Comprehensive analysis of compatibility between QZSS and GPS in Asia-Pacific region: Signal quality, RTK and PPP

Abstract

The Japanese QZSS, with four operational satellites in orbit, provides positioning, navigation and timing services as a GPS-complementary system. However, the compatibility between GPS and QZSS has not been adequately investigated so far. In this paper, their compatibility, from the aspects of signal quality, GPS + QZSS inter-system RTK and GPS + QZSS combined PPP is comprehensively analyzed. First, the signal quality comparison using 10-day zero-baseline observations shows that although QZSS has slightly lower carrier‑to‑noise‑density ratio (C/N0) than GPS, it has GPS-comparable accuracies of 36.2 and 7.4 cm on L1 and L5 code observations respectively and 2.4, 2.1 and 2.2 mm on L1/L2, L1/L5 and L2/L5 geometry-free phase observations respectively, and an even better one of 22.3 cm on L2 code observation. Second, the short-baseline experiment testifies that GPS + QZSS inter-system double-difference (ISDD) RTK obtains better positioning performance than traditional double-difference (TDD) RTK in terms of ambiguity-fixing success rate (ASR) and positioning accuracy. This indicates a good compatibility between GPS and QZSS which benefits from their same frequency on L1, L2 and L5. And at 10° cut-off elevation, the GPS + QZSS with ISDD RTK model obtains the ASR improvements of 15.3%, 4.0% and 73.1% over GPS-only on L1, L2 and L5, respectively. The significant improvement on L5 is due to only 12 GPS Block IIF satellites transmitting the L5 signal. Third, the theoretical derivation demonstrates that the inter-system bias (ISB) in GPS + QZSS PPP is composed of the constant part (i.e., receiver-dependent hardware delay difference and AC-dependent clock datum difference) and the variable part. The analysis of ISB characteristics shows that ISBWUM and ISBCOM have the same variation trend and behave stably with a daily variation of 0.4 m, while ISBGBM makes a significant daily variation of 1.4 m. Furthermore, the GPS + QZSS dual- and triple-frequency PPP based on the observations of 40 GNSS stations in Asia-Pacific region show that the GPS + QZSS PPP performance using WUM and COM products is superior to that using GBM product. These results denote that WUM and COM products make GPS and QZSS more compatible in PPP. The four-satellite QZSS system is able to effectively improve the performance of RTK and PPP for GPS in Asia-Pacific region and this improvement increases with increasing cut-off elevation.