A joint research team from the National Time Service Centre (NTSC) of the Chinese Academy of Sciences (CAS) and the Curtin University conducted a study on the influences of Low Earth orbit (LEO) augmentation on the precise point positioning---real-time kinematic (PPP-RTK) positioning and its integrity monitoring. They found that the convergence of the formal positioning precision and its protection levels in the horizontal direction can be significantly shortened.
The study was published in remote sensing.
LEO augmentation has been frequently discussed to improve positioning performance. Its benefits in the integrity monitoring (IM) of the positional results, which find more and more applications nowadays in the safety-related applications like the intelligent transports systems, were less discussed , said by Prof. WANG Kan from the NTSC.
Using 1 Hz simulated data from networks of different scales around Beijing for global positioning system (GPS)/Galileo/Beidou navigation satellite system (BDS) and the planned LEO constellation of CentiSpace with 150 satellites on L1/L5, scientists found that the convergence of the formal horizontal precision can be significantly shortened in the ambiguity-float case, especially for the single-constellation scenarios with low precision of the interpolated ionospheric delays.
In addition to the positioning itself, scientists proposed an IM strategy for multi-constellation PPP-RTK positioning based on global navigation satellite system (GNSS) signals. Based on this strategy, the ambiguity-float horizontal protection levels (HPLs) are sharply reduced in various tested scenarios when augmented with LEO satellites, i.e., with an improvement of more than 60% in the ambiguity-float scenario from 5 to 30 min after the processing start.
The study benefits the GNSS community in two aspects. It first proposed a general IM strategy for the PPP-RTK positioning. Different from the classical IM strategy like the ARAIM, the IM strategy proposed for PPP-RTK considers at one side the bias propagation of, e.g., the multipath effects, within the filter. On the other side, it was designed to consider the ionospheric interpolation errors that are related to both the ionospheric activity and the network scale, which could be an essential factor disturbing the PPP-RTK positioning results and its integrity.
As for the second aspect, the contribution reveals the importance of LEO augmentation to the PPP-RTK positioning and its integrity. With the rapid geometry change brought by the LEO satellites, the convergence time of both the positional results and their protection levels are expected to be reduced.
In the future, the group will further look into the benefits of the LEO-augmentation on the PPP-RTK positioning, especially for those with large-scale networks that are not able to enable the integer ambiguity resolution (IAR) within a short period. LEO-augmentation, hopefully, would help with the fast convergence and the IAR, which may partially replace the function of the missing ionospheric corrections in large-scale networks.
Nation Time Service Center, CAS