Sources of Public, Real-Time, High-Precision Corrections

April 5, 2013  - By 0 Comments

I probably reminisce a bit too much at times, but I can’t help but think back to the ’90s, when obtaining three-meter accuracy via post-processing took a bit of planning. You either had to operate your own GPS base station, or you had to find a publicly available one before you went to the field to make sure you had a source of base station data. Remember, back then publicly available CORS weren’t very common.

Then, towards the late ’90s, there were enough publicly available GPS CORS in the U.S. that you could collect data in the field without knowing where the closest base station was located, but you knew GPS base stations were so prolific that you could find one close enough to use for post-processing without prior planning/coordinating.

Then, sources of real-time GPS corrections started through the same progression. In the ’90s, if you wanted real-time corrections, you either had to operate your own GPS base station and wireless datalink or, if you were lucky you were close to a U.S. Coast Guard beacon transmitter, which were few a far between. OmniSTAR was an option, but subscription was a quite a bit more expensive back then and the equipment was bulky.

Today, post-processing is a no brainer. You don’t even need to have to license post-processing software. Through the National Geodetic Survey’s OPUS, Austraila’s AUSPOS and Canada’s CSRS-PPP, you can collect GPS data anywhere in the world, submit it to one of these free, online processing centers, and have the answer in your email inbox in a few minutes. But, as I’ve lamented more than once over the years, post-processing is a dinosaur. Mind you, it will never go away completely, but it doesn’t belong in the typical mainstream data collection workflow. It just doesn’t make sense.

As it was 20 years ago and as it is today, the challenge with real-time GPS/GNSS data collection is the wireless datalink. If you’ve ever worked with real-time GPS/GNSS data collection and had a unreliable wireless data link between the base and your receiver, you know what I mean. It’s exceedingly frustrating and unproductive. However, when everything is working as designed, the real-time GPS/GNSS data collection workflow is a thing of beauty.

Sources of high-precision real-time GPS/GNSS corrections are still a rather disparate group of public and commercial services that depend heavily on geography and communications infrastructure. For example, in the U.S. there is plenty of wireless coverage (GSM, CDMA, Wi-Fi) in metro areas and along major interstate roads, but there are still vast areas of rural farmland, prairie and desert where wireless networks don’t reach, leaving the choice of either satellite-based communications or setting up your own private wireless communications (UHF/VHF/900 MHz) between a base station and your receiver.

That said, there are more choices for real-time, high-precision GPS/GNSS corrections than ever before. In fact, just last week, the International GNSS Service (IGS) announced that it has started to offer a global real-time PPP data stream for high-precision, dual-frequency GPS receivers via NTRIP. That means anyone with a dual-frequency GPS receiver and an Internet connection can achieve sub-decimeter accuracy anywhere in the world, free of charge. How exciting is that! I think about the regions of the world like South America, Central and Southern Africa, Australia, and parts of Asia that aren’t served well by public SBAS or other free sources of high-precision GPS/GNSS corrections. This service will open up those regions to a new level of real-time, high-precision positioning. There’s one catch though; GPS/GNSS receiver designers have to implement special firmware to use the IGS RT PPP service. Some manufacturers are talking about implementing this, which would be a boon for the high-precision GNSS user community. Global IGS RT accuracy = ~10 cm.

Of course, OmniSTAR, Fugro, Starfire, Veripos have been providing real-time PPP for years  (as well as Terrastar and Trimble more recently) in their respective vertical markets (land and offshore) but it requires an annual subscription fee and specialty hardware (L-band) to receive the signal. The receiver hardware can be prohibitively expensive for some potential users, and their coverage, based on leased communications satellite footprint, isn’t dependent on local Internet connectivity. However, I will say that OmniSTAR subscription pricing is very competitive now, and that a public service like what IGS is offering has no guarantees of availability or accuracy. On the other hand, since commercial services like OmniSTAR are collecting a fee, they have an obligation to service their users. Nevertheless, public, sub-meter SBAS services like WAAS, EGNOS, MSAS, GAGAN, and SDCM are offered to non-aviation users on the same terms as IGS, and those services have worked out very well for our surveying and mapping user community.


Global IGS GPS/GNSS Stations use for Real-time PPP Streaming
Source: European Space Agency (ESA).

Other public sources of high-precision GPS/GNSS corrections are on the rise:

RTK Networks. RTK networks continue to proliferate. In the U.S., many states offer free access to their centimeter-level statewide RTK networks. These are somewhat well-known within the surveying and agriculture community, but not as well known within the GIS community. Many countries also offer regional and country-wide RTK networks. RTK network accuracy = 1-2 cm.


Washington State RTK Network
Source: Washington State Reference Network.

PBO real-time streaming. In the western U.S., UNAVCO’s Plate Boundary Observatory (PBO) maintains more than 1,000 GNSS base stations with many of them broadcasting RTCM3-formatted data. If you’re in California, Oregon, Washington, and the surrounding states, you should take a look at its website. The only requirement is that you have a receiver capable of handling RTCM3 data and you have Internet access in the field. You’ll also need an NTRIP client software (there are free ones available) running on your data collector (smartphone, handheld, tablet). Note that these are single baseline solutions (as opposed to the RTK network solution), so the further you are from the base station, the more error will be introduced. One caveat: be sure you understand which horizontal datum and epoch the particular PBO base station is streaming. PBO real-time streaming accuracy within 20 km = 1-2 cm.


PBO Station Streaming Map
Source: UNAVCO Plate Boundary Observatory.

CORS Streaming. The National Geodetic Survey (NGS) is testing real-time streaming from nearly a couple of dozen CORS sites, mostly in the Eastern U.S. This is very similar to PBO real-time streaming. If you are 50-75 km from the base station, you’ll still be under 10 cm. If you’re within 20 km, you’ll be down to 1-2 cm.


CORS Streaming Station Map
Source: National Geodetic Survey.

SBAS. SBAS (WAAS/EGNOS/MSAS/GAGAN/SDCM) was the first true source of public, country-wide high-precision GPS corrections. What make SBAS so easy is its ergonomics. It’s become such a standard that virtually every high-performance GPS/GNSS receiver designed today has SBAS capability built-in. You don’t need to purchase any extra hardware or software to use it. SBAS accuracy = sub-meter (with a receiver designed to optimize WAAS).


Global SBAS Coverage Map
Source: Geneq, Inc.

There’s no doubt that years from now, we’ll look back and be amused at how “difficult” and expensive real-time, high-precision positioning was. Today, there are many more sources of high-precision, real-time GPS/GNSS corrections than there were ten years ago. In ten years, there may or may not be many more choices for high-precision GPS/GNSS corrections, but certainly the sources will be less complex, more ubiquitous and more convenient than they are today.

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Thanks, and see you next month.

This article is tagged with and posted in Newsletter Editorials, Survey Scene
Eric Gakstatter

About the Author:

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributing editor to GPS World magazine,writing a monthly newsletter on high-precision GPS/GNSS technology. He is also editor of Geospatial Solutions, a weekly newsletter focused on geospatial technologies.

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