RTK - Useful or not?

As a follow on from this discussion, I would like to pose the question - is there any benefit to using RTK GPS on our Ardupilot controled vehicles and what can reasonably be expected? In the thread mentioned, @ChrisOlson who I have to acknowledge, has significant experience related to this topic, seems to be of the opinion that the typical low cost RTK GPS that are becoming quite common place, offer no benefit over the standard GPS such as the M8N. This has not been my experience with rovers and I would like to get the opinion of others and even put it to the test. There are certainly limitations to using RTK and I have also seen people with unrealistic expectations, so what should be expected?

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Yes, I think this came about because a user hovered a helicopter with one of these systems being marketing to the drone industry and expected to see cm accuracy in hover. It don’t work that way.

There’s already RTK network-based drone solutions out there and they cost upwards of $10,000. But the network solutions do improve the distance error and have a known datum for traceability - where you have to either measure your base station or set up at a measured GCP to have traceability on any official survey data with a straight-line system.

So I think there’s some misconceptions as to the abilities of these systems being sold to the average drone consumer for $600-700. They will work quite in a tiny little circle of radio coverage, assuming you set up the base station correctly. But that doesn’t take into account (at least in the US) the FCC 601 license filing application fee ($100), frequency coordination fee ($125), engineering fee - usually around $600 depending on the specs on the radio and whether it not it meets the requirements for the FCC. Just to operate the transmitter.

There is a few FCC Part 15 certified RTK straight-line systems that don’t require a license to operate but they have a useful range of about 100-150 meters.

These cheap drone RTK systems are all the buzzword right now because everybody thinks it’s new technology. In reality it’s well over 30 year old technology, but marketing does wonderful things in selling product to consumers. They will very quickly be replaced by professional-grade systems using cluster or network-based correction for people who actually need RTK capabilities. Since the traceability of these cheap hobby-grade systems just isn’t there.

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For my rovers, I use the S2525F8-GL-RTK GPS receiver. This is about as low cost as it gets for full all-in-one RTK. I use an ESP32/GSM module to feed RTCM to the receiver via NTRIP. Corrections are sourced free of charge from PositioNZ This setup does not use Ardupilot to channel corrections to the GPS as my rovers typically operate without a GCS. The all up cost for this RTK solution including an active dual band antenna is about $125 USD. My rovers spend their time out in agricultural pasture on relatively flat terrain with a good view of the sky. For me, the difference between standard GPS and even just an RTK float solution, means the difference between reliably making it through a farm gate or not. In order to hopefully demonstrate at least some advantage to using RTK, I propose to add an M8N (of which I happen to have a few…) to my existing setup and manually navigate around the markings on a local sports field multiple times, over multiple days and then compare the GPS tracks compared to the ground truth. Hopefully this will be a start at least in determining the difference between what RTK GPS offers over standard GPS. I am open to suggestions that might make the comparison more useful.

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It must be realized that with aircraft GPS is quite reliable and accurate in the air. The closer you get to the ground the less reliable and accurate it becomes due to multi-pathing, etc… So using a local ground-based correction there is no doubt you can improve accuracy on the ground. This was done 25 years ago by John Deere with their GreenStar system for precision farming and tractors and combines with auto-steer.

However, these systems where you used to have to place a ground-based correction unit didn’t last very long in the market. About 2-3 years. The cluster RTK systems came into being about 15-20 years ago. Today, everything is 4G cellular (except for a few VHF/UHF clusters still operating in parts of the country) for the local correction.

The network correction systems provide a certified datum for repeatability and traceability of data. However, they are not free to use - they require a subscription fee. Except for Iowa which now has a free government-sponsored network-based RTK system.

The old WAAS-based correction system for GPS is still used by aviation for basic navigation.


Chris, some of the correction systems you discuss are currently pretty US centric. A lot of countries do not have comprehensive systems in place along the lines of what you are describing. That said, It is still my position that these cheap RTK GPS can offer an advantage anywhere. Using either your own local base station or any of those available within a few tens of Km with an internet connection. The issue of radio licencing is not particularly relevant. Most countries have frequencies set aside for this type of public use at power levels that allow a LOS range well over 1Km. I am pretty sure that this would cover the area of interest for the majority of people here. In my case, I use a cellular connection to the internet and this more than adequately covers my area of interest which in some cases can be well over 100 hectares. Regardless of how old the technology is, I am interested in whether or not there is an improvement to be had using these systems right now. I think the ‘new’ aspect of what we are seeing is the price point at which such systems can be set up, not the technology itself. For me, the $125 I spend (less the cost of a regular GPS) to get more accurate position information is the difference between my project being successful or not. Compared to the rest of my spend on hardware and in time, that is pretty insignificant. I am sure that there are others in a similar situation that could benefit.


