This is blog 1 in the series “Real-Time GNSS Corrections: Productivity, Profitability, and Practical Considerations”, written by Jason Evans, Portfolio Manager, Trimble Positioning Services, NYS Registered & Licensed Land Surveyor. The blog series discusses important considerations for your GNSS surveying workflows: When should you use a base station? Are you missing the opportunity to gain productivity by including correction services in your workflows? Can differential base-rover RTK benefit from correction services? Reading the full series will not only help you to answer these questions, it will enable you to calculate the cost savings of a real-time GNSS correction subscription.
The ubiquity of real-time networks and globally available GNSS correction services herald a new era where traditional base-rover workflows may not be part of the surveyor's daily work as they once were. There are many situations where differential real-time kinematics (RTK) using a base-rover pair of GNSS receivers may be the best—or only—choice. However, many surveyors have already realized, such instances are becoming fewer, and the alternatives can bring additional time and cost savings. There have been great advances in alternatives to base-rover workflows—networks and correction services—but also in high-precision GNSS technology overall. If you haven’t already “gone baseless,” now might be the right time to visit the idea.
In the early days of GPS-only, high precision came with a premium on time. For instance, in the pre real-time era, where postprocessing was the only option, workflows involved many steps—and a lot of waiting; waiting for enough satellites to be in view, waiting for hours of observations, waiting hours or days to do additional sets of long observations, waiting for hours while static data files trickled through a serial cable, waiting days for precise ephemeris availability for download, and waiting to be back in the office to process and adjust. Despite the time required, GPS was an improvement over legacy terrestrial surveying workflows. Then along came real-time differential (DGPS), and real-time kinematics (RTK) with on-the-fly (OTF) initialization giving surveyors and mappers many more options such as those discussed by Gavin Schrock in his article “GPS/GNSS Corrections: The Right One for Your Needs”1.
The first commercial differential RTK base-rover system was released by Trimble in 19942. These RTK systems from the earliest days connected the base and rover via dedicated, and often built-in, radios. By the end of the 1990s, network RTK, like Trimble VRS Now™ was developed, and started spreading rapidly in the form of local, regional, statewide, and nationwide real-time networks (RTN) that provided real-time GNSS corrections. These RTNs have evolved to be referred to as a new form of “utility”, and in many cases, to also serve as essential geodetic infrastructure.
Current Trimble Services
Developments over the past 20 years have brought even greater availability of real-time correction services that provide adequate accuracy for nearly all surveying tasks including construction site surveying. This includes broad regional commercial RTN, such as Trimble VRS Now service, and global precise point positioning (PPP) correction services, such as Trimble CenterPoint® RTX correction service.
The Trimble VRS Now service delivers instant access to RTK level positioning services using a network of permanent, continuously operating reference stations covering large areas3 of North America, Europe, Australia, and New Zealand, and continues to grow with dense networks of reference stations. Precise point positioning (PPP) services, such as Trimble CenterPoint RTX real-time GNSS correction service, are global in scale and offer real-time, centimeter-level corrections almost anywhere on the planet—even in places such as the Australian outback, the American Great Plains, and remote areas of African Rwanda, where there may not be cellular data and internet access.
GNSS receivers that leverage correction services have delivered high precision and accuracy at a faster rate, simplified common workflows, and provided instant ties to geodetic reference frameworks. In many instances, surveyors and mappers can start working as soon as they get out of their truck. Correction services can mean less up-front investment as only a single rover is needed, or a base-rover pair can be repurposed as two rovers.
A quick look at Trimble VRS Now and Trimble RTX correction services:
Accuracy: 2 cm (< 1 in) horizontal / 3 cm (< 1.5 in) vertical
- gives instant access to RTK positioning services using a network of permanent, continuously operating reference stations.
- has existed for over 20 years and provides a dependable network RTK correction for the rover receiver.
- requires a two-way cellular data connection.
Accuracy: 2 cm (< 1 inch) horizontal / 5 cm (<2 inch) vertical
- advanced PPP technology that provides real-time, centimeter-level positions.
- enables true mobility allowing the work without the constraints of a local base station or VRS network, with high-level accuracy delivered worldwide via satellite or Cellular/IP.
- provides a globally available RTK correction for the rover receiver.
The case for a base—or not
Let’s look at reasons why some surveyors and mappers have been hesitant to go baseless and the Trimble solutions that need to be considered.
There are other elements of GNSS workflows to consider, including: time, equipment costs, the error budget, site control, geodetic references. In subsequent blogs we examine these elements and how the evolution of the technology and the aforementioned services warrant a reexamination of legacy workflows.
A few additional notes to consider: Trimble VRS Now and Trimble RTX services are staffed 24 hours a day, 7 days a week to monitor network performance. Trimble correction services have an uptime rating of 99.99 percent, which is probably an improvement for most work crews, unless they have never forgotten to charge a battery or turn the radio on before driving off (never!). And finally, the number one blunder in GNSS workflows that has remained a constant since the start: the height of the receiver/antenna, especially the base. When receiving a correction, reference station heights have been rigorously measured and remain unchanged. Out in the field, sometimes the measure-ups are miscommunicated or done inconsistently: “Do I measure to the ARP, or bumper? Where is the measure up mark on this one? Where is the note in the field book? Where is the field book?”…
It’s sometimes tough to reach the accuracy level of a differential base-rover RTK solution over a short baseline or with great changes in height and therefore potentially great changes in atmospheric conditions. Your base-rover will likely be a dependable tool in your toolbox for years to come, but don’t depend on one tool for every job where the potential for cost-savings is evident.
In the next blog post, The Value Proposition of Going Baseless, we take a closer look at some typical workflows.