I am a newbie, but I can confidently say that for a practical application of ArduROVER where accuracy of at least 30 cm or so is needed, RTK seems to be essential. I have a zero-turn lawn mower that I am using to mow large fields. I still need to perfect the tuning to some degree but I know that if I lose RTK fixed, the mower misses streaks. With RTK fixed, it mows very precisely (except for my tuning issues). Just my two cents.
Now here is an excellent example of the same thing as precision ag application. Pass to pass accuracy. Since you have a static setup where the base can get an accurate surveyed static GPS solution, it works. If youāre getting more floats than you like, try getting your base antenna higher to keep a cleaner signal and reduce multi-path errors close to the ground.
Multi-pathing affects RTK corrections just as much as it does DGPS.
Thatās a VERY cool setup, BTW. I might have to build one of those.
Hi Kenny, very interesting and well done! There are many similarities in our applications. What GPS unit are you using? Where are the corrections sourced from? How are they sent to your rover? Do you use a companion computer?
Thanks! I still have a lot of improvements, especially just making the installation more robust, but it is so much fun to see it run (and useful!) that I canāt seem to stop to make it better! I guess this winter, maybe I will find the time to do so. I have nearly 100% RTK Fixed with my setup. I have my base antenna about 15 feet off the ground with no close objects.
And I agree, that the key for RTK being useful and not much trouble is really when you can setup the base once and your done. Although, I find it not much trouble at all to do another survey-in if I need to move it. But actually, I am really not concerned about absolute latitude-longitude accuracy. What is more important is that it is relatively close to Google Earth images (in Mission Planner). So what I have done is installed the base at the corner of an object I can identify accurately on Google Earth. Then I keyed in that position in the base.
I hope to do more videos soon explaining the changes I have made since my first couple of overview videos. Butā¦ having too much fun right now.
Butā¦I am a BIG RTK proponent!
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Oh yes, I have to admit this is a good one!
Have you ever left your base set for 10 hours or so and see if Google Earthās georeferencing was actually good? At 10 hours the static GPS solution should be down to less than 20cm. Iāve found instances where Googleās georeferencing has been off by 30 meters. They use USDA imagery in the US quite a bit, which I shoot a fair portion of. But sometimes when the imagery is processed obliquing of the camera, or the GPS unit on the camera (the camera has itās own RTK GPS) going to RTK float throws it off.
Chris, if that is truly your position, then wouldnāt you have problems advocating the use of Ardupilot generally? The problem of marketing hype is universal. From my point of view, education is the best cureā¦
If one thing is for certain - you can not rely on the accuracy of Google Maps on the scale we are talking about! Even when some points on a given image are close, distortion due to perspective and vertical elevation can be significant. Just look at some tall building and how the roof outline can be significantly different from its footprint on the ground. Like Chris says, sometimes you can be tens of meters offā¦ @ktrussell, is there any reason you could not determine the limits of your area of interest by using the āsave way pointā function and by driving the mover around the perimeter?
Chris, my base stays on all the time. It has been on for many days. I really have only begun using it for serious mowing in the last week or so. Before that I was just tuning and working out bugs in the middle of a field. I canāt say how accurate or stable the Google Earth images are for me from day to day. I am being very cautious and only letting the mower work in large open areas at the moment.
And Jim, I absolutely can and have used the save way point feature. I expect that will be the way I will do things when I get a routine established.
Kenny, a great example of the benefits of rtk 
Why? ArduPilot is an excellent autopilot. And I do have problems advocating use of certain features of ArduPilot, which I believe depend too much on automation in critical flight stages for helicopters. Especially from a safety standpoint with the larger more powerful piston machines I fly. And Iām quite vocal about why I think that. The development push is to have the system be fully autonomous, which is fine. But it breeds complacency with a system that must be supervised with manual intervention required when it screws up. And one of the major areas where it screws up is related to the weak link - GPS. The system depends too heavily on it. GPS is not 100% reliable and when a system depends on it the system is therefor not 100% reliable. It requires a human operator. Itās not to Terminator III-Rise of the Machines quality yet.
Why? Because RTK is an excellent mechanism for improving GPS accuracy but you are concerned that people are ignorant, have unrealistic expectations and that there could be unintended consequences. The same rationale extends to Ardupilot itselfā¦
Letās not say āignorantā as that seems rather harsh. Letās say not aware of the limitations.
I tend to think along the lines of aircraft applications because thatās what I do. With ArduPilot and helicopters I strongly advise against using GPS augmented flight modes close to the ground because it is not reliable. Can RTK being used at a takeoff/landing zone for autonomous helicopters improve the reliability like the FAA once proposed for precision approaches (and eventually abandoned because it doesnāt work reliably)? Now, that I donāt know because Iāve never tried it. It seems to me that thereās other things could be done in the software to improve that, like limiting the satellite constellation to a certain number of degrees above the horizon below a certain altitude. Or totally disconnecting the GPS solution altogether close to the ground and just use inertial nav for the autopilot. Or use other types of sensors like optical, sonar, radar.
I might have to buy one of these CUAV systems and experiment with it to see if it can do a precision approach around obstacles for a helicopter, and do it reliably. In full-size it has been proven not to work with much more advanced equipment. But letās say a person operates a fleet of autonomous helicopters that do, say, some sort of delivery service in emergency situations or something. Can a prepared staging area with a takeoff/landing zone be done where the helicopters have to fly a prescribed precision approach to landing for traffic seperation? And use the local RTK correction solution to make the approach and landing reliable? The helicopters would be flying at slow speed on the approach. And transitioning from a safe altitude to critical altitude where DGPS is no longer reliable. That would be a fun experiment. I can think of a couple practical applications for that.
Thanks for this great discussion guys, I apprecite your experiences and knowledge, as Iām sure others do.
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Here is a simple comparison I have done using my RTK system and an M8N:
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A great discussion, thanks guys for the detail you have put into the RTK question.
I would like to add one piece of information for others reading this blog.
The discussion is about SINGLE FREQUENCY GNSS systems, L1 to be precise.
This is the common consumer grade GNSS gear that is so cheap these days.
RTK can certainly help these systems and get you down to the 5cm to 10cm range as @jimovonz has so thoughtfully shown in the great work he did with his example.
But single frequency GNSS cannot resolve the ambiguities involved with satellite location, no matter how good the RTK or location service fix is.
For Sub cm GNSS you will still need a multi frequency GNSS system to resolve those ambiguities.
I just wanted to add this to clarify the context of the discussion as being about single frequency GNSS only.
Thanks for taking the time to air this subject more.
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Gentlemen, I am a fixed wing operator, large combustion engines and I am very pleased to have this type of information made available. I thank all of you for your time and effort here. As Chris had pointed outā¦ I was about ready to get a GNSS with RTK for my Ground Control Station in anticipation of those 1 cm accurate spot landings. Thanks for saving the time, money and disappointment. This being saidā¦ I think a 20 to 30 cm radius landing would still beat the 3 meter alternative. If I can set up an RTK linked to a cell phone and get a fix in 2 minutes as Jim Ovonz has done in his sample comparison then that is a worthwhile move in my opinion. as Chris said, itās network based. I have to admit I dont have an in depth understanding of this yet, my question remains at to the what are the network provided fixes one must pay for and the potential of RTK being ineffective due to the fact the aircraft is moving and if speed is a factor as the aircraft flies at 16 to 22 meters per second. Anywayā¦ good information. Kenny Trussell, awesome gear thereā¦ please keep sharing those videos as I need one of those out here in Thailand and yours is inspirational. Thx gents. Craig.
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