On the feasibility of resolving Android GNSS carrier-phase ambiguities

High-precision navigation using low-cost handsets has profound potential for mass-market applications, which has been being boosted by the release of raw GNSS data from Google Android smart devices. However, integer ambiguity fixing for centimeter-level GNSS positioning is prevented by the unaligned chipset initial phase biases (IPBs) found within Android carrier-phase data. In this study, we thus investigate the temporal behaviors of those chipset IPBs using zero baselines where smart devices are linked to external survey-grade antennas, and find that the IPBs are generally stable over time as the mean standard deviation of single-epoch IPB estimates derived from continuous carrier-phase data is as low as 0.04 cycles for all satellites. Unfortunately, these chipset IPBs differ randomly among satellites and change unpredictably if carrier-phase signals are re-tracked, discouragingly suggesting that the chipset IPBs cannot be pre-calibrated or even calibrated on the fly. We therefore have to presumably correct for them in a post-processing manner with the goal of inspecting the potential of Android GNSS ambiguity resolution if hopefully the IPBs can be gone. For a vehicle-borne Nexus 9 tablet with respect to a survey-grade receiver located 100-2000 m away, we achieve the first ambiguity-fixed solution within 321 s and finally 51.6% of all epochs are resolved; the ambiguity-fixed epochs can achieve a positioning accuracy of 1.4, 2.2 and 3.6 cm for the east, north and up components, respectively, showing an improvement of 30%-80% compared to the ambiguity-float solutions. While all smart devices above are connected to external survey-grade antennas, we find that a Xiaomi 8 smartphone can be coupled effectively with a miniaturized portable patch antenna, and then achieve commensurate carrier-phase tracking and ambiguity-fixing performance to those of a commercial μ-blox receiver with its dedicated patch antenna. This is encouraging since a compact and inexpensive patch antenna paired with smart devices can promote the democratization of high-precision GNSS.

Performance Analysis of Positioning Solution Using Low-Cost Single-Frequency U-Blox Receiver Based on Baseline Length Constraint

With the rapid development of the satellite navigation industry, low-cost and high-precision Global Navigation Satellite System (GNSS) positioning has recently become a research hotspot. The traditional application of GNSS may be further extended thanks to the low cost of measuring instruments, but effective methods are also desperately needed due to the low quality of the data obtained using these instruments. Thus, in this paper, we propose the analysis and evaluation of the ambiguity fixed-rate and positioning accuracy of single-frequency Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) data, collected from a low-cost u-blox receiver, based on the Constrained LAMBDA (CLAMBDA) method with a baseline length constraint, instead of the classical LAMBDA method. Three sets of experiments in different observation environments, including two sets of static short-baseline experiments and a set of dynamic vehicle experiments, are adopted in this paper. The experiment results show that, compared to classical LAMBDA method, the CLAMBDA method can significantly improve the success rate of the GNSS ambiguity resolution. When the ambiguity is fixed correctly, the baseline solution accuracy reaches 0.5 and 1 cm in a static scenario, and 1 and 2 cm on a dynamic platform.