GNSS Next: Taking Cues from the "Other Field Work"
Professional Surveyor Magazine - July 2012
View Part 1 here
By Gavin Schrock, PLS
“We have a choice: to plow new ground or let the weeds grow.”
—Virginia Department of Agriculture report for fiscal year 1958-1959
I began an examination of the subject of real-time Precise Point Positioning as a likely successor to RTK in this magazine’s June 2012 cover article, “After RTK.” My continuation here is based on the idea that much of the development of early adoption solutions has been from the field of precision agriculture. This might seem counter-intuitive for surveyors who inhabit a world of the highest precisions, but we might be in for a surprise.
High-precision positioning is traditionally expensive. An old (unattributed) quote posits, “I can measure that line for $x, but that last centimeter will cost you $100x.” The value of high precision has not been lost on our fellows in the “other field.” The term “precision agriculture” has grown to include not only precision in monitoring crop health and application of fertilizers and water but also precision in positioning for plowing, planting, weeding, application of chemicals, and harvesting. Farms have been using GPS for basic navigation for as long as (if not longer than) many surveyors; more recently they’ve added GPS machine guidance. For the most part, however, these uses have been for applications requiring precision lower than surveyors would consider useful.
Agriculture as Early Adopters
Solutions for Precision Agriculture are helping pave the way for high-precision real-time PPP for surveying and other applications. GPS (and, more recently) GNSS adoption in agriculture (Ag) has grown to represent usage, infrastructure, and technology investments that dwarf that of surveying GPS/GNSS usage and infrastructure. While there are agricultural uses for the low-precision DGPS beacons, or sometimes WAAS (widely used for navigation but less often than other methods for precision Ag), you might be surprised to find out that there are estimated to be thousands of RTK bases used specifically for agriculture spread across the United States and Canada—more than the NGS CORS and local/regional RTN combined.
These arrays are mainly hosted by Ag dealers and manufacturers and provide corrections through a variety of permanent and ad hoc radio networks along with some innovative correction relaying services. In addition, the use of RTN for precision Ag is increasing in states with substantial Ag presence, on the “hosted RTN” services (manufacturer-hosted, originally survey-centric RTN like Trimble VRS Now and Leica’s SmartNet). Finally there are the Ag-centric, Ag manufacturer-hosted, satellite-delivered correction services like Navcom’s (Deere and Company) Starfire service, OmniSTAR, and Trimble’s CenterPoint RTX. That is a lot of investment and usage, and that might lead non-Ag folks to wonder: What does the farm really need with centimeter precisions?
Cost-benefits for Ag
High precision might be pricey, but it can yield substantial cost savings. While a surveyor might be able to realize 10% to 30% cost savings in RTK over conventional methods (of course only where appropriate … calm down), the farmer may, in some cases, see only a fraction of a percentage in cost benefit per unit in comparing certain manual tasks with GPS-guided alternatives. But, if you consider the sheer scale of agricultural operations, the relentless cycle of overlapping planting and harvesting seasons, the hungry markets, and the fluctuations in fuel and fertilizer costs, then these modest per-unit savings can add up.
Sub-meter DGPS positioning does not cut it when considering that, by adding a little more precision, the farm can gain an extra row or more per acre for some crops by going to decimeter-grade solutions, for example. Or, with RTK the farm could gain the ability to reduce fertilizer applications by precisely targeting planted strips a few inches wide. The technology of high precision got cheaper and much easier to apply to more elements of farming.
Back to real-time Precise Point Positioning. As outlined in my previous article: Some of the expected advantages of PPP over RTK and RTN are the prospects of (eventually) needing much smaller arrays of CORS. And, there would no longer be a need to transmit bandwidth-hogging observations and resultant corrections between base and rover (or RTN); this is because the PPP “error states” products (i.e. clock, orbit, and iono/tropo models) can be merged into a very small message for purely broadcast transmission over wide regions, even by satellite.
