The emerging Internet of Things (IoT) applications, such as smart manufacturing and smart home, lead to a huge demand on the provisioning of low-cost and high-accuracy positioning and navigation solutions. Inertial measurement unit (IMU) can provide an accurate inertial navigation solution in a short time but its positioning error increases fast with time due to the cumulative error of accelerometer measurement. On the other hand, ultrawideband (UWB) positioning and navigation accuracy will be affected by the actual environment and may lead to uncertain jumps even under line-of-sight (LOS) conditions. Therefore, it is hard to use a standalone positioning and navigation system to achieve high accuracy in indoor environments. In this article, we propose an integrated indoor positioning system (IPS) combining IMU and UWB through the extended Kalman filter (EKF) and unscented Kalman filter (UKF) to improve the robustness and accuracy. We also discuss the relationship between the geometric distribution of the base stations (BSs) and the dilution of precision (DOP) to reasonably deploy the BSs. The simulation results show that the prior information provided by IMU can significantly suppress the observation error of UWB. It is also shown that the integrated positioning and navigation accuracy of IPS significantly improves that of the least squares (LSs) algorithm, which only depends on UWB measurements. Moreover, the proposed algorithm has high computational efficiency and can realize real-time computation on general embedded devices. In addition, two random motion approximation model algorithms are proposed and evaluated in the real environment. The experimental results show that the two algorithms can achieve certain robustness and continuous tracking ability in the actual IPS.

Kalman-Filter-Based Integration of IMU and UWB for High-Accuracy Indoor Positioning and Navigation

Autonomous orbit determination is the ability of navigation satellites to estimate the orbit parameters on-board using inter-satellite link (ISL) measurements This study mainly focuses on data processing of the ISL measurements as a new measurement type and its application on the centralized autonomous orbit determination of the new-generation Beidou navigation satellite system satellites for the first time The ISL measurements are dual one-way measurements that follow a time division multiple access (TDMA) structure The ranging error of the ISL measurements is less than 025 ns This paper proposes a derivation approach to the satellite clock offsets and the geometric distances from TDMA dual one-way measurements without a loss of accuracy The derived clock offsets are used for time synchronization, and the derived geometry distances are used for autonomous orbit determination The clock offsets from the ISL measurements are consistent with the L-band two-way satellite, and time–frequency transfer clock measurements and the detrended residuals vary within 05 ns The centralized autonomous orbit determination is conducted in a batch mode on a ground-capable server for the feasibility study Constant hardware delays are present in the geometric distances and become the largest source of error in the autonomous orbit determination Therefore, the hardware delays are estimated simultaneously with the satellite orbits To avoid uncertainties in the constellation orientation, a ground anchor station that “observes” the satellites with on-board ISL payloads is introduced into the orbit determination The root-mean-square values of orbit determination residuals are within 100 cm, and the standard deviation of the estimated ISL hardware delays is within 02 ns The accuracy of the autonomous orbits is evaluated by analysis of overlap comparison and the satellite laser ranging (SLR) residuals and is compared with the accuracy of the L-band orbits The results indicate that the radial overlap differences between the autonomous orbits are less than 150 cm for the inclined geosynchronous orbit (IGSO) satellites and less than 100 cm for the MEO satellites The SLR residuals are approximately 150 cm for the IGSO satellites and approximately 100 cm for the MEO satellites, representing an improvement over the L-band orbits

Initial results of centralized autonomous orbit determination of the new-generation BDS satellites with inter-satellite link measurements

The BeiDou Navigation Satellite System is expected to provide a global positioning and navigation service by 2020. To achieve this goal, the new-generation navigation satellites that have been launched since March 2015 are equipped with inter-satellite links (ISLs), with the objective of testing new navigation signals and the ISLs themselves. Using these new-generation navigation satellites and several ground facilities in China, a combined orbit determination experiment was carried out during August 2016. The orbit mechanical model, orbit determination method, and accuracy evaluation method used in this experiment are presented here. The accuracy of the combined orbit determination method is evaluated, and the performance-related improvements resulting from the ISLs are analyzed. The performance of orbit determination has been increased about 37–76% for different satellites in orbit-only signal-in-space range error (orbit-only SISRE).