What I’m wondering is if anybody has ever taken one of these units to a location with surveyed GCP’s, set the base station random and leave it set for the half day or so that it takes for the static GPS solution to properly survey the base station. Then fly a survey over the area, see how many RTK-floats you get and compare the data to actual survey data from the GCP’s.

How accurate really is it? I’d be highly surprised if you can get much better than 50-60 cm horizontal.

There’s only one in the US - 902 MHz frequency-hopping spread spectrum at no more than 20dBm. What frequency is the radios using in these things? 440? If so, you need a license and frequency coordination for it.

Chris, you are a little US focused. In EU you can use up to 500mW with 868Mhz without any license, which gives you quite a range. But the most important thing that you are overlooking is that you don’t need RTK. All you need is PPK.

Around 30min of data collection at the base station, and a nearby correction station data (not online but downloadable later) you can determine your base location with less than 50mm precision. (tested a number of times, verified against survey grade measurements). If there is no nearby correction station, then most providers offer virtual correction station locations, precision is still below 50mm with them.

Once you have your base location, just fly,and calculate your positions later, based on the collected raw gps observations. Calculated positions relative to the base station position will have less than 10mm precision. It is not good for flying with this precision, but perfect for surveying.

So we are getting a bit closer to reality with this statement. You are basically saying that you don’t know how accurate these units are yet you are making blanket assertions as to their usefulness. Even 50cm accuracy is a big improvement over what is otherwise available for some. Why are you determined to try and deny the utility of this type of result?

Only one if you disregard WiFi and Cellular mobile? Not all units even come with a radio. Even those that do still likely have the facility to interface whatever correction transmission facility that is most appropriate for the particular application. All of the cheap single frequency RTK receiver modules I know of at least have a serial port for receiving correction data.

Andras, PPK definately has its place but there are many applications where real time location is required, It doesn’t help my rover at all if I can determine the reason why it failed to navigate a farm gate 24 hrs after it hit the fence. There are plenty of UAV applications where real time location accuracy is required also.

OK I see.
To find the gate, have you tried navigation via optical markers ? (like precision landing)

Andras, I used a gateway as an example but the environment where my rovers are working requires accuracy for navigating a number of different obstacles. I am working on an auxiliary vision system that hopefully will include identifying gates along with a host of other objects as well, but at the moment I am just working on SLAM.

Because 50-60cm can be done with regular WAAS-corrected GPS.

The point is these systems are being sold as providing +/- 1 cm and this may indeed be achievable with a stationary survey unit. But ArduPilot is used in various types of vehicles. Once you start moving it introduces errors. So people are buying these systems believing they’re going to get +/- 1 cm in their drone or whatever. But the debate between farmers using precision ag is endless on whether or not WAAS or the subscriiption RTK networks are better. The ag industry is by far more experienced with precision GPS solutions than even the survey business. And the ag industry operates these high-end units in machines and aircraft. When these first ground units appeared on the market it was the only correction system there was. But then the FAA came up with WAAS and various commerical entities came up with RTK clusters and networks. It was quickly replaced with better equipment, and now it’s being sold to drone enthusiasts that are believing it is the best thing since sliced bread, and they are believing it’s the latest technology.

And that’s the point :grinning:

Saying this is no different than saying RTK “can” achieve 1-2cm. Significantly more than half the planet is not covered by WAAS

So, my take on what you have said so far…

If you live in the US, and are operating a commercial business flying precision missions charging multiple thousands of dollars an hour with profitability dependant on covering as much ground as possible in as little time as possible then your typical low end single frequency RTK GPS receiver tied to a LOS base station is not going to be of much benefit?

There is a whole bunch of folk round these parts that just don’t fall into that use category and your blanket statements on the usefulness of RTK GPS just don’t apply.

And that’s the point :grinning:

That is correct. It’s called EGNOS in Europe. SDCM in Russia, MSAS in East Asia.

I work in a business where I’ve seen these systems come and go. And I’m aware of their limitations. But I’m not going to get involved in a religious debate over it just because it became the new buzzword in the drone world when it’s been commonplace in other industries for years.

Still big chunks of the world missing… There is nothing religious about acknowledging the utility of a technology regardless of how old it is. The simple fact of the matter is, there are no such options here where I am and RTK does offer an improvement. I am sure I am not the only one. Is there really any harm in acknowledging this?

