Being a drone enthusiast, you must have probably heard of terms like PPK, RTK, GCPs and more. Some well-known drones in the market – M300 RTK and Phantom 4 RTK, for instance – carry these acronyms in their title. But – What exactly are PPK and RTK? What are RTK and PPK drones used for? Which one gives the best results? Are these questions confusing you? No worries, this article is for you! Simply put, the major distinction between them happens during the positional corrections; RTK corrects while in flight, and PPK corrects later. Either way, both methods fix the location errors of drone mapping data and eliminate the necessity for GCPs, resulting in absolute precision as low as cm (sub-inch).
Drones are proving to be invaluable assets across various industries – Public Safety, Oil and Gas, Energy and Utilities, Construction, and Agriculture – to mention a few. Drone-powered solutions are economical, practical, efficient, and accurate alternatives to existing workflows. Always remember that the efficiency of your survey is tied to choosing the appropriate correction methods. However, before we get into the core of the matter, it is vital to understand the concepts of GPS, GNSS and GCPs.
What are GPS and GNSS?
The Global Navigation Satellite System (GNSS) is a network of satellites that broadcast time and orbital data required for navigation and location measurements. In simple terms, GNSS receivers precisely calculate their own location by measuring the distance to three or more satellites. In contrast, the Global Positioning System (GPS) is a form of GNSS that is also the most frequently used GNSS in the world. GPS is a navigation system that synchronises location, velocity, and time data for air, sea, and land movement using satellites, a receiver, and algorithms. It delivers worldwide positioning and timing information in real time.
The fundamental distinction between GPS and GNSS is that GNSS-compatible equipment can use navigational satellites from networks other than the GPS. More satellites translate to greater receiver accuracy and reliability. All GNSS receivers are GPS-compatible; however, GPS receivers are not always GNSS-compatible.
What are GCPs?
Ground Control Points, or GCPs, are designated points on the ground with a defined geographic location. They are often necessary for aerial survey applications as they further escalate the positioning and accuracy of the mapping outputs. In an aerial survey, drones are deployed to capture images of the survey region autonomously; and if used, GCPs must be visible in these aerial photographs. The photos are then processed in the cloud using drone mapping software.
Laying GCPs in the mission area can be time-consuming; the task requires a crew, vehicles and equipment along with paperwork requirements for authorisations and permissions. Fortunately, alternatives such as PPK and RTK drones are here now to help with this time-consuming operation. Now back to our primary question, “Which drone technology -RTK or PPK- provides the better mapping experience?” The answer depends entirely on your specific missions, including the local network connectivity. But either way, RTK and PPK have several advantages and disadvantages; and I’m here to dissect this topic and assist you in determining which is genuinely best for your drone mapping team.
What are RTK Drones?
RTK or Real Time Kinematic Drones are equipped with an inbuilt GNSS RTK-receiver that collects data from satellites and a base station to correct image location in real time as the drone flies.
With a data connection like GSM or WiFi, an RTK drone will connect to a base station or a Virtual Reference Station (VRS) via the device in control of the flight. The camera positions are determined in real-time with an accuracy of up to two or three centimetres in both horizontal and vertical directions. In a nutshell, if all signals are steady when the drone lands, data with absolute precision is available for post-processing into mapping survey findings. Benefits of using RTK drones:
- Save time as it eliminates the necessity of placing GCPs in difficult-to-reach locations
- The drone’s real-time GPS positioning is used to make real-time corrections; and eliminating the need for post-flight GNSS processing
- Better waypoint navigation, beneficial for challenging settings
- The RTK-assisted autonomous landing
However, due to the data-connection dependency, RTK positioning has its own downsides, such as radio link outages, signal blocks and so on. In addition, long distances between the drone and the base station might create signal interference, resulting in loss of correction data and a reduced percentage of precise camera positions in flight. In such scenarios, you can opt for PPK corrections if your drone is capable enough.
It may appear to be convenient to receive revised data during a flight. However, under real-world situations, where barriers might block or interrupt signals and flights may stretch beyond the strength of a signal, RTK technology has flaws that should be considered. RTK achieves the same absolute precision as PPK in the best case—when the link between the base station and the drone remains continuous.
