Unmanned aerial vehicles (UAVs) are widely used due to their low cost, multitasking capabilities, excellent mobility, high efficiency, and low radiation. They find extensive applications in various aspects of military and civilian production. Because GPS offers all-weather, high-precision, and automatic measurement capabilities, most UAVs for mapping currently rely on GPS for positioning and navigation. However, the single-point positioning accuracy of GPS in UAV flight control is insufficient. In the past, a large number of image control points were used to correct image distortion. However, in certain terrains such as mountains, valleys, rivers, etc., it is challenging for field personnel to deploy image control points. In order to reduce workload, eliminate the need for most image control points, and enhance aircraft positioning accuracy, RTK (Real-Time Kinematic) and PPK (Post-Processed Kinematic) technologies can achieve centimeter-level accuracy.
We will now compare and analyze the two technical principles of RTK and PPK to find a more suitable method for GPS aerial positioning.
- Working Principle of RTK:
RTK (Real-Time Kinematic) measurement systems typically consist of three parts: GPS receivers, data transmission systems, and software systems for dynamic measurements. RTK measurement technology is based on carrier phase observations and has fast, high-precision positioning capabilities. Carrier phase differential measurement technology can obtain real-time three-dimensional positioning results of the measuring station in a specified coordinate system and has centimeter-level positioning accuracy.
The working principle of RTK measurement is as follows: place one receiver on the base station and another receiver on the carrier (referred to as the rover station). The base station and rover station simultaneously receive signals sent by the same GPS satellite. The obtained observations are compared with known position information to obtain GPS differential correction values. Then, this correction value is transmitted in real-time via a radio data link from the base station to the rover station of the public satellite to refine its GPS observations. This process enables obtaining a more accurate real-time position of the rover station after differential correction.
Currently, mainstream RTK manufacturers can achieve plane positioning accuracy of 8mm + 1ppm and elevation accuracy of 15mm + 1ppm. There are two main communication methods between the base station and the rover station: radio and network. Radio signals are stable, while network signals have a longer transmission distance, each with its own advantages.
- Working Principle of PPK:
PPK (Post-Processed Kinematic, GPS Dynamic Post-Processing Differential) technology operates by using a base station receiver for simultaneous observations and at least one mobile receiver for simultaneous observations of GPS satellites. In other words, the base station remains stationary. After initialization of observations, the rover station moves to the next point to be determined. It is necessary to maintain continuous tracking of satellites during the relocation to resolve the ambiguity of the satellites' entire week to the scheduled time.
The data simultaneously received by the reference station and the rover station are linearly combined in the computer to form virtual carrier phase observations, determining the relative positions between receivers. Finally, the known coordinates of the reference station are introduced to obtain the three-dimensional coordinates of the rover station.
PPK technology is the earliest GPS dynamic differential technology (also known as semi-dynamic method, quasi-dynamic relative positioning method, stop-and-go method). Its main difference from RTK technology lies in the fact that real-time data transfer (as required by RTK) is not necessary between the base station and the rover station. Instead, the positioning data collected by the two GPS receivers are jointly processed after the measurement to calculate the corresponding time and the coordinates of the moving station. The rover station is not subject to strict restrictions. Its advantages include high positioning accuracy, high operational efficiency, a large working radius, and simple operation.
III. Comparison Between RTK and PPK:
Common between RTK and PPK:
1.operating modes. Both technologies use a combination of reference stations and rover stations.
2.Initialization is required before operation for both technologies.
3.Both can achieve centimeter-level accuracy.
Differences between RTK and PPK:
1.Communication Method:
RTK requires radio or network communication to transmit differential data.
PPK does not rely on communication technology and can record static data.
2.Positioning Method:
RTK uses real-time positioning, allowing you to view the coordinates and accuracy of measurement points on the rover station at any time.
PPK is a post-processing method, and the coordinates of the point cannot be observed on-site. Results are obtained through post-processing.
3.Working Radius:
RTK operation is limited by the communication station, with a typical distance of not exceeding 10 km.
PPK can operate at a radius of up to 50 km.
4.Effect of Satellite Signals:
In RTK operations, it is easy to lose lock, especially when near obstacles like large trees.
PPK, once initialized, is less prone to losing lock.
5.Positioning Accuracy:
RTK: Plane accuracy of 8mm + 1ppm, elevation accuracy of 15mm + 1ppm.
PPK: Plane accuracy of 2.5mm + 0.5ppm, elevation accuracy of 5mm + 0.5ppm.
6.Positioning Frequency:
RTK: Frequency of differential data sent by the base station and received by the rover station is generally 1-2 HZ.
PPK: Maximum positioning frequency can reach 50HZ.
Comparative Analysis:
1.UAVs have high flying speeds, requiring a high positioning frequency. PPK's support for 50HZ positioning frequency meets this demand.
2.RTK provides real-time location information, while PPK allows solving epoch data over a period, improving the fix rate and achieving higher resolution accuracy.
3.RTK requires radio or network communication modules, while PPK does not, reducing the UAV's burden and extending its flight life.
4.RTK has a limited operating distance, while PPK can reach up to 50 km, making it suitable for large-scale operations in areas like power lines, highways, railways, pipelines, etc.
Conclusion:
Although RTK can provide real-time high-precision location information, there are significant technical challenges in its current application for UAVs. PPK's major advantage over RTK is its ability to be processed afterward, addressing satellite loss-of-lock issues and improving positioning accuracy through fusion and reverse filtering. PPK technology, with its development and compatibility with UAVs, holds significant promise in the field of unmanned aerial surveys, surpassing traditional RTK measurements in efficiency and effectiveness. PPK is poised to become a cornerstone technology for UAVs in mapping applications.