RTK Experiments

Copyright © 2022, William D. Ricker, and licensed CC-BY-SA 4.0 or later CC-BY-SA.



We get a false sense of accuracy from phone, car, hiking GPS.

“Stay on road” and perhaps inertial corrections let the GPS display cover for the erratic measurements.

If we mark a point with a GPS and then ask for directions to get back there, it likely takes us to the right place close enough, as buildings/driveways are almost far enough apart to be mostly non-overlapping in error. Most people don’t need 1m, 0.1m, or 1cm accuracy.

If you download the “tracks” off a GPS that was left on in a parked car, you’ll see a random “star” as the position computed varies this way and that. A disturbingly large star, that shows that only with a very long term average could you compute a 1m position. My Garmin GPS peering through a windshield-shaped hole with trees and house partially blocking what little sky it could see drew a 50×15m (150×50’) error star for us. If the nearest house-lots weren’t vacant, many of the points in the star if chosen at random (whenever one hits the “save” button) the position wouldn’t obviously be associated with my house! But the centroid of the star-cloud is roughly correct (slightly offset away from the house by i presume reflections).

Driveway Error Star

Native GPS precision/resolution in Civilian mode is 30m; a 5 minute average can improve that to 5m, 12 hours average to maybe 1m? Dual Frequency L1+L2 and DGPS correction signals can improve that slightly, since the ionosphere bends L1 and L2 bands differently, but not by much. Military grade only improves on codeless L1+L2 20%. (They didn’t want us that close but algorithms are winning.)

If one wants to map things smaller than a house accurately, or things hard to see from medium distance away (10-50m) even dual L1+L2 & classical DGPS/WAAS isn’t good enough.

Which is why surveyors use GPS equipment that is capable of correcting for all the errors by using a continuous error stream from a near-by (practically) identical receiver at a known location. (Known to ±1cm preferably.)

Professional GPS

These devices cost less than new cars, but I’ve seen used cars for less lately. With a long-term occupation, it can get a ±1cm accurate position on the tripod, and then can share that accuracy with the portable GPS on a surveyor’s rod.

RTK is the latest walkabout form of enhanced GPS/GNSS position solution.

It’s called Real Time Kinematic because it corrects in real time, when moving.†

Simplified RTK network (from wikimedia © 2022 TS Eriksson - Own work CC BY-SA 4.0 )

In the simplest form, a base station on a precisely known location records phase data on received GPS/GNSS sat signals, both L1 and L2 bands, and notes the corrections - and sends them to one or more rovers subscribers - most simply via direct data radio if nearby, as illustrated. One can insert the Internet (via cellular data in field) and consolidating servers for (a) added complexity (b) only buying one unit.

Subscription may be commercial, open/free, private, or public by e.g. MassDOT, MaineDOT (free, registration required). One can connect to the nearest station of the network by name (having looked up which is nearest) or a network may provide a Virtual base by weighted-averaging several nearest bases to derive a virtual correction for your location instead of just giving the nearest. (This requires disclosing your location to the Network.)

†( Why bother calling it RTK? Because there is an alternative phase-difference enhancement scheme, requiring same high-end GNSS receiver hardware, that is non-real-time, non-kinematic, that compares logged received phases - logging for 12+ satellites in both L1 and L2 bands - at both a known base-station and an unknown station, logged over many hours, unmoving, with the log comparison and calculation done later to get just one (nearly) perfect location solution for the unknown base. Can be < ±1cm if close enough to the known base which was already known that well. This is great for professional surveyors setting up a chain of known positions extending from the nearest public base for their future use in their remote territory, moving their own base unit around. There’s also a compromise that doesn’t use Internet or data radios, so is not real-time but is still kinematic: One can also log raw data while moving around and capturing approximate GPS points, and compare raw field logs to base logs later, back in the office, to get corrected positions for the points collected in the field. )


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