No harm at all. The thing is that the RTK correction system was never designed for navigation. But that’s what the drone industry is trying to use it for. So when we have users that expect it’s going to make their drone hover accurate +/- 1 cm, that’s not the way it works. But that’s what’s advertised.

Most people don’t realize they are already using an augmented GPS system with the augmentation signal coming from space. What introduces errors is various phenomena like multi-pathing of the signal. RTK does not mitigate problems like multi-path. Or interference from trees, buildings, structures, or even signal interference from shared radio services. So for navigation on an autonomous vehicle that’s moving around - obstructions, different attitudes (and antenna offsets) it is no better than regular GPS. This is why it was never used for navigation. It is primarily a survey and precision application tool that can mitigate GPS position errors with a properly set and surveyed base station for greater precision in survey measurements or pass-to-pass accuracy for precision application in agriculture.

So let’s look at your rover application where you’re trying to make it thru a gate and hit the fence instead. Being you’re on the ground you’re getting a lot of error due to various factors that affect GPS. But that gate opening has a very specific set of geographic coords that don’t change. So you set your RTK base station and if you let it only calculate the static GPS solution for 10 minutes you are no more accurate (+/- 2-3m) than regular GPS. Because your base station does not yet know its precise location. That base station has to sit there and get the static GPS solution for about 10 hours before it can provide the corrections necessary to hit the same wheel tracks every time, accurate to about +/-10-20 cm.

And it’s only going to happen if the base station is set correctly. And that includes antenna offsets and taking care to mitigate all the common problems that affect GPS accuracy. Anybody who has ever set one up on a professional basis can attest to this. To get the actual +/- 1 cm takes a pro that even measures the antenna offset.

This is where the clusters and networks came in. They are more accurate, providing a pre-surveyed correction, and mitigate the problems with having to have a surveyed static GPS solution for a local base station. If you are a surveyor and go to a site you want to set up and start surveying right away. You might be in an area where there is no previously surveyed GCP’s. And you don’t have time to wait for your ground station to get a surveyed static GPS solution. The clusters and networks provide that (for a fee) instantly. And you don’t even have to own a base station. And that’s what make me raise eyebrows with the drone industry selling these things for navigation.

So what I’m pointing out is the inherent limitations of GPS, and that includes any of the augmentation systems. The average user who thinks RTK with a local base station is going to provide +/- 1 cm in relation to geographic features just by plugging in a ground station and go fly has been misled. It is no better than regular GPS unless it is set up by a pro that knows what they’re doing.

Even if you survey a particular set and go back to that same set later, it takes extreme care in setting the antenna for the base station to get repeatable results.

So let’s consider the average drone flyer that goes someplace and on this flight they want to fly their drone precisely between a couple trees with only a few cm to spare on each side. So they do the flight plan in the ground station and upload it. Set up their RTK base station by setting the antenna on an upside down 5 gallon bucket. Take off, flip into auto, the drone gets to the trees and smacks dead into a tree. Why? Because Google’s maps aren’t 100% accurate. And even if they were, when the drone flew between the trees it got GPS position error. And even if it didn’t get GPS position error due to signal refraction or multi-path, the RTK base station only knew where it was on the face of the earth accurate +/- 1-2 meters because it wasn’t properly surveyed and set up right.

May as well just use regular GPS. You get the same result.

Chris, first of all, thank you for sticking with the conversation even though I am somewhat vigorously defending a differing opinion.

I agree that in a general sense, navigation over large areas that are not already covered by a correction system, is not so suitable for RTK GPS. That is not to say that there are not many situations where RTK is IS suitable.

Other than the speed at which you might leave the effective influence of a base station, I am not aware of any limitations of RTK per se, that apply to a navigation. As with all GPS, there are situations where it just isn’t going to work and we have to manage these regardless.

Let me tell you about my rover application since you have gone out of you way to make it sound quite cumbersome and ineffective. My ‘base station’ does not require any setup at all as it is part of a continuously operating network that is freely available as I have already posted. This base station is set up on a first order survey mark and as such, is about as accurate as you can get. I have used this base station at up to 30km away with better than 10cm accuracy with a RTK fix solution. I can plonk my rover down any where within approximately 3000 square kilometres and achieve this with no prior setup. Communication is via a cellular data. Right at this point, within minutes of arriving, I can record way points using the rover itself and have it reliably repeat the intended path. Using this same RTK system I can survey the operational environment to establish navigation boundaries and automate complete coverage of pasture within a field. Generally my rover knows where it is to better than 10cm with a fix solution and better than 1m with a float solution. With standard GPS this entire system just was not reliable enough. I would have to leave wide margins around fences, would have large holes in my coverage and could not reliably navigate autonomously through gateways between fields. The RTK system I use costs less than $100 more than standard GPS.

