processing.

In addition, Jalbrzikowski recommends establishing a “parking lot” check mark with multiple long-static observations submitted to OPUS Share. This is practice is commonly utilized by surveyors, to have mark handy at their office to check their equipment before a day of work. And it can also be used in OPUS Projects the check the quality of your local RTN.

The criteria for acceptance of observations, whether via the standard GPS on Bench Marks method or the GVX-OPUS Projects method is that the results of each session do not vary by more than five cm. Gillins would like to see three sets of observations, to truly prove the fidelity of each, and if time and logistics permit, this could be a valuable practice. After all, if you can do a few more steps without a major time commitment to ensure you are submitting good data, this will directly improve the quality of the geodetic reference framework you will use moving forward.

RTN Online Post Processing

Another tool that could potentially be used for faster, multi-constellation submissions to GPS on Bench Marks is a service commonly implemented by RTN. Many RTN have online post-processing (OPP) services that perform traditional baseline processing. These are often multi-constellation, and with the community CORS spacing being denser than the NCN, the observation times can be much shorter than for standard OPUS. For instance, in WA (where incidentally all CORS have been Blue Booked) a five-minute 1Hz observation via OPP will typically match a 24-hour OPUS. Users will often submit multiple five-minute sessions. Outputs often include XML and PDF reports that include all bases used, satellites used, baseline lengths, the number of observations used, and residuals. OPP could also be handy for the RTN-based method when cell coverage is spotty or non-existent.

There are two things that would need to happen before this could be considered as a viable method for something like GPS on Bench Marks: the “alignment” service for the “community CORS,” and for the RTN software developers to enable a GVX format export. RTN operators have already submitted requests to their respective vendors for this addition. Resultant GVX files could then be uploaded to OPUS-Projects and submitted to NGS for review.

Time Window Separations

Another aspect of GPS on Bench Marks that could potentially be changed to bring greater efficiency is reducing the time separation between observations.

“We recommend waiting a minimum of three hours between observations to allow the satellite geometry to change enough to be considered an independent measurement,” said Gillins. Long time separations have always been recommended for GPS observations. E.g., “Repeat on a different day at a different time of day.” However, such rules were developed when there was only one constellation. Just how much could four constellations change the rules?

Do you need to wait to select different geometries? Not necessarily. If you have, for instance 35 satellites in view, you could select two subsets of 12 to 20 each. Indeed, some GNSS rover manufacturers have put such a utility in their field software—different geometry subsets at the click of a button. Further study would be needed before such approaches could be proven to meet NGS criteria, but it is an intriguing prospect to consider. 

The subject of time windows has been studied. An academic study commissioned by the Ordnance Survey of the UK yielded the recommendation that the separation between GNSS observations should be 20-45 minutes. Note that they were only using GPS + Glonass at the time. Additionally, their tests indicated that “Window separations of greater than 45 minutes do not typically provide appreciable further improvement to the determined coordinates.”

The NGS will study this as well, using data from what would essentially be “continuously operating rovers.”

“If we log single-epoch solutions continuously, for 24 hours, maybe 48 hours, we could window different scenarios,” said Gillins. “What does it look like in two hours, one hour, etc. And among other things, we could see what separation is needed to properly account for multi path errors.”

There is no timeline or firm commitment to revise recommendations, but in the interim, if the observation times can be reduced by supporting multi-constellation processing and accepting RTN derived vectors, even with the current three-hour separation, surveyors could hit more Bench Marks than we can today.

Faster!

Imagine the time savings that M-PAGES should bring. The time savings the RTN/GVX/OPUS Projects method can bring. And perhaps OPP and the possibility that time separation windows could be reduced. Add these up, and GPS on Bench Marks could get quite a boost. Gillins mused that surveyors could set out do five-minute observations on the way out, and the second set on the way back, covering a lot of marks even in a single day. The present GPS-only can be quite a time burden, and if that handicap could be overcome, it should result in greater participation, more data, and a better NSRS to come.