Real Time Kinematics (RTK) 2
Updated: Jun 13, 2020
In my first article on the subject of Real Time Kinetics (RTK) I explained the principle of operation and how RTK gives us more accurate positioning. In this article I'll explain how and why RTK technology found its way onto drones.
GPS very soon were incorporated in commercial Small Unmanned Aircraft (SUAS), following their military counterparts. GPS enables accurate flight positioning, automatic hovering and autonomous flight.
A drone with an integral GPS receiver, camera positions are trilaterated from satellites so their positions are identified by several distance measurements. Normally, you can expect an inaccuracy of up to a few meters, especially in Z orientation depending on the equipment quality and the satellite geometry.
From early on drones have been used in survey and mapping with the aid of clever software that stitches aerial images together known as photogrammetry. The software uses the GPS data embedded in the EXIF data to help align the images and to produce a georeferenced map.
The accuracy of traditional survey techniques was improved by the use of Differential GPS (DGPS) which is essentially a system to provide positional corrections to GPS signals. DGPS uses a fixed, known position to adjust real time GPS signals to eliminate pseudorange errors. An important point to note is that DGPS corrections improve the accuracy of position data only.
Before RTK enabled SUAS accuracy was improved through the use of Ground Control Points (GCP). Ground Control Points, or GCPs, are marked points on the ground that have a known geographic location. In aerial mapping a drone can be used to autonomously collect photos of the survey area. If used, GCPs must be visible in these aerial photos. The photos are then processed using drone mapping software to create 2D orthomosaics, 3D models, digital elevation models (DEMs), and 3D point clouds from drone images. For aerial survey applications GCPs, are typically required as they can enhance the positioning and accuracy of the mapping outputs. The disadvantage of using GCPs in the time it takes to locate, mark and georeference each of the points using DGPS or RTK ground equipment.
A commercial need for higher precision and quicker aerial mapping necessitated the advent of RTK enabled drones, some with integral receivers and others as an add-on package,
An RTK enabled drone will connect to a Base Station or a Virtual Reference Station (VRS) which augments flight control. The camera positions are calculated in real-time, referring to ground references, the locations of which are accurately measured for a long period of time. The calculation helps correct the camera positions up to two to three centimetres, or even less of accuracy, both horizontal and vertical.
In my next blog on RTK, I'll explain the various ways RTK can be used with drones.