Instantaneous ambiguity resolution techniques are methods employed to achieve real-time high-accuracy positioning. The advent of the Chinese BeiDou system enables us to evaluate the performance of the combined GPS/BeiDou/QZSS dual-frequency ambiguity resolution and BeiDou three-frequency ambiguity resolution. It is known that the increasing number of satellites used can increase the reliability as well as the availability of single-epoch real-time kinematic (RTK) information. Therefore, performance improvement of single-epoch RTK by adding BeiDou satellites is strongly expected because many BeiDou satellites are operated in Asian regions. The first objective of this study is to conduct an initial assessment of the single-epoch RTK performance, as well as standalone positioning/code relative positioning using GPS/BeiDou/QZSS. The second objective of this study is to evaluate the performance of the longer-baseline single-epoch ambiguity resolution using the three-frequency observation data. Furthermore, the possibility of future single-epoch RTK service is discussed in this paper.

Improving GPS positioning accuracy requires an understanding of the inner workings of GPS receivers. However, the necessary hardware and software for research is prohibitively expensive. It is almost impossible to modify the correlator and functions of signal acquisition and tracking in commercial GPS hardware. The software GPS receiver allows us to access the inner workings of the receiver without significant time or expense. The present paper introduces a prototype software GPS receiver developed by us, which consists of a commercial RF-module and PC-based signal processing software. In addition, the software GPS receiver is shown herein to enable evaluation and mitigation of the code multipath error with the outputs of a software multi-correlator, which can be implemented easily in a software GPS receiver, with the aid of maximum likelihood criteria.

In this study, the most recent topics related to the precise global navigation satellite system (GNSS) positioning technology are discussed. Precise positioning here means that the position can be estimated with centimeter-level accuracy. Technologies supporting precise GNSS positioning include an increase in the number of positioning satellites and the availability of correction data. Smartphones are now capable of centimeter-level positioning. For correction data, real-time kinematic positioning (RTK)-GNSS, which has primarily been used in surveying, and the new precise point positioning-real-time kinematic (PPP-RTK) and PPP, are garnering attention. The Japanese Quasi-Zenith Satellite System was among the first to broadcast PPP-RTK and PPP correction data free of charge. RTKLIB has long been popular for both real-time and post-processing precise positioning. Here, I briefly present a method for improving this software. Precise positioning technology remains crucial as the use of GNSS in highly reliable applications, such as advanced driver-assistance systems, autonomous drones, and robots, is increasing. To ensure precise positioning, improving multipath mitigation techniques is essential; therefore, key factors related to these techniques are discussed. I also introduce my efforts to develop software GNSS receivers for young researchers and engineers as a basis for this purpose. This study is aimed at introducing these technologies in light of the most recent trends.

The increasing demand for navigation and automation has led to the development of a number of accurate and precise navigation applications that make use of the Global Navigation Satellite System (GNSS) and additional sensors. One of the precise navigation techniques in GNSS, the real-time kinematic (RTK) technique, is well known. In this method, once the correct integer ambiguities are found in the carrier phase observation data, position can be determined to within 10cm. In particular, the advent of QZSS and BeiDou satellites can increase the availability of RTK-GNSS (relative to RTK using only GPS). It is understood that the increasing availability of RTK-GNSS will improve the performance of the integration of GNSS with additional sensors because the errors due to the inertial measurement unit (IMU) accumulate as time goes on. On the other hand, GNSS tends to suffer from multipath errors, especially in urban environments. To overcome this problem, a method was developed for improving RTK-GNSS using a low-cost IMU and conventional vehicle speed sensors. In this study, the quality of the complete observation data was assessed based on the carrier-to-noise ratio and satellite elevation angle, and the least-squares ambiguity decorrelation adjustment method and the ratio test were used to obtain fixed positions. We used speed information obtained from Doppler measurements as an alternative source of information; information from the IMU and vehicle speed sensor (integrated with the RTK-GNSS via a Kalman Filter) was used when there were no visible satellites. We also used the IMU and vehicle speed sensors to detect wrong fixes in the RTK-GNSS. A position and orientation system for land vehicles (Applanix) was used to estimate the reference positions. During GNSS outages, it is important to accurately determine the last heading of the car for precise navigation. In this study, it was found that GNSS Doppler-based direction data are required to obtain better direction information. The results of the experiment demonstrate that our proposed method is, to some extent, beneficial as an alternative to the conventional RTK-GPS in an urban environment.