Using PPK in UAVS for georeferencing

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#1
Is there a surveyor out there who could help me understand fully how PPK in UAVS works?

I understand most of the theory but I do not understand where the GNSS receiver fits in? I am okay that a RTK system needs a GNSS receiver for real time satellite connection, but PPK does not use real time satellite connection. So what is the GNSS receiver for in that case?

I would appreciate if someone could lift the fog.
 
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#2
Is there a surveyor out there who could help me understand fully how PPK in UAVS works?

I understand most of the theory but I do not understand where the GNSS receiver fits in? I am okay that a RTK system needs a GNSS receiver for real time satellite connection, but PPK does not use real time satellite connection. So what is the GNSS receiver for in that case?

I would appreciate if someone could lift the fog.
------------------

Hi, Real Time Kinematic vs Post Processed Kinematic will both produce excellent results, as will accurate Ground Control Points. Real Time Kinematic, does just that, it processes the GPS data in real time, with the use of a fixed, base station GPS reciever & UHF/VHF radio transmitter at the known control point (that broadcasts the GPS correction) and a UHF/VHF receiver on the drone (that receives the correction). RTK is very expensive, it produces excellent results, and it will produce the fastest results.

Where as PPK uses uses a fixed GPS control point base station with NO radio system, all of POST processing and correction of the GPS signal is done in the office computer, after the images have been collected. RTK is less expensive, it will produce excellent results and all of the GPS data will need to be POST PROCESSED in the office to correct the GPS location data (more time in the office).

For the average "Joe-pilot", similar / better results can be obtained via accurate Ground Control Points. Here is a good article that explains the process & results in layman's terms:

Do RTK/PPK drones give you better results than GCPs? - Pix4D

With ACCURATE, well spaced, Ground Control Points, you can equal or exceed the precision and accuracy of RTK and PPK. GCPs will be the most labor intensive, as setting accurate GCPs can take a lot of field time, but the results are excellent.
 
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#3
------------------

Hi, Real Time Kinematic vs Post Processed Kinematic will both produce excellent results, as will accurate Ground Control Points. Real Time Kinematic, does just that, it processes the GPS data in real time, with the use of a fixed, base station GPS reciever & UHF/VHF radio transmitter at the known control point (that broadcasts the GPS correction) and a UHF/VHF receiver on the drone (that receives the correction). RTK is very expensive, it produces excellent results, and it will produce the fastest results.

Where as PPK uses uses a fixed GPS control point base station with NO radio system, all of POST processing and correction of the GPS signal is done in the office computer, after the images have been collected. RTK is less expensive, it will produce excellent results and all of the GPS data will need to be POST PROCESSED in the office to correct the GPS location data (more time in the office).

For the average "Joe-pilot", similar / better results can be obtained via accurate Ground Control Points. Here is a good article that explains the process & results in layman's terms:

Do RTK/PPK drones give you better results than GCPs? - Pix4D

With ACCURATE, well spaced, Ground Control Points, you can equal or exceed the precision and accuracy of RTK and PPK. GCPs will be the most labor intensive, as setting accurate GCPs can take a lot of field time, but the results are excellent.
Thanks for taking the trouble to reply. So in a PPK flight, if all of the post processing corrections are done back at the office, what is the GNSS receiver used for? You see, in all the sketches and explanations I have seen, I have the impression that the PPK drone gets its position from satellites, not from the GNSS receiver. I still don't understand what part the GNSS receiver plays. Does the GNSS receiver connect with the PPK drone at some point during the workflow?
 
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#4
Thanks for taking the trouble to reply. So in a PPK flight, if all of the post processing corrections are done back at the office, what is the GNSS receiver used for? You see, in all the sketches and explanations I have seen, I have the impression that the PPK drone gets its position from satellites, not from the GNSS receiver. I still don't understand what part the GNSS receiver plays. Does the GNSS receiver connect with the PPK drone at some point during the workflow?
I guess u need another GNSS receiver attched at drone to process PPK. Hence u correct every take with a base station.
 
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#5
The PPK solution needs two gnss receivers. One at the ground called base station and one in the uav called rover. The base is stationary in a known location and the rover moves around collecting points which in case of uav the points are every photos position. Then taking the base and rover rinex files you process them using special software and determine the position of the points or photos in our case with cm accuracy. This way you don’t need gcp’s. In my opinion as a surveyor you will need at least 2 or 3 gcp’s to be confident about the results.
 
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#6
Is there a surveyor out there who could help me understand fully how PPK in UAVS works?

I understand most of the theory but I do not understand where the GNSS receiver fits in? I am okay that a RTK system needs a GNSS receiver for real time satellite connection, but PPK does not use real time satellite connection. So what is the GNSS receiver for in that case?

