Why PPK System Is More Popular Than RTK System?
UAVs have a wide range of applications, there are many features such as low cost, multi-tasking, no casualties, good mobility, high efficiency, and low radiation. They are widely used in military and civilian production.
Because GPS has all-weather, high-precision and automatic measurement characteristics, the current UAVs used for mapping are basically GPS positioning and navigation.
The GPS single-point positioning accuracy of the UAV flight control is too poor. Previously, a large number of image control points were used to correct the distortion of the image.
However, some special terrains (such as mountains, canyons, rivers, etc.) are difficult to lay out like control points. In order to reduce the workload, reduce most of the image control points or even do not need to control points, it is necessary to improve the pos point of the aircraft. Accuracy, RTK technology, and PPK technology can achieve centimeter accuracy.
Below we proceed from the two technical principles of RTK and PPK and carry out comparative analysis to find a more suitable method for GPS aerial positioning.
RTK operating principle
The RTK (Real Time Kinematic) measurement system generally consists of three parts: a GPS receiving device, a data transmission system, and a software system that implements dynamic measurement.
The RTK measurement technology is based on carrier phase observation. The carrier phase differential measurement technology with fast and high-precision positioning function can obtain the three-dimensional positioning result of the measurement station in the specified coordinate system in real time and has centimeter-level positioning accuracy.
The RTK measurement works by placing one receiver on the base station and the other receiver on a carrier (called a mobile station). The base station and the mobile station simultaneously receive the same GPS satellite at the same time.
The transmitted signal, the observation obtained by the base station is compared with the known position information, and the GPS differential correction value is obtained.
The corrected value is then transmitted to the mobile station of the common-view satellite through the radio data link station to refine its GPS observations, thereby obtaining a more accurate real-time position of the mobile station after the differential correction.
At present, the mainstream manufacturer RTK has a positioning plane accuracy of 8mm+1ppm and an elevation accuracy of 15mm+1ppm. The communication modes between the base station and the mobile station mainly include two types of stations: radio and network.
The radio signal is stable and the network signal transmission distance is long, which has advantages.
PPK System operating principle
The PPK (Post-Processing Kinematic) system works by using a base station receiver for simultaneous observation and at least one mobile receiver to perform simultaneous observations on GPS satellites; that is, the base station remains continuous.
Observed, the initialized rover will move to the next to-be-determined point, and continuous tracking of the satellites needs to be maintained during the relocation process in order to transfer the full-circumference ambiguity to the to-be-determined point.
The data received synchronously by the base station and the rover are linearly combined in a computer to form a virtual carrier phase observation, determine the relative position between the receivers, and finally introduce the known coordinates of the base station to obtain the three-dimensional coordinates of the rover.
PPK system is the earliest GPS dynamic differential technology (also known as semi-dynamic method, quasi-dynamic relative positioning method, Stop and Go method), and its main difference with RTK system is in the base station and the rover It is not necessary to establish real-time data transmission like RTK, but after the positioning observation, the joint processing of the positioning data collected by the two GPS receivers is performed to calculate the coordinate position of the mobile station at the corresponding time.
There is no strict limit to the distance between the base station and the rover. It has the advantages of high positioning accuracy, high work efficiency, large working radius, and easy operation.
The Similar of RTK and PPK System
1. The operation mode is the same. Both technologies use the reference station plus rover mode of operation.
2. Both technologies need to be initialized before the job.
3. both can achieve centimeter accuracy.
The Difference Between RTK and PPK
1. Different communication methods. RTK system requires a radio or network to transmit differential data; PPK technology does not require the support of communication technology and records static data.
2. The way of positioning operations is different. RTK uses real-time positioning technology to see the coordinates and accuracy of the measurement points at any time. The PPK positioning belongs to the post-processing positioning.
The coordinates of the points are not visible at the scene, and the results need to be processed afterward to see the results.
3. The working radius is different. RTK operations are restricted by communication stations. The working distance is generally less than 10 km. The network mode requires an area covered by network signals. With PPK technology, the general operating radius can reach 50 km.
4. The degree of influence by satellite signals is different. In the RTK operation, if it is near the obstacles such as big trees, it is very easy to lose the lock; while in the PPK operation, after the initialization, it is generally not easy to lose the 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. the positioning frequency is different. The RTK system base station transmits differential data and the frequency received by the mobile station is generally 1-2 HZ, and the PPK positioning frequency is up to 50 Hz.
1. The flight speed of the drone is very fast, which requires a 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 location information in real time. PPK can solve the epoch data in one cycle by post-processing method, which can not only improve the fixed rate but also solve the calculation accuracy more.
3. RTK needs to use radio or network communication module, PPK is not needed, reduces the load of the drone, increases the aircraft lifetime.
4. RTK working distance is limited, PPK working distance can be as far as 50km, PPK will be the best choice in long-distance and wide-ranging working areas, especially strip-shaped areas, such as transmission lines, roads, railways, oil and gas pipelines.
Although RTK can provide high-precision position information in real time, there are currently many technical problems in the application of drones. The biggest advantage of PPK over RTK is that it can be processed afterward, reverse Kalman Filter, so that it can solve the problem of some satellites losing lock, and improve the positioning accuracy through the fusion solution (Forward & Reverse).
PPK technology is a space positioning technology that is more suitable for being mounted on drones with the development of technology. UAV technology is becoming more and more mature, and a wide range of operations are much more efficient than traditional RTK measurement. PPK technology has obvious broader development prospects in the field of drones.
PPK technology is bound to become the mainstay of drones in surveying and mapping.