Survey Perspectives – Early February 2008

February 7, 2008 - By Eric Gakstatter

DOT Throws NDGPS a Life Preserver

It appears the US Department of Transportation has bought the Nationwide Differential GPS system (NDGPS) another year. The FY09 Presidential Budget Request was released earlier this week, and it contains a line item in the Research and Innovative Technology Administration (RITA) budget for NDGPS in the amount of $4.6M for operations and maintenance of the current system until October 2009. There is no budget item for the planned build-out of NDGPS. The budget request is subject to approval by Congress, but most likely this will go through.

The funding request is neither a thumbs-up nor a thumbs-down for NDGPS. The FY09 $4.6M budget request for NDGPS merely means that the DOT hasn’t figured out what to do with NDGPS yet, and the pain of having to fund a decommissioning program outweighs the $4.6M to keep it running for another year.

I think it’s the right decision. That may be intriguing to some of you who have followed my criticisms, but they have principally been directed at the stewards of NDGPS, not the program itself. RITA, regardless of how incompetent it has been at trying to understand this, needs more time to have a chance of comprehending how NDGPS is used.

Last year, RITA was funded $400,000 for a “needs assessment” of NDGPS. In other words, the administration is supposed to study and understand who is using NDGPS. Their primary attempt at this was opening a formal docket for accepting public comment last fall. You can read the Federal Register Notice here.

With an initial deadline for public response of October 1, 2007, the responses were very weak; about 30 comments were collected. The deadline was ignored by DOT, and more comments have been trickling in, with the last one posted January 28, 2008. As of February 4, 2008, there were 124 comments. However, because the explanation in the docket was written so poorly, some of the comments are not about NDGPS and obvious confusion exists between NDGPS, CORS, and OPUS. I read through every comment submitted.

After culling out the statements from by people who didn’t understand NDGPS or made meaningless comments, nearly one-third of the responses in favor of NDGPS were from National Park Service employees. Several submissions represented federal and state government users, from agencies such as the USDA, state DNRs, state DOTs, state geodetic surveys, and county and local governments. It’s hard to assign a number of users to those sorts of submissions, though. For example, in the USDA comment, it claims to have 7,000 GPS receivers in use nationwide, but you and I know that only a very small percentage use the NDGPS stations being considered for decommissioning. The USDA commenter also wrote that the loss of CORS “would have a severe impact on high-accuracy positioning.” Well, that’s not the case, so discounts the credibility of the agency’s support.

It’s sad that a pioneering GPS program such as NDGPS is being treated as it is today. Whether you support NDGPS or not, it has earned a fair shot — and it’s not getting it. That’s why I agree with the decision to fund it for another year while RITA pulls itself together. It will be very interesting to read the results of RITA’s $400,000 “needs assessment” report that was due to be completed January 30, 2008. If it’s anything like the joke of a report entitled “NDGPS Study” that was presented last fall at the CGSIC meeting in Ft. Worth, just go ahead and shoot me now.

Since the RITA docket failed to communicate to the public just what effect the loss of 26 NDGPS site would have for both NDGPS users and CORS/OPUS users, I’ll attempt to spell it out here, as clearly and concisely as possible.

What’s at Stake?

If the 26 NDGPS sites cease to operate, you will not be able to receive DGPS corrections from these sites.

Map of current DGPS and NDGPS sites:

Map of DGPS system minus the 26 NDGPS sites:

Following is a list of the 26 NDGPS sites on the chopping block:

Hackleburg, AL (HAC)
Flagstaff, AZ (FST)
Bakersfield, CA (BKR)
Chico, CA (CHO)
Essex (Fenner), CA (CAE)
Pueblo, CO (PUB)
Macon, GA (MCN)
Hagerstown, MD (HAG)
Pine River, MN (PNR)
Billings, MT (BIL)
Polson, MT (PLS)
Greensboro, NC (NCG)
Medora, ND (MDR)
Whitney, NE (WHN)
Albuquerque, NM (ABQ)
Austin, NV (AST)
Hudson Falls, NY (HDF)
Klamath Falls, OR (ORK)
Seneca, OR (ORS)
Hawk Run, PA (HRN)
Clark, SD (CLK)
Dandridge, TN (TND)
Hartsville, TN (HTV)
Summerfield, TX (SUM)
Myton, UT (MYT)
Spokane, WA (SPN)

What Alternatives Exist?

If you depend on one of the above sites for DGPS corrections (not CORS or OPUS but beacon corrections), what are your alternatives if the site is shut down?

1. The easiest choice is to switch to WAAS as a correction source. Most receivers are WAAS-enabled and, like NDGPS, it’s free. However, you’ll need to reconcile the horizontal datum difference between the two. NDGPS uses NAD 83(CORS96) and WAAS uses WGS-84(G1150). I’ve done this many times; it’s not difficult, but it needs to be done or you will introduce 1+ meter error.

Caveat emptor. Some GPS receivers handle WAAS better than others. Check for firmware updates from the manufacturer of your equipment. Also, some receivers don’t handle WAAS well when you are working under tree canopy or around buildings.

