Post-processed kinematic (PPK) and real-time kinematic (RTK) are not new technologies. They’ve been used in terrestrial surveying for years to improve the accuracy of GNSS data. Both methods correct the location of drone mapping data and remove the need for GCPs, bringing absolute accuracy down to cm (sub-inch) range.
However, when you factor in obstacles and environmental conditions common to aerial surveying, the difference between a PPK drone and an RTK drone becomes more important to understand. In fact, choosing the best correction method for your surveys will save you time and project costs. But how do you know which is best? The answer can be found in learning about how each method works, including its inherent strengths and drawbacks.
What is an RTK drone (and how does RTK work)?
- An RTK drone carries an onboard GNSS RTK receiver that gathers data from satellites and a stationary base (ground) station to more accurately correct image location, in real time as it flies
- Satellite data—by itself, and in any case—is error-prone due to tropospheric delays, etc., providing a maximum accuracy of about 1 meter (3.3 ft)
- The data from a ground station is factored in to correct satellite signal error, bringing accuracy down to cm (sub-inch)-level range
- In the case of RTK technology, uninterrupted communications are required from the GNSS base station, through the drone base station to the drone
- When the drone lands, if all signals were constant, data with absolute accuracy is available for post processing into mapping survey results
What is a PPK drone (and how does PPK work)?
- A PPK drone flies with an onboard GNSS PPK receiver that gathers data from satellites and logs it for retrieval after the flight
- Satellite data—by itself, and in any case—is error prone due to tropospheric delays, etc., providing a maximum accuracy of about 1 meter (3.3 ft)
- The satellite data from a GNSS receiver on a nearby base (ground) station is collected and, after the flight, it’s factored in with the drone data to correct satellite signal error, bringing accuracy down to cm (sub-inch)-level range
- In the case of PPK technology, neither GNSS base station to drone base station, nor drone base station to drone correction data communications are required; only the telemetry between drone and drone base station is needed
- When the drone lands, a correction process needs to be applied through a suitable software, e.g., WingtraHub. Data with absolute accuracy is then available for post processing into mapping survey results
RTK vs. PPK overview
Because it is more reliable, PPK is often suggested as a back-up for RTK that fails. So why use RTK in the first place?
RTK Pros
- Correction complete immediately after flight
- Offers real-time, highly-accurate position of the drone
- Removes need for GCPs if the connections are all constant throughout a survey
RTK Cons
- Success of the mission depends on several technologies, thus is more prone to failure, i.e., it requires stable connection between GNSS base station (via cable, bluetooth, etc.) or CORS network supporting VRS (via mobile network) to drone ground station, and then via telemetry link to drone
- Short-term loss of lock equals long time to re-initialize and more data lost
- Mission beyond telemetry range, e.g. extended visual line of sight (EVLOS) or beyond visual line of sight (BVLOS), is impossible
- No post-flight correction data available, i.e., flight area limited by signal strength
- Any error in the base station position must be corrected after the flight, which reduces real-time advantage
PPK Pros
- Your GNSS base station doesn’t need to be connected to a drone ground station
- Any CORS can be used—in place of a GNSS base station—with or without VRS
- Way simpler setup—no need for so many constantly-working telemetry links
- Mission beyond telemetry range, e.g. extended visual line of sight (EVLOS) or beyond visual line of sight (BVLOS), is possible
- Some sources say PPK is more accurate because you can use data to forward and backward process around any gaps to correct them
- Removes need for GCPs
PPK Cons
- Correction happens after flight