There are a number of issues to consider when choosing a positioning solution — price point, hardware requirements, precision requirements, personnel training considerations, sky-view limitations, the need for relative or absolute accuracy, availability of services in a given location, etc.
“There’s still a lack of understanding of why you use one type, what the limitations are, and where you have to start thinking about the other,” says Mackie. That’s the biggest education piece we run across. It’s not so much talking about the technology or how it works, it’s talking about when to use which.”
There are resources, though, both formal and informal to help you decide, and this article only scratches the surface. The manufacturers often post case studies, and while those might be motivated by marketing, at least you get to see real-world examples, and maybe some like your planned application. And the proliferation of social media, like LinkedIn and work-type-related pages, also help users find examples.
Choosing the right solution is application by application. For example, why do construction companies often put up a base at construction sites for RTK instead of RTN or PPP? “Many construction companies are working with fine vertical. They are always going to try and use the solutions that give that absolute best vertical solution,” says Mackie. “If they’re roughing something in, there’s no worry at all for using a network or even some kind of PPP. But once again, for much finer grading, short-baseline RTK is extremely reliable. Assuming the base doesn’t get stolen. And it’s a known technology that provides the accuracy they need.”
Then there are the numerous choices of PPP positioning services providers and multiple flavors from each. Which to choose for which application? For an insider’s view, I asked Michael Bruno, an engineer who has worked in both the realms of differential and PPP solutions and is now a program manager in Trimble’s Advanced Positioning group. Like other PPP providers, they offer various tiers of PPP solutions. Theirs are all under the blanket term “Trimble RTX”, for different end uses, differentiated by precision ranges and price points.
“We have one flavor that is the same kind of precision users were used to with DGPS, for sub-meter, we call it ViewPoint RTX,” says Bruno. “This is good for the kind of field mapping folks do for certain GIS layers, asset inventory, rough mapping. And it’s at a low price point.” For farmers looking for half-meter pass-to-pass precision, there is RangePoint RTX, “This is widely used in Agriculture; there are broad areas of the US and Europe for instance where the majority of our ag users are using RangePoint RTX to achieve better performance than SBAS in the region. It is very affordable for the benefits they receive.”
“For some asset inventory and mapping, users are looking for tighter than sub-meter, but do not need survey grade, for this, there is FieldPoint RTX,” says Bruno. Again, the precision goes up, the price goes up. And then there is the tier that is essentially survey grade, centimeters. “CenterPoint RTX is our top tier for real-time, and we also have an online portal for automated post-processing for situations where you might not have a view of the L-Band sats (those GEO sats deliver the RTK PPP data),” adds Bruno. “CenterPoint RTX gets you 2cm once it is converged, and this is where the newer ‘fast’ service comes into play. All of the RTX tiers come from data we get from our global base network, and the old convergence times were measured in 10s of minutes. Years ago, we started adding data from more stations, like in the U.S. Mid-West where we have a lot of ag users, and these semi-dense networks did not have to be as close together as RTN stations but provided enough additional data to get the convergence times down to well under a minute.” The fast service now covers most of Europe and the U.S.
If PPP is now capable of survey-grade precisions, are providers urging users to simply move to PPP instead of differential solutions? Like some other providers, Trimble has PPP services, operates their own RTN (e.g., VRS Now), and also sells software for public and private RTN. Are these services at odds, or do they complement each other depending on user needs? I asked Bruno how they would advise different users on which to use. “CenterPoint RTX, especially now that it converges fast, can do many of things that users are used to with RTN, but there are limitations, like keeping the (GEO) sats in view,” says Bruno. “RTX has big advantages for areas where there is no cell coverage, as RTN rely on a cell to send corrections. But where there is cell coverage, and an RTN, surveyors for instance should simply use the RTN, and the PPP can fill in seamlessly where there are cell coverage holes.” This type of top tier, fast PPP are growing in emerging markets, says Bruno. “Our biggest user base is from ag, especially for RangePoint RTX, but now we are seeing a whole wave of different users for CenterPoint RTX. You can imagine uses cases like asset inventory and management some autonomous delivery services, and automotive autonomy development. The sky is the limit.” An example of the latter that Mackie mentioned was the Super Cruise system, making use of RTX and installed on many new Cadillacs, has just reached the milestone of over 6 million miles of hands-free driving.