One of the most important points when making photogrammetric flights with drones is that the information captured is georeferenced. In this article we will explain the RTK and PPK systems and which we recommend from Helixnorth.
The vast majority of drones used for photogrammetry applications have a GPS antenna for autonomous navigation. The same GPS information is usually applied to the georeferencing of the images, which are usually accurate to within 2-3m both horizontally and vertically.
Currently, this level of GPS accuracy is a standard, which depending on the project or needs may be sufficient. If this is not the case, we must apply control points in our photogrammetric software so that the resulting products have the required precision and accuracy.
Fig. 1. Checkpoint for a photogrammetric flight.
RTK and PPK.
GNSS receiver manufacturers have long been able to miniaturize and reduce the cost of the most advanced receivers, thus facilitating their integration into systems as small as drones.
They usually have two modes of operation: RTK and PPK, but what are the main differences, and which is better?
Both have in common that they require two sets of data to work: data from the Rover (in our case the drone) and data from a Base. That base can be your own and installed over a known point in our flight zone or a permanent base belonging (in the case of Spain) to the National Geographic Institute.
Fig. 2 Own base installed in the flight zone.
Fig. 3 Network of permanent stations of the Spanish IGN and the Portuguese RENEP.
Those data sets obtained by both antennas must be processed by an algorithm to yield a solution, in this case the position of the drone with centimeter accuracy at the time the image was taken. In this way, we will obtain accuracies with errors of a few centimeters.
Fig. 4 Chart of errors on a PPK flight.
The difference lies in how both options (RTK and PPK) obtain that solution, since while the RTK does it in real time, the PPK is done in post-processing. Let’s explain both options.
Fig. 5 Operating diagram of an RTK system.
RTK (Real Time Kinematics) indicates that it is a system that works in real time.
In this image we can see that both our base and our drone receive readings from the different satellite constellations available. But we can also see how a constant data link is required between the base and the drone in real time so that it can obtain that centimeter solution we need.
Fig. 6 Operating diagram of a PPK system.
In contrast, PPK (Post Processed Kinematic) performs post-processing of data.
In the image we see that both the base and the drone receive readings from the different satellite constellations. But, unlike the RTK, there is no constant data link between the drone and the base, since both record in a memory that information that we will have to download after the flight, usually in RINEX format files, to postprocess and obtain our solution.
So, which one to choose?
In the beginning, both systems should obtain the same result or solution, since the origin of the data (satellites) is the same.
Photogrammetry is an application that requires post-processing of the data (images), so adding the PPK process does not slow down the workflow.
From Helixnorth we recommend PPK, although the GNSS receivers we use are also upgradeable to RTK.
RTK needs corrections of its base in real time. This is done through a data link that may be susceptible to momentary interruptions. In addition, if it is BVLOS flights the interruptions make the drone receiver go from a FIX solution (accuracy 1-3cm) to a SINGLE solution (accuracy 2-3m). To this we should add possible problems or errors derived from a bad parking of the base, making our position divergent or inconsistent.
On the other hand, the RTK in proper operation will provide us with accurate data without the need for post-processing.
On the other hand, the PPK is a more robust solution, since it does not depend on a constant data link. In addition, field implementation is faster, as we do not necessarily need to know our base position before the flight and can be done after it.
We can also improve our position by adding extra permanent base data or perform post-processing with different data to improve our position and accuracy.
If you want to find out more about the PPK system we use at Helixnorth, click here.