Haojun Li

Performance of Real-Time Precise Point Positioning

The IGS Real-time Pilot Project (IGS-RTPP) provides real-time precise orbits and clocks, which support real-time positioning for single stations over large areas using the Precise Point Positioning (PPP) technique. This paper investigates the impact of real-time orbits, network configuration, and analysis strategies on real-time PPP implementation and demonstrates the real-time PPP performance. One month of data from the IGS network is analyzed in a real-time simulation mode. Results reveal the following: (1) In clock estimation, differential approaches are much more efficient than the zero-differenced approach. (2) The precision of IGS Ultra rapid (IGU) orbits could meet the IGS-RTPP requirement for precise clock estimation and PPP positioning. (3) Considering efficiency and precision, a network with 50 stations is recommended for the IGS-RTPP. It is demonstrated that the real-time satellite clock precision is 0.1 ns supporting hourly static PPP with a mean precision of 2–3 cm in the North component and 3–4 cm in the other components. Kinematic PPP assessed with onboard GPS data collected from a buoy provided mean coordinate precision of 2.2, 4.2, 6.1 cm in the North, East and Up directions, compared to the RTK solutions.

Application of Inter-system Hardware Delay Bias in GPS/GLONASS PPP

GPS applies Code Division Multiple Access technique in signal coding, while GLONASS’s signal is produced with Frequency Division Multiple Access technique. The differences in signal frequency results in inter-system hardware delay bias for GPS/GLONASS receivers. Subjecting to these hardware delays, strategies and models of GPS/GLONASS PPP based positioning needs to be modified. We derived a GPS/GLONASS combined PPP positioning models by introducing inter-system hardware delay biases. Several scenarios were simulated to test the introduced models using the observation of IGS stations. The results show that: ① Adding a couple of GLONASS satellites can improve the positioning accuracy in the environment without enough GPS satellites being tracked; ② The inter-system hardware delay bias is stable on daily base, and it could be predicted and be fixed in the GPS/GLONASS combined PPP.

SHA: The GNSS Analysis Center at SHAO

Today, most precise GNSS products, including orbits and clocks, are provided by the International GNSS Service (IGS) and its Analysis Centers (ACs). Each AC provides its products to the AC Coordinator (ACC) for combination. ACs develop their own software packages by implementing different strategies, which as a result improving the robustness of the combined products. Following the IGS AC strategy and to fulfill the requests of satellite missions in China, we set up the GNSS Analysis Center at Shanghai Astronomical Observatory (SHAO). Currently our GNSS routine analysis includes: Global GPS+GLONASS data processing, GLOBAL+CMONOC GPS data processing. In the first routine, we use ~110 global stations, of which ~50 have GLONASS observations, to derive the integrated and consistent GNSS products. In the second routine, we combine the IGS network used the first routine and the Crustal Movement Observation Network of China (CMONOC) network, GPS only solution is performed using ~300 stations. This paper introduces the details of the Analysis Center and presents the latest results.

Impact of GPS differential code bias in dual- and triple-frequency positioning and satellite clock estimation

The features and differences of various GPS differential code bias (DCB)s are discussed. The application of these biases in dual- and triple-frequency satellite clock estimation is introduced based on this discussion. A method for estimating the satellite clock error from triple-frequency uncombined observations is presented to meet the need of the triple-frequency uncombined precise point positioning (PPP). In order to evaluate the estimated satellite clock error, the performance of these biases in dual- and triple-frequency positioning is studied. Analysis of the inter-frequency clock bias (IFCB), which is a result of constant and time-varying frequency-dependent hardware delays, in ionospheric-free code-based (P1/P5) single point positioning indicates that its influence on the up direction is more pronounced than on the north and east directions. When the IFCB is corrected, the mean improvements are about 29, 35 and 52% for north, east and up directions, respectively. Considering the contribution of code observations to PPP convergence time, the performance of DCB(P1–P2), DCB(P1–P5) and IFCB in GPS triple-frequency PPP convergence is investigated. The results indicate that the DCB correction can accelerate PPP convergence by means of improving the accuracy of the code observation. The performance of these biases in positioning further verifies the correctness of the estimated dual- and triple-frequency satellite clock error.

Estimation and evaluation of the triple-frequency GPS satellite inter-frequency clock bias

A new estimation method is presented to detect and remove the potential outliers in the triple-frequency GPS inter-frequency clock bias (IFCB) estimating. The estimated results of the robust estimation (RE) can successfully mitigate the influence of outliers on IFCB estimation, IFCB modelling and IFCB application. The difference between the constant parts of IFCB estimated with observations and the differential code bias (DCB) (P1–P2) and DCB (P1–P5) products is computed and this difference shows that the DCB products cannot completely replace the constant parts of IFCB. The characteristic of the IFCB modeling is discussed based on the 5- year results. The estimated IFCB is evaluated and its impact on the precise point positioning (PPP) is analysed. The PPP results show that the influence of the IFCB on the static positioning is different with that of kinematic mode.

iBeacon/WiFi Signal Characteristics Analysis for Indoor Positioning Using Mobile Phone

Mobile phone equipped with WiFi, Bluetooth, MEMS-IMU, and other sensors is a perfect platform to provide location based service for personal users. For positioning using wireless signals, the fingerprints of received signal strength (RSS) measurements from access points (APs) are typically used, which are called fingerprinting methods. The main characteristic of the fingerprint positioning technologies is to calculate the similarities between the observed RSS and the known RSS fingerprints. Compared with WiFi, the iBeacon APs are with lower cost, smaller size, and more flexible location, so it is much more suitable and economic for applications where enough WiFi APs are not available. In this paper, the RSS noise characteristics of iBeacon and WiFi signals are compared. The fingerprinting positioning performance by using iBeacon and WiFi RSS measurements are further presented and evaluated.