Abstract
Drone mapping (UAV) has become very popular recently. This method is actively used in various industries, such as agriculture, construction, environmental monitoring, mining, etc. However, not all operators know positioning technologies and their advantages and disadvantages. This article will help fill this gap and help conduct missions most effectively.
The following sections will be covered: Introduction to Positioning Technologies and Choosing the Right Technology for Your UAV. The conclusion will summarize the benefits of using all technologies.
Section 1. Introduction to Positioning Technologies
GNSS
GNSS (Global Navigation Satellite System) is a global satellite positioning system used to determine any point on Earth’s exact location, speed, and time. GNSS includes several satellite systems (GPS, GLONASS, Galileo, and BeiDou) to provide global coverage [1].
Each system has satellites in Earth orbit that continuously transmit signals with information about their location and time. A GNSS receiver receives signals from multiple satellites (usually four or more). It uses the time it takes for the signal to travel from the satellite and its coordinates to calculate its location (latitude, longitude, and altitude). GNSS accuracy is affected by atmospheric distortion, multipath, satellite clock and orbit errors, and the number of visible satellites.
To improve accuracy, the following methods are used (these are not all possible, but the most frequently used when mapping from a drone):
- Ground Control Points (GCPs) [2].
- RTK (Real-Time Kinematic) [3].
- PPK (Post-Processing Kinematic) [4].
Let’s look at each of them in more detail below.
GCPs
Ground control points (GCPs) are physical markers installed on the earth’s surface and used to improve the accuracy of georeferencing data collected from drones or other aerial photography. GCPs are essential in mapping and surveying, ensuring the correction and accuracy of resulting images and models.
The points must be visible on aerial photographs or point clouds, so bright or contrasting markers (such as black and white crosses) are often used for easy identification, as shown in Figure 1.
Surveyors have placed markers on site and measured their precise coordinates using a high-precision GNSS receiver (such as RTK or PPK) to obtain sub-centimeter accuracy. The coordinates of the points are usually defined in a coordinate system appropriate to the project.
Figure 1. Ground control point.
The drone flies along a pre-planned route, collecting overlapping images of the area or scanning the terrain using LiDAR.
In specialized software (e.g., Pix4D, LiDAR360) [5, 6], points are identified in images/point clouds, and their coordinates are used to adjust and georeferencing the data. This minimizes errors and distortions, increasing the overall accuracy of the data.
The disadvantage is the labor intensity of placement and measurement of ground control points (in some cases, installation of points can be complex, for example, in dense forests, mountainous or swampy areas), as it requires time and professional equipment, which increases the costs and duration of work. In addition, it is not always possible to accurately measure the coordinates of the points due to poor GNSS signal penetration.
Benefits of GCPs | Disadvantages of GCPs |
– High positioning accuracy – Cheapness – Allows you to control the quality of data | – Require manual installation, which is not always possible due to the terrain – Requires professional equipment for accurate measurements – Do not work in real-time |
RTK
Real-time kinematic (RTK) is a high-precision coordinate determination method used in surveying, mapping, and navigation to improve the accuracy of data obtained from GNSS. RTK allows centimeter-level accuracy to be achieved in real-time [7].
– Processes and corrects GNSS data in real-time. The base station sends corrections to the rover during flight, and the coordinates are corrected in real-time.
– A stable communication channel (radio signal or mobile communication) between the base station and the mobile receiver is required to transmit data in real-time. In areas with poor coverage or interference, RTK may need to work better.
– Allows sub-centimeter accuracy, but data quality depends on signal stability. Short-term accuracy deviations or data gaps may occur if the signal is lost.
– Ideal for applications that require real-time and operational accuracy. However, setting up a base station and maintaining a stable signal may require additional effort.
– RTK systems can be expensive, especially if a stable connection to a mobile network or the use of radio modems over long distances is required.
Advantages of RTK | Disadvantages of RTK |
– Accurate data can be obtained immediately after the flight – To receive corrections, you can use a network of base stations, such as CORS. – High positioning accuracy
| – A stable connection between the base station and the payload is required. – Short-term loss of connection means long re-initialization time and data loss. – Flight area is limited by signal magnitude |
PPK
Post-processing kinematic (PPK) is a method of high-precision positioning using GNSS data with subsequent post-flight data processing. PPK is similar to RTK but differs in that the data is corrected after the flight rather than in real-time [7].
