The Long-Range Navigation-C (Loran-C) system is an internationally standardized positioning, navigation, and timing service system based on a 100-kHz carrier. The development of the system antenna has always been one of the key research objects in the development of its receiver. In this article, a broadband compact small active magnetic antenna used to receive Loran-C signal outdoors and indoors is designed, which can be divided into a magnetic antenna part and an active circuit part. In the antenna part, this article designs a new “H”-shaped magnetic antenna with a high quality factor from two aspects: material selection and simulation of antenna shape. In the active circuit part, the amplifier and filter are designed by an operational amplifier, and the frequency selection is realized by a single-tube double tuning circuit to improve the signal-to-noise ratio (SNR). Through the above designs, the effective reception of the Loran-C signal is realized outdoors and indoors. Finally, the whole Loran-C active magnetic antenna is processed and tested. It can completely receive a Loran-C signal, with a clear pulse group, and the frequency point of the signal is 99.21 kHz. The length of the antenna is 2.75 cm, and the size of the circuit board is $10\times3$ cm. The SNR of the outdoor environment is 12.04 dB and that of the indoor is 16.48 dB.

Demand for Global Navigation Satellite System (GNSS) applications in the urban environment has experienced a remarkable growth in recent years. However, the received signals are subjected to various urban channel impairments, like shadowing and multipath fading. Therefore, the decoding performance is different from that in open-sky conditions. In this paper, a two-state Land Mobile Satellite (LMS) channel based on the Markov process is used to model the urban channel properties, and then, the analysis of decoding performance in terms of frame error rate (FER) in the LMS channel is performed by evaluating the effect of three major influencing factors, specifically, coding and interleaving in the GNSS message, terminal speed, and satellite elevation angle. Extensive simulations are conducted on BDS-3 B1C B-CNAV1 message and GALILEO E5a F/NAV message. The results validate the excellent error correcting performance of the nonbinary low density parity check (NB-LDPC) code of the B-CNAV1 message and the effectiveness of interleaving in both of the messages in urban condition. Furthermore, it also shows that decoding performance improvement can be achieved with higher terminal speed and higher elevation angle in urban scenarios.

This work investigated concatenated coding schemes for Global Navigation Satellite System (GNSS) signals in order to increase their error correction capability in urban environments. In particular, a serial concatenated code that combines an outer Reed–Solomon (RS) code with an inner low-density parity-check (LDPC) code was designed, and the performance was investigated over the land mobile satellite (LMS) channel for characterizing multipath and shadow fading in urban environments. The performance of the proposed concatenated coding scheme was compared to that of a B-CNAV1 message, in which two interleaved 64-ary LDPC codes were employed. The simulation results demonstrate that the proposed concatenated code can obtain a similar error correction performance to the two interleaved 64-ary LDPC codes in both the additive white Gaussian noise (AWGN) and LMS channels at a lower complexity level.

With the development of global navigation satellite systems (GNSS), multiple signals modulated on different sub-carriers are needed to provide various services and to ensure compatibility with previous signals. As an effective method to provide diversified signals without introducing the nonlinear distortion of High Power Amplifier (HPA), the multi-carrier constant envelope multiplexing is widely used in satellite navigation systems. However, the previous method does not consider the influence of sub-carrier frequency constraint on the multiplexing signal, which may lead to signal power leakage. By determining the signal states probability according to the sub-carrier frequency constraint and solving the orthogonal bases according to the homogeneous equations, this article proposed multi-carrier constant envelope multiplexing methods based on probability and homogeneous equations. The analysis results show that the methods can multiplex multi-carrier signals without power leakage, thereby reducing the impact on signal ranging performance. Meanwhile, the methods could reduce the computation complexity. In the case of three different carriers multiplexing, the number of optimization equations is reduced by nearly 66%.

The automatic modulation recognition (AMR) algorithms based on deep learning (DL) have achieved high classification accuracy by automatically extracting deep features from massive data. However, in real-world scenarios, sufficient training data is always difficult to collect, which affects the performance of DL-based models. As a type of data augmentation algorithm, Generative Adversarial Networks (GANs) can generate artificial data similar to the given real data and thus solve the problem of insufficient data, whereas the training process of GANs is also affected by limited number of data samples. Inspired by the successful application of transferring GANs in the field of image generation, this paper employs transfer learning-based GANs to enlarge the training data by generating the constellation diagram images of radio signals, which can effectively solve the problems of divergence and model collapse. We feed the enhanced dataset into a CNN model for modulation recognition and the experimental results demonstrate that our proposed method achieves a performance improvement ranging from 1% to 5.1% compared with the result of the original limited training data.

Frequency standard comparison measurement has important practical significance for the rational use of frequency standard in engineering. This paper was devoted to the study of frequency standard comparison measurement based on classical dual mixing time difference method. However, in the actual system design and implementation, the commonly used counter was discarded and the phase difference was measured by a digital signal processing method based on Field Programmable Gate Array (FPGA). A miniaturized 10 MHz frequency standard comparator with good noise floor was successfully developed. The size of the prototype circuit board is only about 292.1 cm². The experimental results showed that the noise floor of the frequency standard comparator was typically better than 7.50 × 10⁻¹²/s, and its relative error of phase difference measurement was less than 1.70 × 10⁻⁵.