2. If you don’t require real-time corrections when you’re in the field, then you can choose to post-process your data. Post-processing software is fairly automated these days, but inconvenient nonetheless.

3. If you absolutely need submeter positioning in real time and your receiver isn’t capable of providing that via WAAS, there are several options.

OmniSTAR is a commercial provider of submeter and decimeter corrections. It may or may not work where you work, however, because it’s got a line-of-sight limitation. If you’ve got a GPS receiver with an OmniSTAR receiver already built in (e.g., Trimble ProXRS), then it would be relatively painless for you to try it. I seem to recall that OmniSTAR has a trial program of sorts.

RTK networks are popping up all over the country. Some are able to provide submeter corrections to mapping receivers via a mobile phone. Mobile phone data plans are relatively inexpensive, and you may even be able to rent one from a local GPS dealer when you need it. Most RTK networks charge a subscription or membership fee, but it doesn’t hurt to ask how they could accommodate you.

Believe it or not, it’s not that hard to take control by setting up your own portable base station and broadcast corrections. Yes, you need two GPS receivers (one to generate the corrections), and you need a way to get data from one receiver to the other (UHF radios, spread-spectrum radios, NTRIP, etc.), but it’s doable. It’s a little painful to put the system together, but once you’ve done it, you’re set for life. You don’t rely on anyone else.

Effects on CORS/OPUS Users

In shutting down the 26 NDGPS sites, one piece of collateral damage would be the loss of CORS and OPUS for post-processing using those sites. Is it an issue? For CORS and OPUS users, it’s not; for OPUS-RS users, it might be. I’ll explain.

First, let’s get definitions out of the way. When I write CORS, I’m referring to accessing RINEX data for L1 C/A post-processing. That’s you folks who use a Trimble Pathfinder, ProXR, etc., and post-processing the data to obtain meter-level accuracy. When I write OPUS and OPUS-RS, I’m referring to the National Geodetic Survey’s Online Positioning User Service, whereby you submit L1/L2 data and have their OPUS post-processing software reduce your data to centimeter-level accuracy and return corrected coordinates to you.

For CORS users, the loss of your favorite NDGPS site won’t affect you, except that you’ll have to use either the next-closest CORS site or a regional reference station from the US Forest Service or state/local government. There are a ton of them around, so that shouldn’t be a problem.

For OPUS users, the loss of the NDGPS sites won’t affect you. OPUS provides good results when using sites that are 500, 600, and even 700 kilometers away. If you go to http://www.ngs.noaa.gov/OPUS and click on Recent Solutions, you’ll see solutions from as far away as South America. I interviewed Dr. Dru Smith from the National Geodetic Survey in September 2006, and even back then, he said the days of needing to “use your favorite CORS” station are over. The OPUS software, he said, is designed such that an increased baseline distance is not an issue to be concerned with given the high density of CORS stations.

For OPUS-RS users in certain areas, the loss of the NDGPS sites may affect you. The difference between OPUS and OPUS-RS, to the user, is that OPUS occupations require a minimum of two hours, whereas OPUS-RS only requires a minimum of 15 minutes of occupation time. But a limitation of OPUS-RS is that the user position must be within 250 kilometers of three CORS; those three CORS stations must surround the user position (think good geometry). In certain regions, that will create a problem for users.

NGS has already conducted preliminary studies, determining that CORS coverage for OPUS-RS users in some regions of the country is deficient even with the NDGPS sites still active. Northern Maine, northern Minnesota, North and South Dakota, Iowa, Nebraska, Montana, Wyoming, Idaho, and northeastern Washington have been identified as deficient regions for OPUS-RS users, according to Dr. Richard Snay of NGS. Decommissioning the NDGPS sites in those areas would magnify the problem. On a positive note, Snay did say that NGS will soon be adding several CORS from the Minnesota Department of Transportation, so that will help OPUS-RS users in the region.

What’s the solution for the OPUS-RS users who would be affected if the DOT decommissions the 26 NDGPS sites? The easiest, and only, solution I’d recommend is to revert back to using the original OPUS program. This means planning for two-hour occupation times instead of 15 minutes. Secondly, I’d start lobbying your state DOT, county, and whoever else might be interested in setting up a cooperative CORS site in your area.

In summary, the impact of shutting down the 26 NDGPS sites has a minimal impact on CORS/OPUS/OPUS-RS users.

Back to the Budget

The FY09 NDGPS funding request is still only good enough to stop the bleeding for another year; it doesn’t solve the problem. When its study is completed, I seriously doubt RITA is going to find enough transportation applications to justify continuing to fund NDGPS under the DOT umbrella. Realistically, it’s going to be up to federal and state government users in the affected regions to pony up the funding. You can bet that no private entities are going to contribute significant funds, if any at all. They’ll find another solution before going down that road

Listed below are some of the major government supporters (or associations who represent government agencies) that submitted public comments in support of NDGPS. I think it will be up to them, and others, to come up with at least the Operations/Maintenance budget of approximately $5 million annually to sustain (not build out) the NDGPS as it is today.