A comparative analysis of navigation signals in BDS-2 and BDS-3 using zero-baseline experiments

The Chinese Beidou navigation satellite system (BDS) has provided regional and global navigation and positioning services for the users via the BDS-2 and BDS-3 satellites. However, BDS-2 and BDS-3 send navigation signals on different frequencies; three of those are identical to assure system compatibility and interoperability. To comparatively analyze these navigation signals, we carried out three zero-baseline experiments with five brands of receivers, including Trimble, Septentrio, NovAtel, ComNav and Unicore. BDS-2 and BDS-3 multi-frequency data were collected during these experiments. The data were processed to evaluate the code and carrier phase measurement noises and investigate the inter-system biases (ISBs) on the three identical frequencies. The effects of the ISB on ambiguity resolution (AR) and position estimation (PE) were also demonstrated using the zero-baseline data with one Trimble receiver and one Septentrio receiver. The results show some new findings: (1) the code and phase noises in BDS-3 are smaller than those in BDS-2 on the same frequencies, and the three new signals B1C, B2b and B2a have comparable noises to the present signals. Besides, millimeter-level code noises are achieved on B3I and B2a signals in NovAtel receivers; (2) the code ISBs on B1I, B2I/B2b and B3I between BDS-2 and BDS-3 were found and distinguished, while no phase ISB could be found. The code ISB seems to be receiver-related and can be as large as 1 m in heterogeneous receivers. It is stable during a whole day but may also vary due to a restart of the receiver. No code and phase ISB on all three frequencies is observed for homogeneous receivers; (3) the code ISB will hamper the reliable AR and precise PE. The HMW (Hatch–Melbourne–Wübbena) combination on B1I and B3I is severely biased by about 0.6 cycles when double differencing between BDS-2 and BDS-3. After ISB calibration, the ratio value of wide-lane (WL) AR has a slight increase, and the accuracy of DD B1I code positioning has improved more than 10%, but few improvements can be seen in the fixed solution. It should be noted that although the inter-system code bias (ISCB) between BDS-2 and BDS-3 is detected in the baseline data, this bias should be considered in the absolute positioning as well.