Globalization highlight: orbit determination using BeiDou inter-satellite ranging measurements

Various types of onboard atomic clocks such as rubidium, cesium and hydrogen have different frequency accuracies and frequency drift rate characteristics. A passive hydrogen maser (PHM) has the advantage of low-frequency drift over a long period, which is suitable for long-term autonomous satellite time keeping. The third generation of Beidou Satellite Navigation System (BDS3) is equipped with PHMs which have been independently developed by China for their IGSO and MEO experimental satellites. Including Galileo, it is the second global satellite navigation system that uses PHM as a frequency standard for navigation signals. We briefly introduce the PHM design at the Shanghai Astronomical Observatory (SHAO) and detailed performance evaluation of in-orbit PHMs. Using the high-precision clock values obtained by satellite-ground and inter-satellite measurement and communication systems, we analyze the frequency stability, clock prediction accuracy and clock rate variation characteristics of the BDS3 experimental satellites. The results show that the in-orbit PHM frequency stability of the BDS3 is approximately 6 × 10−15 at 1-day intervals, which is better than those of other types of onboard atomic clocks. The BDS3 PHM 2-, 10-h and 7-day clock prediction precision values are 0.26, 0.4 and 2.2 ns, respectively, which are better than those of the BDS3 rubidium clock and most of the GPS Block IIF and Galileo clocks. The BDS3 PHM 15-day clock rate variation is − 1.83 × 10−14 s/s, which indicates an extremely small frequency drift. The 15-day long-term stability results show that the BDS3 PHM in-orbit stability is roughly the same as the ground performance test. The PHM is expected to provide a highly stable time and frequency standard in the autonomous navigation case.

Performance of the BDS3 experimental satellite passive hydrogen maser

Time synchronization of new-generation BDS satellites using inter-satellite link measurements

Improvement of orbit determination accuracy for Beidou Navigation Satellite System with Two-way Satellite Time Frequency Transfer

Precise orbit determination and rapid orbit recovery supported by time synchronization

Satellite-station two-way time comparison is a typical design in Beidou System (BDS) which is significantly different from other satellite navigation systems. As a type of two-way time comparison method, BDS time synchronization is hardly influenced by satellite orbit error, atmosphere delay, tracking station coordinate error and measurement model error. Meanwhile, single-way time comparison can be realized through the method of Multi-satellite Precision Orbit Determination (MPOD) with pseudo-range and carrier phase of monitor receiver. It is proved in the constellation of 3GEO/2IGSO that the radial orbit error can be reflected in the difference between two-way time comparison and single-way time comparison, and that may lead to a substitute for orbit evaluation by SLR. In this article, the relation between orbit error and difference of two-way and single-way time comparison is illustrated based on the whole constellation of BDS. Considering the all-weather and real-time operation mode of two-way time comparison, the orbit error could be quantifiably monitored in a real-time mode through comparing two-way and single-way time synchronization. In addition, the orbit error can be predicted and corrected in a short time based on its periodic characteristic. It is described in the experiments of GEO and IGSO that the prediction accuracy of space signal can be obviously improved when the prediction orbit error is sent to the users through navigation message, and then the UERE including terminal error can be reduced from 0.1 m to 0.4 m while the average accuracy can be improved more than 27%. Though it is still hard to make accuracy improvement for Precision Orbit Determination (POD) and orbit prediction because of the confined tracking net and the difficulties in dynamic model optimization, in this paper, a practical method for orbit accuracy improvement is proposed based on two-way time comparison which can result in the reflection of orbit error.

Satellite-station time synchronization information based real-time orbit error monitoring and correction of navigation satellite in Beidou System

Construction of a BDSPHERE solar radiation pressure model for BeiDou GEOs at vernal and autumn equinox periods

For sub-meter precise orbit determination of GEO satellites, solar radiation pressure (SRP) perturbation is the major error and also the main factor that restrains the accuracy of orbit prediction. Compared with MEO satellites, GEO satellites are easier to be influenced by SRP perturbation due to their high altitude orbit feature and the magnitude achieves 10–7 (relative to the central gravity). Solar pressure modeling relates to multiple factors, including the satellite platform, attitude controlling, etc., thus it is difficult to accurately model it. It is also the biggest error term that affects the accuracy of precise orbit determination (POD) for medium and high Earth orbit satellites. This paper completes the construction of the time series of the 3.5-year model SRP using the recent 3.5-year real data from COMPASS using the post-processing method. Meanwhile, this paper discovers a secular variation trend and a half-year period of the Cr coefficient. The parameters variation features relating to the seasonal solar activities are investigated. Estimated SPR parameters and periods of solar activities are analyzed, as well as the relationship among the POD accuracy. Therefore, some fruitful conclusions have been got, which helps to establish and refine the COMPASS GEO SRP model.

Shan Wu

Papers

Improvement of orbit determination accuracy for Beidou Navigation Satellite System with Two-way Satellite Time Frequency Transfer

Precise orbit determination and rapid orbit recovery supported by time synchronization

Satellite-station time synchronization information based real-time orbit error monitoring and correction of navigation satellite in Beidou System

Construction of a BDSPHERE solar radiation pressure model for BeiDou GEOs at vernal and autumn equinox periods

Application Characteristics Analysis of the T20 Solar Radiation Pressure Model in Orbit Determination for COMPASS GEO Satellites