RTK and PPK for Drone Surveys in the UK: Real-Time and Post-Processed Positioning Guide

Understanding RTK and PPK

Standard GNSS receivers on consumer and prosumer drones typically achieve positional accuracy measured in metres. For professional survey work, that is not sufficient. RTK and PPK are differential GNSS techniques that correct for atmospheric and orbital errors, bringing positional accuracy to the centimetre level.

RTK — Real-Time Kinematic

RTK works by receiving correction data in real time during the flight. The drone's GNSS receiver compares its own satellite observations with corrections from a known reference point — either a local base station set up on site or a network of permanent reference stations.

In the UK, the main correction networks available for RTK include:

• OS Net: Ordnance Survey's network of permanent GNSS reference stations across Great Britain. Access to real-time corrections is available through OS Net's NRTK (Network RTK) service.
• SmartNet: A commercial NRTK service operated by Leica Geosystems, providing corrections via mobile data connection across the UK.
• Local base station: Setting up your own base station on a known survey point on site. This avoids reliance on mobile signal but requires additional equipment and a surveyed reference point.

The key advantage of RTK is that you know your positioning accuracy during the flight. If the correction link drops, you are immediately aware that the data from that portion of the flight may not meet specification.

PPK — Post-Processed Kinematic

PPK uses the same differential correction principle but applies the corrections after the flight rather than in real time. The drone records raw GNSS observation data (typically in RINEX format) during the flight, and this data is later combined with reference station observations to compute corrected positions.

PPK has several practical advantages in UK conditions:

• No real-time data link required: The drone does not need a mobile signal or radio link to a base station during flight. This makes PPK particularly valuable in rural or remote areas of the UK where mobile coverage is unreliable.
• Higher reliability: Because corrections are applied in post-processing, every epoch can be resolved, including any that might have been missed during a brief RTK signal dropout.
• Flexible reference data: RINEX data from OS Net stations can be downloaded after the flight, so you do not need to arrange real-time access in advance.

RTK vs PPK — Which to Choose

The choice between RTK and PPK depends on site conditions and project requirements:

• Choose RTK when you have reliable mobile signal or can set up a local base station, and you want real-time confirmation of positioning accuracy during the flight.
• Choose PPK when mobile signal is unreliable, the site is remote, or you want the highest confidence that every image position is accurately resolved.
• Use both together for maximum reliability — fly with RTK corrections active but also log raw RINEX data so that PPK can be applied as a backup or cross-check.

Base Station Considerations

If using a local base station rather than a network correction service, several requirements apply:

• The base station must be set up over a point with known coordinates in the project's coordinate system (typically OSGB36 for UK work).
• The base station position can be established by static GNSS observation, connecting to a nearby OS Net station or known survey control point.
• The base station should have a clear view of the sky with no obstructions above approximately 15 degrees elevation.
• The baseline distance (distance between base station and drone) should be kept within the operating limits of the equipment — generally, accuracy degrades with increasing baseline length.

RINEX Data and Post-Processing

For PPK workflows, the raw GNSS observations must be recorded in a compatible format. RINEX (Receiver Independent Exchange Format) is the standard format used across the industry. Key considerations:

• Ensure the drone's GNSS receiver is configured to log raw observations at an appropriate rate.
• Download corresponding reference station data for the same time period — OS Net RINEX data is available from Ordnance Survey.
• Use PPK processing software to compute corrected positions for each image capture event.
• Check the solution quality — fixed integer solutions provide the highest accuracy, while float solutions indicate lower confidence.

Do You Still Need GCPs with RTK or PPK?

RTK and PPK can significantly reduce the number of GCPs required, and in some cases eliminate them entirely. However, best practice in the UK survey industry is to include at least a few independent check points to verify the accuracy of the final output, regardless of the positioning method used. For guidance on ground control, see our GCP best practice guide.