Special Error Sources and Correction Methods for GPS L2P（Y） Signals in GNSS-IR Technology

Objectives With the advancement of global navigation satellite systems (GNSS), GNSS remote sensing technology has been undergoing steady expansion. GNSS interferometric reflectometry (GNSS-IR) has emerged as an integral part of this domain, which allows for remote sensing of the surrounding environment using traditional geodetic receivers. It offers several advantages, including cost-effectiveness, high levels of automation, all-weather operability, continuous monitoring capabilities, and stable frameworks. However, a distinct type of error associated with global positioning system (GPS) L2P signal has been observed. In spectral analysis, this error exhibits a “bimodal” behavior and subsequently contributes to inaccuracies in GNSS-IR retrievals. Presently, the underlying causes of this error source remain unclear, and there are no corresponding correction strategies available. This limitation constrains the utilization of GPS L2P signal within GNSS-IR applications. Given the widespread deployment of dual-frequency GPS tracking stations worldwide, optimizing the utilization of GPS L2P signal has the potential to enhance the performance of these tracking stations in GNSS-IR applications. Hence, this study aims to analyze the origins of the “bimodal” error in GPS L2P signal within the context of GNSS-IR technology and proposes corresponding correction strategies.

Methods First, the Lomb-Scargle periodogram (LSP) method is used to analyze the spectrum of signal-to-noise ratio (SNR) arcs of the L2P signal, and the phenomenon of “double peaks” in LSPs is observed. According to the tracking mode of L2P, the mathematical relationship between the frequencies of the double peaks should be related to the ratio of L1 wavelength and L2 wavelength. Then, the bimodal error in L2P inversion is corrected according to this mathematical relation.

Results The analysis draws upon data from four stations situated in proximity to distinct aquatic environments. It investigates the spectral characteristics of SNR data under different L2P tracking modes and find the bimodal errors linked to various tracking modes influenced on water level/tide inversions. A bimodal error correction strategy is introduced. The results conclusively demonstrate that the proposed error correction strategy effectively mitigates L2P bimodal errors, thereby enhancing the utility of GPS L2P signals in GNSS-IR water level/tide retrievals. No matter which tracing mode is adopted, the LSP diagram of L2P(Y) signals has the phenomenon of “double peaks”. This phenomenon leads to some inversion values not related to L2P signal, but to L1 signal. However, under different tracking modes, the influence of bimodal error is different. Under half-code tracking mode, the SNR arcs of L2P signal are seriously polluted by L1 signal; while under the non-code tracking mode, the SNR arcs of L2P signal are mildly polluted by L1 signal.

Conclusions The proposed correction strategy can effectively correct the bimodal error of L2P(Y) signals. Especially in the half-code tracking mode, this method makes the inversion result of L2P(Y) signals from unusable to usable.