Print Recently, the research team from the National Time Service Center (NTSC), Chinese Academy of Sciences (CAS), discussed the impact of uncalibrated LEO(Low Earth Orbit ) satellite code biases on LEO-augmented PPP, bringing the LEO-relevant benefits from perfect assumptions back to reality.
This study was published in the international journal “GPS Solutions” on December 15, 2025, entitled “The impact of LEO satellite hardware delays on LEO-augmented precise point positioning”.
LEO satellites possess high signal strength and rapid geometric changes, significantly improving the geometry of traditional GNSS observations and accelerating Precise Point Positioning (PPP) convergence. However, current research often relies on an optimistic perfect non-error assumption to estimate the benefits of LEO for PNT services, neglecting the fact that the estimated LEO satellite clocks typically contain the code biases of the GNSS receiver onboard LEO satellite, which differ from those actually needed by ground users, i.e., the clocks containing the code biases of the transmitter downlinking LEO navigation signals. This bias term could also be influenced by the temperature of LEO satellite and suffer from time-varying effects. In the early stages of LEO system development, the number and distribution of ground stations capable of receiving LEO navigation signals are limited, making continuous and precise in-orbit calibration of the aforementioned time-varying LEO hardware delays difficult. Therefore, the impact of these delays on LEO-enhanced PPP performance urgently requires systematic evaluation.
According to Prof. Kan Wang, the team leader of the LEO-augmented PNT from the NTSC, the research helped to reveal part of the reality of the future LEO-augmented PNT. Despite all the model-based benefits to be brought by LEO satellites, various LEO-related errors could largely degrade the actual performance of LEO-augmented PNT services, including the sharply shortened convergence time in PPP.
40 MGEX stations were used to conduct the LEO-enhanced PPP experiments under different error configurations, integrating real GNSS data with simulated LEO observation data. The results showed that real-time LEO satellite orbital and clock errors weaken the PPP convergence speed and the positioning accuracy, but the impact of the aforementioned LEO code biases is more significant. Under the condition of small code biases (C0+P0), the convergence time of static PPP in the vertical direction was shortened from 14.7 min under GNSS-only conditions to 3.1 min; however, when the code biases increased to C40+P12 conditions, the convergence time rebounded to 9.8 min, and the advantage of LEO augmentation is significantly weakened. For some stations, excessive code biases also led to a decrease in positioning accuracy; for example, the vertical positioning accuracy of the static PPP at the BRUX station increased from 0.024 m to 0.034 m. The results indicate that the uncalibrated code biases of the transmitter downlinking LEO navigation signals could limit the gain effect of LEO augmentation on PPP performance.
As noted by one of the reviewers, this study addresses an important topic of LEO-augmented GNSS PPP positioning.
Fig 1. 90th percentile lines of the static (left) and kinematic PPP without (black lines) and with LEO augmentation (other colored lines) under various settings for uncalibrated LEO satellite code biases (Image by Ye et al. 2025)