– Data processing occurs after the flight is completed. GNSS data is recorded by the base station and the rover and then combined and corrected in specialized software. This improves accuracy, but the data is only available after post-processing.
– Does not require real-time communication between the base station and the rover, making it less dependent on communication conditions, which is helpful in remote or challenging environments.
– Provides the same or even higher accuracy since the data is processed after the flight, and correcting all errors and distortions during recording is possible. PPK can be more reliable under challenging conditions since all data is available for analysis and correction.
– Suitable for tasks where high accuracy is important, but real-time data is not critical.
– Although the PPK equipment includes a base station and a mobile receiver, the lack of real-time requirements can reduce communication costs and provide greater mobility (CORS network data or other services can be used). However, post-processing of data requires time.
Advantages of PPK | Disadvantages of PPK |
– It does not require the connection of the base station to the payload. – You can use a network of base stations. – PPK can be made more accurate through data processing algorithms. | – Correction occurs after the flight.
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Section 2. Choosing the Right Technology for Your UAV
The choice of one or another depends on the mission’s conditions, the budget, and the user’s time. In this section, we will consider the intricacies of using one or another technology, which will help us better understand and make a choice.
Since the missions are based on GNSS, we will not consider them; instead, we will focus only on methods for increasing accuracy.
Characteristic | RTK | PPK |
Processing time | In real-time | After completing the mission |
Communication requirement | A stable communication channel is needed | It does not depend on the connection |
Accuracy | Sub-centimeter (depending on signal) | Sub-centimeter (more reliable) |
Application | Tasks that require instant accuracy or good conditions | Tasks where terrain conditions may interfere with free signal transmission |
Price | High (due to the need for a stable connection) | Moderate (lower communication costs) |
Thus, both technologies allow for very high accuracy. The choice between them depends on the operating conditions, the need for real-time, and the availability of a stable signal. If it is additionally necessary to control the quality of the data or make adjustments to the processed data, then it is worth considering ground control points.
Conclusion
Choosing the right positioning technology for your drone depends on many factors, including accuracy requirements, terrain conditions, equipment availability, and budget. Selecting the right positioning technology for your UAV – whether GCP, GNSS, PPK, or RTK – depends on various factors, including the accuracy required, the survey conditions, the size and complexity of the site, and the project budget. Ground control points (GCPs) are traditionally used for sub-centimeter accuracy but can be labor-intensive and time-consuming to install.
RTK and PPK technologies that work with GNSS data avoid setting points on the ground, providing the ability to obtain highly accurate georeferencing in real time (RTK) or post-processing (PPK). RTK is ideal for real-time surveys in areas with good GNSS coverage, while PPK is more effective in environments with variable connectivity. Thus, the optimal technology depends on the project objectives: RTK and PPK offer significant advantages for large and inaccessible areas, while GCP remains reliable when maximum accuracy is required.
References
[1] Wikipedia. “Satellite Navigation.” Wikipedia, 19 Mar. 2020, en.wikipedia.org/wiki/Satellite_navigation.
[2] “Ground Control Points | U.S. Geological Survey.” Www.usgs.gov, www.usgs.gov/landsat-missions/ground-control-points.
[3] “Real-Time Kinematic Positioning.” Wikipedia, 5 June 2022, en.wikipedia.org/wiki/Real-time_kinematic_positioning.
[4] Wikipedia Contributors. “Differential GPS.” Wikipedia, Wikimedia Foundation, 7 Sept. 2024.
[5] “Pix4D.” Pix4D, 2011, www.pix4d.com/.
[6] “LiDAR360 Software and Real-Time Point Cloud Display.” Www.greenvalleyintl.com, www.greenvalleyintl.com/LiDAR360/.
[7] Jemai, Mohamed, et al. “A Height Accuracy Study Based on RTK and PPK Methods Outside the Standard Working Range.” International Journal of Advanced Computer Science and Applications, vol. 14, no. 5, 1 Jan. 2023, https://doi.org/10.14569/ijacsa.2023.0140520. Accessed 26 Jan. 2024.
