For the non-surveyor, understanding the differences between PPK and RTK (and which one to choose as your automated data processing solution) can be tricky. First, they’re both very similar-looking acronyms. On top of that, it’s not always easy to understand which one is right for your worksite. Our recent blog post “How it Works: PPK vs RTK” tackles the technical elements of each process to explain the differences.
In this post, we’ll take a high-level look at each construction data processing solution. We’ll explain the pros and cons of each, including why Propeller uses PPK.
How real-time kinematic (RTK) processing works
“RTK” stands for real-time kinematic. The term describes both onboard drone hardware and a data processing solution, which will be important later on when we talk about PPK.
Onboard RTK drone hardware was revolutionary when it first hit the market in the 1990s. RTK is a differential GNSS (global navigation satellite system) technique used to capture survey-grade positional data. In essence, drones with onboard GNSS equipment communicate with satellites and a fixed base station to triangulate their location and correct their positional data in real time as they fly.
Why does satellite data need correction in the first place? It might sound surprising, but satellite data is error-prone due to tropospheric delay and other issues that arise when you’re communicating with objects hovering thousands of miles above the earth.
So, to make sure that the drone’s RTK receiver records accurate geocoordinates, another GNSS receiver (also called a “base” or “base station”) is placed on solid ground. It sits—without moving—on a point with known coordinates. Incorporating the base station boosts the accuracy of RTK from about 1 meter to a centimeter-level range.
In order to do so, the base continuously records signals from positioning satellites and communicates those signals to the survey drone via a radio, cellular, or satellite network link. The drone’s onboard GNSS system can then compare its positional data to the more accurate base station, which allows it to correct atmospheric, orbit, and timing errors that affect accuracy.
The result is an accurate measurement of the distance between the drone and the base in real time—hence, real-time kinematic. From there, it’s easy math. By adding the relative distance to the base station’s known coordinates, we can accurately pinpoint the drone’s position in space at any given time within just a few centimeters.
While RTK systems are highly accurate, there are some significant drawbacks every team should know about.
In an RTK workflow, your level of accuracy depends on an uninterrupted connection between the base station and the drone—a connection that can be breached fairly easily. Signal loss can occur for several reasons: patchy or unreliable signal, flying behind obstructions like trees or buildings, antenna orientation during a turn, and so on. When that happens, data is unreliable until the drone regains its connection with the base.
- Data processing is complete immediately after flight
- Real-time, accurate drone positional data
- Requires stable connection between base station and drone
- Short-term loss of connection means significant loss of data
How post-processing kinematic (PPK) works
Let’s get the confusing part out of the way: PPK is a data processing workflow. Unlike RTK, it’s not hardware. To muddle things even further, PPK workflows actually use RTK drone technology.
So why is a workflow that uses an RTK-enabled drone called “PPK”?
PPK stands for post-processing kinematic. Propeller’s PPK workflow uses RTK drone hardware to collect accurate positional data during a survey flight. But as the name implies, the actual processing of that data occurs after the flight is complete, not in real time.
In a PPK workflow, the drone still attaches geocoordinates to each image based on an onboard RTK GNSS unit. And a base unit or base station still records positional information, allowing us to triangulate the drone’s position in space—though you have more flexibility with the type of base that can be used in PPK.
Here’s where RTK and PPK part ways.
The two sets of GPS data (one from the base, one from the drone) are matched using timestamps on the photos. The more accurate positional data from the base then corrects the drone data to produce a dataset that also achieves centimeter-level accuracy.
For more in-depth info on how PPK works, check out this post.
With PPK, there’s no need for the base station and the drone to maintain contact during flight, because all the data is recorded and reconciled after the fact. This is a huge advantage to sites with unreliable signal, large sites, or busy teams who don’t want to worry about recharging batteries mid-flight.
- No need to maintain connection between the RTK base station and the drone during flight
- Use AeroPoints in conjunction with the Propeller Corrections Network or user-provided RINEX observations, in place of a GNSS base station or an NTRIP correction source provided over mobile data
- Much simpler setup than RTK, and fewer points of failure
- Data is processed after the flight, so there is some processing time between flight and data output
- Some systems require RTK to enable terrain following functionality
Why Propeller uses PPK
“Real-time” might sound flashier than “post-processing,” but PPK is actually more reliable than RTK, with the same level of accuracy.
That’s because you can correct any errors after the fact, once those gaps are known. Because a constant connection between the base and drone is not required with PPK, you don’t have to worry about reduced accuracy if a signal is interrupted. PPK allows you to fly longer distances with technology that’s easier to use.
And with Propeller, the “post-processing” is all done by our team, so there’s nothing for you to manage. It’s often as simple as placing an AeroPoint on the ground, flying your drone for 10 minutes, and connecting to WiFi after the flight. We handle the rest.