This is exactly what certain providers are already doing for Ag users. While the dream of PPP in real time without infrastructure is still years away, the way is being paved by researchers from academia and Ag-industry service providers that have developed fully functional interim solutions, such as PPP-RTK and other variations. While this still involves at least some infrastructure, less is required, and new options are open for transmission of these much-more compact messages.
Innovative Service Providers
Several Ag service providers have been delivering such solutions, based at least in part on these concepts, for many years. By installing their own tracking networks of CORS worldwide and, more densely, in the target Ag-market regions, they produce their own clock and orbit products and worldwide and regional ionospheric and (in some cases) tropospheric models. They deliver these messages via their own satellites or leased channels on the satellites of others. None of these service providers would be expected to reveal the secrets of their own solutions, but they have revealed some notable elements of interest about their respective approaches.
Navcom’s Starfire system has operated a satellite-based augmentation service (SBAS) since 1994. Part of the legacy, Starfire system is a worldwide, sub-meter navigation service like the FAA’s WAAS. But, in the intervening years, they have added other service levels to provide higher-precision positions, both stand-alone or to “carry on” certain levels of precision when the tractor goes out of connection with an RTK base/radio, with the satellite messages taking over. More recently, Navcom has implemented a proprietary clock and orbit real-time estimator, Kalman filtering, for both GPS and Glonass, and carrier phase ambiguity fixing—a mix of both PPP and RTK approaches—getting down to 5cm precisions (after convergence) worldwide for many applications.
Solutions of this type—including many variations developed by academia, scientific research groups of many countries, as well as commercial developers—have been deployed to serve not only precision Ag but marine navigation, marine construction, oil and gas industries, resource mapping and management, mining, and public safety. But the limitation of some current solutions is the trade-off between precision and speed. Yes, you can get centimeter precisions, but the convergence time (time needed to achieve optimal precision) can take 1 to 45 minutes or more. Yet, even today the various service providers are working diligently to overcome this precision/speed conundrum. It is no surprise that the development was targeted at Ag uses first for several reasons: The current state of precision/speed trade-off is more palatable to Ag (who can live with lower performance in both), and Ag is such a huge market.
One example of a service provider who began their foray into the world of real-time PPP with Ag and marine markets (yet whom most surveyors would more readily associate with surveying) is Trimble. Trimble’s Terrasat development team in Munich, headed by Dr. Herbert Landau, was a pioneer in RTK/RTN, with the development of Trimble’s VRS (known as Trimble CenterPoint VRS to Ag markets) among their accomplishments.
The Terrasat team has developed CenterPoint RTX technology, a system that consists of a worldwide array of their own tracking stations that provide orbit determinations and clock estimations. These determinations include a bias estimation and a carrier-phase ambiguity-resolution process and are passed along to the rover via satellite or wireless internet. The rover uses this data to achieve ambiguity fixes in a PPP-RTK way. There are also other elements in play that their own tracking can improve upon (such as improvements over some public-domain orbit products). To provide the more-localized atmospheric models, they have set up arrays of CORS in heavy Ag areas (e.g. parts of the Midwest) on a spacing of 100km or more. Initializations for precisions in centimeters are less than a minute—maybe not “surveyor ready” yet, but well on track.
Some elements of PPP-RTK have already been applied for high-precision, real-time surveying for over a decade. It is standard practice for RTN to apply advanced orbit products to their solutions. Taking this a step further, Dr. Gerhard Wübbena and his team at Geo++ apply multiple “error state” products (in the manner of PPP) to their RTN solution called GNSMART; the results are then translated in the “correction”-type formats that RTN rovers typically use as an example of PPP-RTK.We should not be surprised if the first waves of surveyors to go “real-time PPP” are clients of offshoots from these services pioneered for Ag uses. Considering the sheer scale of the Ag markets and the respective successes of these providers and others: we are getting a glimpse of what may be in store for surveyors and other high-precision end users. These developments are definitely something to keep an eye on.
Gavin Schrock, PLS, is a surveyor, technology writer, and operator of an RTN. He’s also on our editorial board.
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