I would appreciate if someone could lift the fog.
You've gotten some "sorta accurate" info so far. But to your question.

The GPS receiver in your phantom (or any other off the shelf drone, including DJI's current RTK offerings) is not very accurate. It will record the position of the drone when it snaps an image, but in may be meters off from where it "really" is in 3d space. So, a more accurate gps receiver, commonly referred to as a "gnss receiver", is installed on the drone and a timing connection is set up so that the exact millisecond the image is captured is logged on the gnss receiver. You do not need to have an additional gnss ground station as was mentioned above because the Post Processing is most commonly done with the log provided by a CORS network station closest to the subject location.

Back at the office, so to speak, the "Post Processing" begins. You use the log from the CORS network to adjust the positions, based upon time, in the log that was recorded by the gnss on the drone. Then, you use proprietary software, or a script of your own making if you're handy, to go into your captured images exif data and take out the gps data recorded by the drone's gps and replace it with the gps data that was captured by the gnss receiver and corrected by the CORS station log.

Now, when you process your images in Drone Deploy or Pix4d, or Agisoft, you will end up with centimeter accuracy instead of up to several meter accuracy because the location of the drone when it snapped the picture is now very accurate rather than being up to meters off.

This is how PPK works in a nutshell and why you need a gnss (very accurate) gps receiver on the drone to make it work. The factory gps receiver on the drone is not near accurate enough. In addition, capturing the exact millisecond the image is recorded is critical.
 
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#7
The PPK solution needs two gnss receivers. One at the ground called base station and one in the uav called rover. The base is stationary in a known location and the rover moves around collecting points which in case of uav the points are every photos position. Then taking the base and rover rinex files you process them using special software and determine the position of the points or photos in our case with cm accuracy. This way you don’t need gcp’s. In my opinion as a surveyor you will need at least 2 or 3 gcp’s to be confident about the results.
Thank you, Sir. I understand the system perfectly now. It is very much appreciated. References to the system in other media are pretty vague. I can now read on with confidence.

I appreciate your comment also about GCPs. That was another question on my mind.

Thanks to all of you who took the time to respond to my OP.

Stephen
 
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#8
Thank you, Sir. I understand the system perfectly now. It is very much appreciated. References to the system in other media are pretty vague. I can now read on with confidence.

I appreciate your comment also about GCPs. That was another question on my mind.

Thanks to all of you who took the time to respond to my OP.

Stephen
Actually, vgo195's description is most commonly used in the "RTK" model not "PPK". Glad you understand though. ;)
 
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#9
My description fits both RTK and PPK. The use of COORS is not free in all countries and requires a subscription. Also the common PPK drones including some modified Phantom 4 Pro’s use L1 only gnss receivers so small baselines are preferred that’s why I propose the use of a base station. The nearest COORS station could be Km’s away and if there is no VRS option you can end up with bad results. Except if you install a L1/L2 receiver in the drone which will cost a lot.
Of course there are more details in the whole PPK solution but Unique is not a surveyor so a simple explanation of how the system works is better for him IMHO.
 
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#10
It's all good. Actually, the OP only asked about the need for a precision gps on the drone anyway.

Here in the US, CORS subscriptions are mostly not free, but the logs are almost always free and the coverage is quite good. And the beauty of using network logs (if available at a workable cost) is that no surveyed or "known points" are required on the job site at all.
show up, fly the site, and leave.
 
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#11
Thanks Guys. I really appreciate your expertise. Your profession is something I always respected and I am glad to have had the opportunity to learn from you. My interest is in Reality Modelling and the whole '3d models from photos' (and LiDAR hybrid). I am no Pro, but the advent of half decent cameras on quadcopters has opened up an interest for me. I didn't even know what RTK was until a year ago.

Thanks again to all.
 
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#12
My description fits both RTK and PPK. The use of COORS is not free in all countries and requires a subscription. Also the common PPK drones including some modified Phantom 4 Pro’s use L1 only gnss receivers so small baselines are preferred that’s why I propose the use of a base station. The nearest COORS station could be Km’s away and if there is no VRS option you can end up with bad results. Except if you install a L1/L2 receiver in the drone which will cost a lot.
Of course there are more details in the whole PPK solution but Unique is not a surveyor so a simple explanation of how the system works is better for him IMHO.
is it possible to extract rinex log from P4A or P4P?
 
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#14
is it possible to extract rinex log from P4A or P4P?
I wish it could but unfortunately i haven’t found anyone do it.
To be able to read raw gps data the receiver has to be capable of this.
The accuracy of the best survey grade receiver in autonomous mode is the same as the low end receivers. What matters in gps accuracy is the combination of good receiver and good antenna and of course the ability to read raw data and process them with another base station.
 

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