A new unambiguous tracking technique based on combined correlation functions for sine Binary Offset Carrier (BOC) signals is proposed in this paper. The key to this method is to exploit two types of local reference signals: the BOC signal and a linear combination of a series of BOC signals with different delays. They are both correlated with the received signals. Then, a correlation function without any positive side peaks is obtained by multiplying the two correlation results to make tracking completely unambiguous. Theoretical analysis and simulation in the tracking stage show that the proposed method has the best code tracking accuracy among the method tracking BOC signals like Binary Phase-Shift Keying signals (BPSK-LIKE), the Pseudo correlation function based Unambiguous Delay Lock Loop (PUDLL), Symmetrical Pulse Ambiguity Removing (SPAR) technique, the method proposed by Shen Feng (SF) and the two methods proposed by Yan Tao (YT-V1 and YT-V2). In multipath environments, the proposed method has the best anti-multipath performance of all the tracking methods mentioned above. In conclusion, the proposed method can completely eliminate ambiguity and has significant performance advantages compared with the methods mentioned above.

In addressing the limitations of the self-attention mechanism, particularly in handling local features and channel adaptivity for modulation recognition tasks, this paper presents the Window Attention Convolution Network (WACN). The proposed approach partitions the input In-phase and Quadrature (IQ) signals into multiple local windows. Within these windows, attention computation constructed by deep convolution, deep dilated convolution and pointwise convolution is used to analyze local features and extract important information. Comprehensive evaluations based on four publicly available datasets demonstrate the superiority of WACN in terms of classification accuracy, comparing to other seven state-of-the-art models based on CNN, RNN, Transformer and hybrid networks. It is worth noting that on the HisarMod2019.1 dataset, WACN’s recognition accuracy is more than 85% at the lowest SNR, and the overall recognition accuracy is more than 95%. At 0dB, WACN significantly enhances accuracy compared to traditional modulation recognition methods. The implementation details and source code are available at github1.

To better understand and evaluate the GNSS positioning performance, it is convenient to adopt corresponding measures to reduce the impact of errors on positioning. A GNSS positioning error estimation scheme based on an improved multi-layer perceptron model is proposed. The multi-head attention mechanism and gating operation are integrated into the multi-layer perceptron model to adaptively select and filter features, enhancing the model’s ability to understand input features. First, the original positioning error of the satellite is obtained through the Kalman filter positioning method. The data are then preprocessed to extract available features. Finally, the features are input into the constructed model for training and testing to obtain the estimated positioning error value. Two types of comparative experiments were completed. The performance of the presented model is evaluated by the root mean square error. Experimental results show that the proposed method performs well in terms of performance indicators, and has obvious advantages over other state-of-the-art methods. In particular, the root mean square error of the presented method in the first dataset is 0.239 m, which is $39.2\%$ and $17\%$ lower than the current state-of-the-art long short-term memory network and convolutional neural network, respectively. The presented method can provide higher-precision estimated values for studying the GNSS positioning error estimation problem.

Alternative Binary Offset Carrier (AltBOC) modulation signal has the constraint of an equal power allocation scheme. Thus, it is not flexible enough to meet different requirements. To solve this problem, we propose a General AltBOC (GAltBOC) modulation. The proposed technique can achieve the same function as AltBOC. Meanwhile, its power allocation ratio can be adjusted when required. The detailed derivation of the GAltBOC modulation is presented, and three representative cases as well as the signal properties are discussed. To further improve the combination efficiency, we develop the Interlacing GAltBOC (IGAltBOC) modulation based on the GAltBOC modulation. The Power Spectrum Density (PSD) and correlation functions of the proposed signals are verified by simulation. The code tracking error and implementation complexities are analysed and compared with existing methods. Results show the proposed signals indeed enhance the flexibility of power allocation ratio with the same level code tracking accuracy. Compared with AltBOC modulation, the proposed signal can reach a higher combination efficiency at the expense of relatively higher implementation complexity. Compared with Asymmetric Constant Envelope Double-sideband (ACED) modulation, the proposed signals have significantly lower complexity of signal generation, which is helpful in simplifying the signal generation in the payload transmitter.

Doppler positioning using the signals of opportunity (SOPs) broadcast by low Earth orbit (LEO) satellites offers a viable alternative to global navigation satellite system (GNSS) positioning. As unauthorized users cannot access high-precision ephemeris of satellites, significant errors arise when calculating the orbits of LEO satellites, leading to a biased estimation of the receiver’s position. In this study, the impact of satellite orbit error on the receiver positioning error is analyzed, and the characteristics of the resulting bias in position estimation are derived. Based on these characteristics, a simultaneous positioning and orbit correction (SPOC) method is proposed. This method jointly estimates both the receiver position and satellite orbit error in radial and along-track directions, while further improving positioning accuracy by weighting measurements based on the statistical characteristics of residual orbit error. Experiments using multiple groups of real Doppler measurements from Iridium-NEXT satellites show that, under an identical altitude constraint, the SPOC method reduces positioning error by 62% compared to the conventional least squares method (CLS), and by 27% compared to the orbit error compensation and weighting (OECW) method.