This is blog 2 in the series “Real-Time GNSS Corrections: Productivity, Profitability, and Practical Considerations” written by Jason Evans, Portfolio Manager, Trimble Positioning Services and 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.
How much time could you save by using a real-time GNSS correction service as part of your regular survey workflow?
So you’re considering taking the leap, and leaving the base in the truck—or even back at the office—in favor of using a real-time correction service? This blog explores how much time and money you can expect to save leaving that base behind. We will review some common workflows and examine how a correction service might work as a replacement for your base, or as a workflow augmentation that could save you time and money. To help you evaluate such options, we’ve included some tables with example calculations of savings and invite you to use your own values to easily determine return on investment.
Switching to a real-time GNSS correction service will deliver varying ROI depending on the type of work, the project-specific challenges regarding establishing control, and the logistics of setting up a base. For many applications where you set up one base per day, you could save up to an hour or more daily, as one surveying and engineering firm noted in a case study about pipeline construction staking1.
If you’re not using a real-time GNSS correction service, you will likely be setting up a GNSS receiver as a base station for traditional base-rover differential real-time kinematic (RTK) surveying, whether measuring points or staking points.
Prior to setting up, you’ve probably determined which positioning/control approach you will be taking for your specific project:
- Known coordinates when setting up over an established control mark with published coordinates on a stated geodetic datum. For instance, in the U.S. this might be a National Geodetic Survey (NGS) monument, state or local monument, or setting up over a job site control point that was provided.
- Unknown coordinates when you require establishing a control point. Establishing coordinates through long, static observations and postprocessing later with office software like Trimble Business Center, or with online postprocessing services like the NGS Online Positioning User Service (OPUS) in the U.S., Trimble RTX Postprocessing, or other PPP services. Many real-time networks also offer online postprocessing services. Establishing base coordinates using a real-time correction service is also an option.
- Unknown coordinates where performing a site calibration is necessary. Setting up a base somewhere safe where it will not be stolen or disturbed, and then measuring existing control points on the project site for orientation.
Different control approaches require different amounts of time to execute, which we’ll examine later in a subsequent blog post: Bases, Correction Services, and Control2, however no matter which method you use to establish the position of your base, you will have to set it up first, so let’s explore what time that requires.
Base and Rover Setup
Generally these steps would apply to all GNSS receivers that are able to be used as a base station if they have an internal UHF or Spread Spectrum / 900MHz radio. These radios are typically broadcasting at around 1 to 3 watts, which is usually capable of pushing the signal ~1.5 to 3 km (1 to 2 miles), depending on topography, interference from competing signals and obstructions such as trees and buildings.
If the site or survey area extends beyond this range, an external radio can be added to increase broadcasting power. For example, in the U.S., these radios tend to be around 35 watts, and while subject to licensing restrictions, are capable of broadcasting ~15 to 30 km (10 to 20 miles). External radios are of subject to the same limitations of topography, interference, and obstructions. Be sure to license your radio to avoid fines levied against pirate broadcasters.
Additionally, you’ll need to calculate the probable error at the extent of job site:
- for horizontal error, 8 mm + 1 ppm would result in an error of 8 mm + 30 mm if you were 30 km away from your base (38 mm, or roughly 1 in. or 0.12 ft)
- for vertical error, 15 mm + 1 ppm would result in a vertical error of 15 mm + 30 mm at the same distance (45 mm, or roughly 1 in. or 0.15 ft).
However, as you increase the distance between your base and rover, using network RTK solutions like Trimble VRS Now will generally perform better over similar baseline lengths. After all, this was one of the reasons why network RTK was developed—to overcome the degradation-over-distance of single-base RTK.
Setting Up an External Radio
After you load the empty cases and bags back into your truck, you’re 20 to 30 minutes into your day, but you’re not ready to be productive just yet. After setting up a GNSS receiver as a base, you will need to set up a second GNSS receiver as a rover, verify that you’re receiving the radio correction signal, and perform some checks to make sure that you’re working accurately.
If you were using a real-time correction such as the Trimble VRS Now or CenterPoint RTX service, you would already have your rover set up and you would have verified that you are working accurately.
Breaking it down
At the end of the day, you’ll need to break down your base station and pack it up to go home. Packing up might include stopping the logging of GNSS data, transferring the file to external storage or to the cloud, drying the gear off after a rainy day, or cleaning it off if you’re on a muddy or dusty job site. You’ll certainly want to coil up cables so that they don’t look like tangled Christmas lights when you take them out tomorrow. You’ll pull batteries so that you can get them on charge overnight. And you’ll close up tripods, disassemble antenna masts, latch the cases, zip the bags, and finally build a protective fortress around your base station control point with indestructible lath and flagging so that you can leave the site knowing that
nothing can possibly disturb that point.
Putting it All Together
This table adds a current typical hourly rate to the set up and tear down times we discussed above, the annualized savings from “ditching the base station” offsets the cost of a real-time correction service.
*USD Based on a recent published rate for an ‘Operating Engineer Heavy/Highway Party Chief’ prevailing wage + supplemental for New York State
or click this link for your own calculation: Online Calculator
Completing the table above with your business numbers shows how the time-cost of using a base station adds up over the course of a year.
There are situations where differential RTK using a base and rover make the most sense; however, treating it as the only solution can add up. Additionally, if you’re paying someone to guard your base from theft or disturbance, obviously the cost will increase. You may also want to consider travel time to/from the base station, or to a checkpoint, or to conduct base station location reconnaissance. There is also the cost of your broadcasting license (where applicable) if you use external radios. Pirate broadcasting without a license can lead to fines, and those can be calculated back to the start of the project.
Finally, consider the opportunity cost of spending time where you could be saving it. How many points could you have staked while you were setting up your base station? What other work could the crew member you left to guard the base be doing? How many measurements could you have taken for your as-built survey instead of driving over to your check point? How many other construction site workers and activities were on hold, waiting for you to get set up, so that you could stake their points?
In the next blog post, Bases, Correction Services, and Control, we examine another element of the value proposition for going baseless.