With regular GPS I do not get the same result.

OK, you’re using the network solution. You’re not using a standalone base station with a VHF/UHF radio link. And you have a surveyed spot for your base. As I have said this has been proven to be more accurate and much faster. It works in my airplanes at 140kts application speed with about 1.5 second latency on actual position vs GPS position.

You are also not using the setup being commonly sold to drone enthusiasts with a standalone straight-line base station with a radio link.

I’m sure you’re aware of all the hassles required with setting one of those up properly.

Chris, at least now we can agree on what a useful RTK setup can look like.
With surveying we setup base stations on regular (if not daily) basis. For the average person who is looking to setup a precisely located base station, it would be all you describe and more. Surveying carries a very large liability risk and as such, there are a multitude of checks and balances in place to verify accuracy. A surveyor will never establish a point in isolation but rather relies on an established network of permanent control marks typically buried in the ground. The predefined relationship between these marks allows us to add additional marks while tracking and minimising error.
Having said that, I am not so sure that a precisely located base station is required to be useful. In a survey job that requires a high degree of accuracy, a base station need not have accurate coordinates. We will often simply place the base station at a convenient, accessible and visible location and have it accept its uncorrected location. As part of the survey we locate and take GPS readings on existing known control marks and this defines the error which can then bee applied to new points of interest to get an accurate location. This is possible due to the RTK solution always being relative to the base station.
When I was initially testing RTK using the M8N and Rtklib, I mistakenly manually entered the base station location using minutes/seconds rather than decimal minutes. The accuracy and repeatability of the solution was fine however it was offset with respect to local survey marks I was testing against. When I switched to using the reported location in the RTCM data, it fixed the offset and I this is when I noticed the entry error. The point being, as long as you have a single fixed reference mark where your base station is set up, you can cover a reasonable area accurately and repeatedly. If it is sufficient to undertake your project solely in these terms and accurate global coordinates are of no particular interest then this can certainly be useful.
As for “not using the setup being commonly sold to drone enthusiasts”, I’m pretty sure that same system I am using is readily accessible using this gear as well. As far as I can see, all these systems accept RTCM corrections via a serial port of some sort. Anyone with a cellphone running an NTRIP client, OTG cable and an FTDI adaptor can have a standalone system using internet based corrections from a local station. Anyone running a GCS with an internet connection can do the same. I forgo the phone and use an ESP32 and a GSM module.
So instead of telling people why it doesn’t work, why don’t we tell then what does work?

Aerial application also carries huge liability risk with the EPA for not being on-target or even applying the wrong rate of a product.

I think we have a couple issues:

  1. the average drone consumer knows they have GPS. And most think GPS is 100% reliable (it’s not)

  2. we have somewhat deceptive marketing going on from manufacturers selling these consumer-grade RTK systems.

So what happens is somebody buys one of these things and sees the 1cm accuracy advertised. Oh wow - it’s way better than regular GPS! So they now expect that their drone is going to hover accurate to 1cm and they’ll be able to navigate around geographic features accurate to 1cm. Except that’s not the way it works.

RTK was not designed to navigate your autonomous vehicle within 10cm of a geographic feature because it can’t. As soon as you have movement involved you have measurement error. RTK was designed to provide repeatability (we call it “traceability”) for surveys and pass-to-pass accuracy for precision agriculture. The instantaneous DGPS solution is repeatable at any one point in time, assuming clear signals. But it drifts. If you make a ground track or pass one day, come back the next, and use the same GPS solution, it may be off by 2 meters. RTK will get that down to sub-meter.

Is the average consumer buying these things going to realize this? No. They don’t know how it works and all they see is the marketing claim.

It is only going to achieve the desired results by a limited set of professionals that know how to set up and use it. And those pros will likely select a more high-end UAV solution from Trimble or Honeywell Aerospace if their job depends on it.

And this is why I said for the people these thing are being marketed to, regular GPS is just as good. That is in no way shape or form trying take away from the capabilities of RTK correction systems. It is a simple observation of the people they are being marketed to not understanding the details of how to use it. So when we have a user that bolts one to a helicopter and hovers it and observes “it doesn’t really seem to be any more accurate than M8N”, of course not. That’s not what it was designed for. So where did that expectation come from? See problem #2 above.