Guide to RTK / PPK for Drones

RTK and PPK are two of the most important positioning technologies used on professional drones for surveying, mapping, LiDAR, construction, infrastructure inspection, agriculture and other applications where ordinary GNSS accuracy is not sufficient. Both technologies can improve drone positioning from metre-level accuracy to centimetre-level accuracy under suitable conditions, but they achieve this in different ways. RTK stands for Real-Time Kinematic, while PPK stands for Post-Processed Kinematic. RTK applies GNSS corrections while the drone is flying, giving the aircraft a highly accurate position in real time. PPK records the positioning data during the flight and applies the corrections afterwards using specialist processing software. For drone operators, manufacturers and geospatial companies, the choice between RTK and PPK depends on the mission. RTK can be ideal when accurate positioning is needed immediately, while PPK can be particularly valuable when operating across large areas where cellular or radio connectivity may be unreliable. Many professional drone systems now support both. ## **What Is RTK for Drones?** Real-Time Kinematic positioning improves GNSS accuracy by comparing the satellite observations received by the drone with observations from a reference station located at a known position. Because the reference station knows exactly where it is, it can calculate errors affecting the satellite measurements and send correction information to the drone. The aircraft receives these corrections during the flight and applies them to its own GNSS measurements. When the system successfully resolves the GNSS carrier-phase measurements, it can achieve what is normally called an RTK fixed solution. Under good satellite and correction conditions, this can provide centimetre-level positioning. The exact accuracy depends on the GNSS equipment, satellite visibility, correction source, distance to the reference station and wider operating environment. ## **What Is PPK for Drones?** Post-Processed Kinematic positioning uses similar GNSS principles, but the correction happens after the drone lands. Instead of requiring the aircraft to receive correction information continuously during the mission, both the drone and reference station record their raw GNSS observations. Afterwards, the datasets are processed together. Specialist software calculates a corrected trajectory for the aircraft and can determine highly accurate positions for camera exposures, LiDAR measurements or other sensor events. Because PPK does not require a continuous correction connection during the flight, it is particularly useful for remote areas, long corridors and BVLOS mapping operations. ## **RTK vs PPK** The most important difference between RTK and PPK is when the correction occurs. RTK calculates the corrected position during the flight, whereas PPK calculates it afterwards. RTK is therefore useful when the drone itself needs highly accurate positioning. Precision landing, accurate navigation, autonomous inspection and real-time geolocation can all benefit from RTK. PPK is primarily focused on the accuracy of the collected survey data. The aircraft can continue flying even if there is no continuous connection to the reference station, as long as the drone records good-quality raw GNSS observations. Neither system is automatically better. The correct technology depends on whether the mission requires high-accuracy navigation in real time, high-accuracy final survey data, or both. ## **Why Standard GNSS Is Not Always Accurate Enough** Standard drone GNSS can be accurate enough for navigation, but surveying applications frequently require much greater precision. Satellite positioning is affected by atmospheric delays, satellite orbit and clock errors, receiver noise, reflections and the physical arrangement of satellites in the sky. These errors can result in a position several metres away from the true location. For a normal visual inspection, this may not matter. For an engineering survey or LiDAR point cloud, however, several metres of error would be unacceptable. RTK and PPK reduce many of these errors because the drone and reference station observe many of the same satellite conditions at approximately the same time. ## **Carrier-Phase Positioning** RTK and PPK achieve their high accuracy partly by analysing the carrier waves used by GNSS satellites. These radio waves are much shorter than the navigation data traditionally used for standard positioning, which allows extremely precise measurements. The challenge is determining exactly how many complete wavelengths exist between the satellite and receiver. This is known as resolving the integer ambiguity. When this ambiguity has been successfully resolved, the receiver can achieve a fixed solution and deliver much higher positioning precision than standard GNSS. ## **RTK Fixed Solutions** An RTK fixed solution means that the receiver has successfully resolved the carrier-phase ambiguities. This is normally the desired state for professional RTK mapping. When the drone has a fixed solution, horizontal positioning can potentially reach centimetre-level accuracy under suitable conditions. Operators should monitor the RTK status throughout the mission. Simply owning an RTK drone does not mean every image or flight segment automatically has centimetre accuracy. ## **RTK Float Solutions** An RTK float solution occurs when the receiver is using carrier-phase data but has not completely resolved the ambiguities. Accuracy may still be considerably better than ordinary GNSS, but it will normally be worse than a fixed solution. If a drone moves close to buildings, trees or other obstacles, the RTK solution can sometimes change from fixed to float. Professional survey workflows should identify these periods and determine whether they affect the required project accuracy. ## **The RTK Base Station** A local RTK base station is a GNSS receiver positioned at a known location. It receives signals from the same satellite constellations as the drone and calculates correction data. The correction information is transmitted to the aircraft using radio, cellular networks or another communications link. A critical point is that the base station itself must have accurate coordinates. If the base position is incorrect by one metre, the entire drone survey can also be shifted by approximately the same amount while still appearing internally very precise. ## **PPK Base Stations** For PPK, the base station does not need to communicate with the drone during the mission. It simply records raw GNSS observations while the aircraft flies. After the mission, the base-station data is combined with the drone data. This makes the field setup less dependent on communications. It also allows the same raw drone observations to potentially be reprocessed later if another reference dataset becomes available. ## **Network RTK** A local physical base station is not always required. Many countries have networks of permanently installed GNSS reference stations that provide correction services. The drone operator connects to the network through an internet connection and receives correction information based on nearby reference stations. This is commonly called Network RTK and can significantly simplify field operations because the operator does not need to establish and survey a local base station for every project. ## **NTRIP** NTRIP is commonly used to deliver RTK correction information over the internet. A drone controller or GNSS receiver connects to an NTRIP service through a cellular or other data connection. The service sends real-time correction information from a GNSS reference network. This is extremely convenient in areas with good cellular coverage, but it introduces dependence on the mobile network. If connectivity disappears, RTK corrections may be interrupted. ## **CORS Networks** Continuously Operating Reference Station networks, commonly known as