What are PPK Drones?
PPK or Post-Processing Kinematic drones attach geocoordinates to each image using an onboard GNSS receiver. Meanwhile, a base unit – a GNSS base station or a CORS network – too collects positional information with more accurate triangulation during flight. When the drone lands, a corrective procedure must be carried out using appropriate software; this programme might be cloud-based or desktop-based, depending on your preferences. Then the two captured sets of GPS data are matched up to rectify satellite signal inaccuracy, bringing precision down to the cm (sub-inch) range.
The RTK Drone system requires four uninterrupted communication lines to function efficiently. 1. The line between satellites and drones. 2. The line between satellites and the GNSS base station or CORS network. 3. The line between the GNSS base station, CORS/VRS and the drone base station. 4. The line between the drone base station and the drone.
The PPK drone system requires two uninterrupted communication lines to function efficiently: 1. The line between satellites and the GNSS base station/CORS network. 2. The line between satellites and drones.
Benefits of using PPK Drones:
- Save time as it eliminates the necessity of placing GCPs in difficult-to-reach locations
- Ensured effective flights even in challenging scenarios where a real-time correction may not be possible
- Retention of GNSS data to verify and reprocess if necessary
- Flexibility in the use of base-stations
- There is no requirement for long-range radio or the risk of data-link outages.
- Accurate GNSS positioning for the drone post-flight
- GPS processing with dual antennas
As already discussed, the downsides of the RTK flight are mainly related to the connection loss due to the obstructions. In such cases, PPK enhances the outcome. That being said, it is also important to note that when the data-link connections are stable, an RTK flight can effectively capture the terrain and provide us with exceptionally accurate findings.
Frequently Asked Questions
1. Is PPK or RTK better?
Well, it depends on the mission at hand. For example, RTK may appear to save time as you get corrected data instantly as the drone lands; however given the link-dependancies, RTK is more prone to failure. On the other hand, PPK systems are more flexible, enabling missions even in the most challenging environments.
Mapping with an RTK/PPK drone is preferable when there is dense vegetation or any other obstacles in your mission area. This is because you always maintain control over high-precision and quality outputs, regardless of where you place GCPs or whether you place GCPs or not.
2. When would you use ground control points in a GIS operation?
There are mainly two reasons to use GCPs: (1) If you need the highest degree of global accuracy or (2) if you are contracted to provide measurements within a certain range of accuracy.
3. What is the difference between Ground Control Points and Checkpoints?
Survey checkpoints are points with established coordinates that are used to validate the accuracy of the survey. On the other hand, Ground control points (GCP) use GPS data to adjust survey models and improve their overall accuracy. It is to be noted that, unlike the GCPs, checkpoints DO NOT affect the data processing.
4. How many Ground Control Points are needed?
Well, the answer depends on the requirements and survey area size. However, mapping and surveying missions would require a minimum of 5 GCPs.
5. What are the downsides of using Ground Control Points?
Certain factors like feasibility, time, cost of the project, and significantly more should be considered before using GCPs. For example, GCP measurements are not always possible on-site due to various factors such as delicate ground textures, inaccessible areas, and other personal safety considerations.
6. Will RTK and PPK drones replace Ground Control Points?
No, drones will not replace the use of GCPs. Even while working with technologies like RTK or PPK, the concept of Ground Control Points is still relevant. Depending on the size of the site, some ground validation may still be required, allowing the survey drone to continue producing highly accurate output. Ideally, Drone Surveys and Land Surveys should go in tandem!
7. Does PPK need a Base Station?
Yes, both RTK and PPK will require a Base Station.
8. Is PPK limited by Line Of Sight?
No, and this is where a PPK drone would outperform an RTK, especially on longer flights. The PPK drones can effortlessly perform Beyond Visual Line of Sight (BVLOS) operations.
To discuss PPK and RTK in more detail and to introduce these surveying methods to your workflows, contact our in-house surveying team or learn more about FEDS Drone Surveys and Mapping.
