Preface
Unmanned aerial vehicles are widely used and have the characteristics of low cost, multitasking, good mobility, high efficiency and low radiation, etc. They are widely used in all aspects of military and civil production.
Because GPS has the characteristics of all-weather, high precision, and automatic measurement, the unmanned aerial vehicles currently used for surveying and mapping basically use GPS for positioning and navigation. The GPS single-point positioning accuracy of UAV flight control is too poor. Previously, a large number of image control points were used to correct the distortion of images.
However, in some terrains (such as mountains, canyons, rivers, etc.), it is difficult for field workers to set up image control points. In order to reduce the workload, most of the image control points are reduced or even none are needed. Therefore, it is necessary to improve the accuracy of pos points of the aircraft. Both RTK technology and PPK technology can achieve centimeter-level accuracy.
In the following, we conduct comparative analysis from the two technical principles of RTK and PPK to find out a more suitable method for GPS aerial positioning.
Operating principle of RTK
RTK(Real-Time Kinematic) measurement system generally consists of three parts: GPS receiving equipment, data transmission system and software system for implementing dynamic measurement. RTK measurement technology is a carrier phase difference measurement technology which is based on the carrier phase observation and has the function of fast and high precision positioning. It can obtain the three-dimensional positioning results of the measuring station in the designated coordinate system in real-time, and it has centimeter-level positioning accuracy.
The working principle of RTK measurement is: one receiver is placed on the reference station, and the other one or several receivers are placed on the carrier (called the mobile station). The Reference Station and the mobile station receive the same time at the same time, for signals transmitted by the same GPS satellite, the observed values obtained from the reference station are compared with the known position information to obtain the GPS differential correction value. Then the modified value is transmitted to the mobile station of the common vision satellite in time through the radio data link radio station to refine its GPS observation value, so as to obtain the more accurate real-time position of the mobile station after differential correction.
At present, the positioning plane accuracy of mainstream manufacturers RTK can reach 8mm + 1ppm, and the elevation accuracy can reach 15mm + 1ppm. There are two main communication modes between the base station and the mobile station: the radio station and the network. The radio station signal is stable and the network signal transmission distance is long, each of which has its own advantages.
Operating principle of PPK
The working principle of PPK(Post-Processing Kinematic, GPS dynamic Post-Processing difference) technology is to use a reference station receiver for synchronous observation and at least one mobile receiver for synchronous observation of GPS satellite; that is to say, the reference station keeps continuous observation, the initial mobile station moves to the next undetermined point, and it needs to keep continuous tracking of the satellite during the process of moving, in order to transfer the entire ambiguity to the undetermined point.
The data synchronously received by the reference station and the mobile station are linearly combined in the computer to form a virtual carrier phase observation, determine the relative position between receivers, and finally introduce the known coordinates of the reference station, so as to obtain the three-dimensional coordinates of the mobile station.
PPK technology is the earliest GPS dynamic difference technology (also known as semi-dynamic method, quasi-dynamic relative positioning method, Stop-and-Walk (Stop and Go) method), and its main difference from RTK technology lies in: between the reference station and the mobile station, it is not necessary to establish real-time data transmission like RTK, but to conduct joint processing after measurement on the positioning data collected by the two GPS receivers after positioning observation, so as to calculate the coordinate position of the mobile station in the corresponding time, and the distance between the reference station and the mobile station is not strictly limited. Its advantages are high positioning accuracy, high operation efficiency, large operation radius and easy operation.
Comparison between RTK and PPK
Similarities between RTK and PPK
1. The operation mode is the same. Both technologies adopt the operation mode of reference station and mobile station.
2. Both technologies need initialization before an operation.
3. Both can achieve centimeter-level accuracy.
Differences between RTK and PPK
1. Different communication modes. RTK technology requires radio stations or networks, and it transmits differential data; PPK technology does not need the support of communication technology but records static data.
2. Different methods of positioning operation. The real-time positioning technology adopted by RTK can see the coordinates and accuracy of the measuring points at any time in the mobile station; PPK positioning belongs to post-processing positioning, and the coordinates of the points can not be seen at the scene, and the results can only be seen after-processing.
3. Different operation radius. RTK operation is restricted by the communication radio station, and the working distance is generally not more than 10km. The network mode needs the area covered by the network signal; Using PPK technology operation, the general working radius can reach 50km.
4. Different degrees of influence by satellite signals. During RTK operation, if it is near obstacles such as trees, it is very easy to lose lock; While during PPK operation, after initialization, it is generally not easy to lose lock.
5. Different positioning accuracy. RTK plane accuracy 8mm + 1ppm, elevation accuracy 15mm + 1ppm PPK plane accuracy 2.5mm + 0.5ppm, elevation accuracy 5mm + 0.5ppm.
6. Different positioning frequencies. The frequency of RTK base station sending differential data and mobile station receiving is generally 1-2 HZ, and the maximum PPK positioning frequency can reach 50HZ.
Relative analysis
1. The flight speed of unmanned aerial vehicle is very fast, which requires high positioning frequency. It is difficult to achieve this condition by real-time navigation with RTK technology. PPK supports 50Hz positioning frequency, which fully meets the demand.
2. RTK provides real-time location information. PPK can calculate the calendar element data in a period through post-processing, which can not only improve the fixed rate, but also improve the calculating accuracy.
3. RTK needs to use radio station or network communication module, while PPK does not need to reduce the load of unmanned aerial vehicle and increase the endurance of the aircraft.
4. The operating distance of RTK is limited, and the operating distance of PPK can be as far as 50km. In the long-distance and wide range of operating areas, especially the ribbon areas, such as power transmission lines, highways, railways, oil and gas pipelines, PPK will be the best choice.
Summary
Although RTK can provide high-precision location information in real time, it has great technical problems in the application of unmanned aerial vehicles. Compared with RTK, the biggest advantage of PPK is that it can be post-processing and Reverse filtering (Reverse Kalman Filter), which can solve the problem of unlocking some satellites and improve the positioning accuracy through Forward & Reverse.
PPK technology is a space positioning technology more suitable for unmanned aerial vehicles with the development of science and technology. With the increasingly mature UAV technology, the efficiency of large-scale operations is much higher than that of traditional RTK measurement. PPK technology obviously has a broader development prospect in unmanned aerial vehicles. PPK technology is bound to become the mainstay of unmanned aerial vehicles in surveying and mapping.
