Anyone looking for a surveying drone will inevitably stumble across the term RTK, short for Real Time Kinematic or real-time movement. But what exactly is it? RTK is a method with which the position of a drone can be determined much more accurately than with normal GNSS.
Motivation: What is the problem with GNSS (GPS and co.)?
Almost every drone has GNSS on board in order to maintain its position. But many people don't realise this: GNSS in itself is relatively inaccurate! Outside of cities, the position can only be determined with an accuracy of 5 to 10 metres. As a result, GNSS alone is not enough to keep a drone stable in one place. In practice, the positioning accuracy of non-RTK drones is "artificially" improved by IMU and visual sensors. However, this does not change the fact that we cannot precisely determine the position of our drone.
But what are the consequences? Drones are a means of acquiring data. More precisely, in the case of photogrammetry , we want high-resolution images with metadata about the current camera position. Obviously, we can only determine the camera position to an accuracy of several metres with normal GNSS. The photogrammetry method acknowledges this with enormously long computing times or the time-consuming laying out and measuring of control points.
How can RTK help?
RTK is a complementary system to GNSS. It utilises the data from several GNSS receivers simultaneously in order to calculate significantly better accuracy. It therefore belongs to the group of so-called differential GNSS, all of which use multiple receivers.
With RTK, drones can achieve positioning accuracies of less than 1 cm horizontally and less than 1.5 cm vertically under ideal conditions. In practice, 3-4 cm accuracy(precision vs. accuracy) is possible without laying out a single control point.
What options do I have for using RTK?
As a basis, you need an RTK-capable drone. The manufacturers usually tell you this in the name. If you have a suitable device (in view), then you still need to decide on a type of correction data reference. You have two options for this:
SAPOS is the satellite positioning service of the German Land Survey and operates a network of reference stations. SAPOS provides correction data via mobile radio. Your RTK drone system should therefore have a SIM card slot in which you can permanently place a SIM card. You can find out which providers and speeds are suitable in our FAQ entry.
Figure 1: RTK with SAPOS
#2 Own reference station
If you do not have sufficient mobile phone coverage or do not want to use SAPOS for other reasons, you can purchase your own reference station . In this case, of course, you do not need a SIM card, but you do need a way to calibrate the station (e.g. total station or total station, more on this in the FAQ article)!
Figure 2: RTK with reference station
We recommend the best of both worlds for maximum reliability and efficiency: Use SAPOS as standard and keep a reference station in the boot as a fallback level.