## 4 Concluding Remarks  
In this paper, GNSS observables from several stations in the Asia-Pacific area were utilized to investigate the CNR, multipath combinations, and geometry-free ionosphere-free combinations of BDS satellites. Their characteristics with time and elevation were analyzed and compared with GPS and Galileo satellites. The CNRs of all except GEO satellites increase significantly with the increase of elevation, and they become stable when the elevation is over 60\({}^{\circ}\). The CNR of B1 observables are the lowest among the three BDS frequencies, while B3 is the highest. The CNR of the GPS L1 observable is remarkably higher than that of L2, while the CNRs of Galileo observables of all three frequencies are at a similar level. At low elevation angles, the magnitude of code multipath combinations and its dispersion become large. The value of BDS code multipath combination can reach 2.0 m. The code multipath combination of BDS MEO satellites shows \"V-shape\" variation during a one-day period. Negative correlations are pronouncedly seen between the code multipath combinations and elevations of all BDS non-GEO satellites, validating the elevation-dependent biases. After a single differencing between two stations, the V-shape could be eliminated. This suggests that the V-shape feature in multipath combination is mainly related to the satellites. Proper modelling of the elevation-dependent trend could help correct the measurement and eventually improve the positioning accuracy with BDS. Diurnal periodicity was found in the GFIF combinations of both BDS GEO and IGSO satellites. The variation range of carrier phase GFIF combinations of GEO satellites is \(-2.0\) to 2.0 cm. The periodicity of carrier phase GFIF combination could be mitigated significantly through a single differencing between two stations. Our analysis indicates that carrier phase GFIF combinations of BDS GEO and IGSO satellites might contain some satellite-related biases. The GFIF time series of C02/C05 show patterns quite different from those of C01/C03/C04. For the BDS GEO satellites C01/C03/C04, the peak values of correlation occur when the lag equals to the local time difference between the two satellites, suggesting that the GFIF combinations' time series of these satellites might vary according to their relative geometries with the sun. The above mentioned results are expected to provide useful reference not only for promoting performances of hardware at both satellite and receiver ends, but also for improving algorithms of high-precision positioning with multi-GNSS. On the one hand, more evident experiments are necessary  
Figure 12: Cross-correlations between the GFIF combination time series (ten days) of different GEO satellites at the stations GMSD (**a**) and CUT0 (**b**), with the red vertical line indicating the local time difference between the two satellites. to strictly explain the above phenomena; on the other hand, we are trying to develop and improve the positioning algorithm based on the current knowledge of BDS observables' characteristics. This work was partially sponsored by the National \"863 Program\" of China (Grant No. 2014 AA123101), partially sponsored by the National Natural Science Foundation of China (Grant No. 41231174 and 41274039) and partially sponsored by China Postdoctoral Science Foundation funded project (Grant No. 2014M562063). The supports of research grant of the Hong Kong Research Grants Council (RGC) (PolyU 5325/12E, F-PP0F) are acknowledged. The authors would like to thank the IGS Multi-GNSS Experiment (MGEX) for providing the GNSS data. The first author gratefully acknowledges [PERSON] at Fugro Intersite B.V. and [PERSON] at Wuhan University, both of whom provided pertinent advice to improve the manuscript. Thanks also go to the three reviewers and the editors for their constructive comments and revisions to the manuscript.