Real Time Kinematic (RTK) is a Global Navigation Satellite Systems (GNSS) enhancement technique that uses a fixed base station (either on site or remote using Networked Transport of RTCM via Internet Protocol – NTRIP – over cellular or Wi-Fi) to transmit correction data to a mobile GNSS receiver such as a “rover” or a Unmanned Aircraft System (UAS). Using the GNSS signal’s carrier wave instead of the so-called pseudorandom code, the base station transmits the phase of the carrier wave it observes from each satellite to the mobile unit. The mobile unit then compares this with its own measurements and calculates its position relative to the base station to within a few millimetres. Assuming the base station is positioned on a known surveyed point, RTK can provide centimetre level absolute accuracy of the mobile unit’s position.

When applied to aerial mapping with drones, RTK can provide centimetre level accuracy of the UAs position when the photograph is taken. The position data can be written into the image file using EXIF and XMP tags which can be read by photogrammetry software to accurately position each camera when creating the point cloud. Traditional mapping using unmanned aircraft relies on ground control points (GCPs) to provide the required accuracy, as standard GNSS receivers commonly fitted in UAS are only accurate to within a few metres. Marking and surveying these points can often take longer than flying the mission itself. RTK can largely eliminate the need for GCPs, however it is standard practice to survey some control to ensure height accuracy, and to use as check points to verify the global accuracy of the outputs.