The positioning system based on the signals of opportunity (SOPs) of low Earth orbit (LEO) satellites can meet the positioning requirements of the receiver in the Global Navigation Satellite System (GNSS) denial environment. However, the received signal does not contain or cannot extract the orbit information and signal emission time of radiation source, which has become the main error source in this positioning system. Taking the Iridium satellites as an example, a Doppler differential(DD)-LEO (DD-LEO) positioning framework is proposed in this paper, and the baseline impact analysis of differential Doppler positioning system (DDPS) and a performance improvement method are carried out. Firstly, the workflow between a base and a rover is introduced, and the error elimination of the DDPS is analyzed. Secondly, the equivalent Doppler error models formed by the orbit error at the base and the rover are established respectively, and the relationship between the baseline and the mean or variance of equivalent Doppler residual in the DDPS is derived. Then, aiming at the problem of poor positioning accuracy due to the inconsistency of space-time reference in the long-baseline DDPS, a signal emission time estimation (SETE) algorithm based on maximum likelihood estimation (MLE) is proposed to further improve the positioning accuracy. Finally, a simulation experiment of the influence of baseline length and direction on the DDPS is implemented, and several groups of field differential experiments with different baseline lengths are completed. The experimental results of long-baseline positioning show that the DDPS can reduce the 3D positioning error by 48.0% compared with the single station without differential positioning. At the same time, the positioning accuracy of the rover station is further enhanced by 86.3% when the base station adopts the SETE algorithm, which verifies the effectiveness of the proposed algorithm.

In order to get rid of the dependence of the navigation and positioning system on the global navigation satellite system (GNSS), radio, television, satellite, and other signals of opportunity (SOPs) can be used to achieve receiver positioning. The space-based SOPs based on satellites offer better coverage and availability than ground-based SOPs. Based on the related research of Iridium SOPs positioning in the open environment, this paper mainly focuses on the occluded environment and studies the Iridium SOPs positioning technique in weak signal environment. A new quadratic square accumulating instantaneous Doppler estimation algorithm (QSA-IDE) is proposed after analysing the orbit and signal characteristics of the Iridium satellite. The new method can improve the ability of the Iridium weak signal Doppler estimation. The theoretical analysis and positioning results based on real signal data show that the positioning based on Iridium SOPs can be realized in a weak signal environment. The research broadens the applicable environment of the Iridium SOPs positioning, thereby improving the availability and continuity of its positioning.

In recent years, some research has been carried out focusing on navigation utilizing signals of opportunity (SOP) transmitted by single low-orbit constellation. However, single constellations usually meet problems such as small visible number and poor geometrical distribution, which limits the positioning accuracy and reliability. In this article, aiming to obtain accurate and reliable position of the user, a scheme fusing the Doppler information from Iridium NEXT and Orbcomm satellites is presented. We first obtain the Doppler frequency of the carrier corresponding to two constellations. Then, Helmert variance component estimation (HVCE) algorithm is proposed to obtain the optimal weights fusing the two constellations. After that, the position of the user is resolved using weighted least-square positioning applying the optimal weights. Finally, the variance of positioning error is derived to evaluate the effectiveness of the system and the performance of the positioning algorithms. The experimental results show that both the mean error and RMSE of the position estimates decreased, which validate the accuracy and reliability of the proposed positioning scheme and algorithms.

Opportunistic positioning utilizing low Earth orbit satellite (LEO) signals mostly adopts Doppler positioning, the performance of which largely depends on the accuracy of Doppler measurement at the receiver. Traditional Doppler measurement methods for Iridium NEXT signals are limited and mostly implemented in frequency domain, which cannot avoid the limitation of FFT operation. Aiming at this problem, this paper proposes a method achieving accurate Doppler measurement of Iridium NEXT signal in time domain, namely phase-time method. This method achieves accurate Doppler measurement by measuring the change rate of the signal phase over time. Experiments were implemented using real Iridium NEXT signals, and the results have demonstrated that compared with the existing method, the Doppler measurement value obtained by phase-time method possesses higher accuracy. Taking the Doppler measurement values obtained by phase-time method as observations, the stability and reliability of Doppler positioning can be significantly improved. The proposed phase-time method is of great significance to LEO-signal frequency estimation in time domain, and further contributes to opportunistic positioning using LEO constellations.

Doppler positioning using the signals of opportunity (SOP) from low earth orbit (LEO) satellites is a supplement and enhancement to the global navigation satellite system (GNSS). However, due to its non-navigation design, it could not obtain high-precision spatial and temporal information of satellite orbit, which significantly affects the accuracy of positioning results. To address this issue, the orbit error equivalent Doppler measurement error (OEEDE) of LEO satellites under the accuracy of auxiliary orbit information is modeled for the first time in this paper. And according to this model analysis, a two-step improved positioning method based on orbit error compensation and weighting (OECW) is proposed to weaken the effect. Experiments using multiple groups of real acquired Iridium-Next and Orbcomm satellite signals have shown that the OECW method achieves higher positioning accuracy than existing methods, especially for the Iridium-Next satellite signals, where the horizontal positioning accuracy is improved by more than 70%. The OECW positioning method proposed in this paper is of great significance for the performance improvement of LEO positioning systems.

The performance of the integrated global navigation satellite system (GNSS) and inertial navigation system (INS) has been proven to be more accurate, reliable, and continuous than stand-alone systems. The development of micro-electronic mechanical system (MEMS) enables inertial measurement unit (IMU) to meet the low-cost and small-size requirements of vehicular navigation. However, the stochastic error characteristics of the inertial sensors and the instability caused by the GNSS signal outages pose a threat to the MEMS-based GNSS/INS land vehicle navigation system. Within this context, we propose the following two-step GNSS/INS integrated architecture at two levels: 1) enhancing the signal-to-noise ratio (SNR) of MEMS-INS raw measurements utilizing a hybrid denoising algorithm with wavelet transform and support vector machine (SVM) and 2) improving the positioning accuracy by a SVM-based data fusion approach which could predict the accumulated error of the MEMS inertial sensors during the GNSS outages. A rotation platform experiment and a field test were carried out, which suggests that the proposed method can effectively eliminate the stochastic errors of MEMS-IMU, and significantly improve the overall positioning accuracy in land vehicle navigation.