The hazards of mixing RTK bases

Single-base RTK is an excellent choice for many uses but mixing different baseline lengths can yield inconsistent results

By Gavin Schrock, PLS

Gavin Schrock, PLS

The surveying lead for a construction firm started getting calls from his crews — suddenly they were not checking in to existing control with the accuracy required. This presented a conundrum and an immediate resolution was needed to stay on schedule. What had changed? A nearby permanent base, part of the regional real-time GNSS network (RTN), had suddenly gone dark, and when the crews switched to other bases, they got the inconsistent results. Time to call the RTN. (See a primer on RTN.)

I have been operating a regional cooperative RTN for 19 years, and I get these kinds of support calls regularly, but typically only from users of the single-base mountpoints. Most RTN provide, via NTRIP casters, both network RTK (NRTK) solutions — such as master-auxiliary, VRS and FKP — and single-base solutions for each base. The base they had been using was down while the roof of the city building on which it is mounted was undergoing some maintenance.

The construction firm, halfway through a multi-year transportation project, had used the base when they established project control, and for layout and as-built tasks. Using the base, which was slightly more than 4 km from the site, the crews were used to seeing check-in results of 0.3′ (9 mm) or better (horizontal). When they switched to different bases, 23 km and 25 km distant, the results were now inconsistent, and in many instances, double.

This was an easy fix. We met on site and checked results using the network solution; it closely matched the results they were seeing from the original base. Until the original base was restored, this would meet their needs.

It made a lot of sense to use the nearby base, as setting a temporary project base on the congested and sky-view challenged site was impractical. Furthermore, the baseline length of 4 km yields excellent results. Single-base RTK is a powerful tool, and a default for many construction projects, provided that:

• the base has an unobstructed view of the sky
• the base is free of nearby multi-path hazards
• the base receiver and the antenna are of the same or better quality as the rovers
• the base receiver and the antenna support the constellations and the signals desired.

In many ways, it is hard to beat single-base RTK. For instance, if you set up a base right on the site, say less than a kilometer away, this should yield the best results possible for RTK, and can be better than network RTK.

However, there are challenges. Single-base, typically “iono-free” solutions common in today’s rovers, degrades over the baseline length. The rule of thumb for many is that the degradation becomes noticeable when baseline lengths exceed 10 km. It is not uncommon for rovers to fix at much longer baseline lengths; 20 km, 30 km, 50 km or more — but results will likely vary from hour to hour or day to day. Changes in ionospheric and tropospheric conditions can bring inconsistencies, particularly over longer baseline lengths.

Network RTK may not beat single-base over very short baselines, but as it uses 5 to 15 bases (depending on the implementation) it can better model in the varied conditions. It can provide great consistency and repeatability, even if an individual base is unavailable, as was the case for this conduction site. There are strengths and weaknesses for both. NRTK brings consistency over a wide area, you do not have to set up (and guard) your own base, and the geodetic values are solved.

If you can have an on-site base, you can under certain conditions see a gain in results. This is especially important for certain applications, such as machine control and precision agriculture, for which tight year-to-year and row-to-row repeatability is key. However, if you may need to use another base at some point, you may be better off starting with NRTK, if it yields the results you seek.

Gavin Schrock is a practicing surveyor, technology writer, editor of xyHt Magazine and operator of a cooperative GNSS network.