Non-geostationary (NGSO) satellites are envisioned to support various new communication applications from countless industries. NGSO systems are known for a number of key features such as lower propagation delay, smaller size, and lower signal losses in comparison to the conventional geostationary (GSO) satellites, which will enable latency-critical applications to be provided through satellites. NGSO promises a dramatic boost in communication speed and energy efficiency, and thus, tackling the main inhibiting factors of commercializing GSO satellites for broader utilizations. However, there are still many NGSO deployment challenges to be addressed to ensure seamless integration not only with GSO systems but also with terrestrial networks. These unprecedented challenges are discussed in this paper, including coexistence with GSO systems in terms of spectrum access and regulatory issues, satellite constellation and architecture designs, resource management problems, and user equipment requirements. Beyond this, the promised improvements of NGSO systems have motivated this survey to provide the state-of-the-art NGSO research focusing on the communication prospects, including physical layer and radio access technologies along with the networking aspects and the overall system features and architectures. We also outline a set of innovative research directions and new opportunities for future NGSO research.

A Survey on Non-Geostationary Satellite Systems: The Communication Perspective

By minimizing the mean squared error, Kalman filter provides optimal estimates with exact system models and precise statistical information of the noises, and the filter may fail or be affected seriously with uncertain models and noises. In the dynamic positioning and navigation field, the filter with the adaptive factor or the fading factor was developed to overcome the drawbacks of the Kalman filter. However, the distinction between these two factors and their performances in the global navigation satellite system (GNSS) and the inertial navigation system (INS) integrated systems is rarely discussed and tested in the literature. The theoretical analysis is implemented between the filters with the adaptive factor and the fading factor, and the performance of both these two filters and the conventional Kalman filter is tested with actual data of the GNSS/INS integrated systems; then, multiple indexes are applied to compare performances of the filtering. For all experiments, the cubature Kalman filter is adopted, aiming at the high-dimensional nonlinear problems. Comparative results demonstrate that the influences of the modeling deviations are well controlled with the adaptive factor or the fading factor, and the filter with the adaptive factor is more likely to avoid the filter divergence and to achieve stable performance in the GNSS/INS integrated systems.

Performance evaluation of the filters with adaptive factor and fading factor for GNSS/INS integrated systems

As the development of traditional scalar tracking loops, different additional enhancement techniques in a GNSS receiver are proposed to help improve the positioning service quality. Up to now, Kalman-based tracking loop has gained much attention due to its potential superiorities, it has been widely exploited and studied in different specific applications. But meanwhile, there is still a lack of the detailed exploration regarding the Kalman tuning and the fully in-depth analysis and validation. Based on our former related work, an improved mathematical expression of Kalman gain and proper tractable models are proposed to help the analysis of Kalman tuning. An explicit tuning rule is proposed step by step regarding every single parameter involved and the validation is carried out in real experiments by the comparison of different receivers with different configurations. The results verify the proposed tuning rule and advantages of KF-tracking with proper setting. Thanks to the accurate and reliable signal parameter estimation, it can dramatically relieve the burden of the following processing modules, like PVT, RTK and other similar algorithms.

/pdf/the-explicit-tuning-investigation-and-validation-of-a-full-4zmcmj643t.pdf

The Explicit Tuning Investigation and Validation of a Full Kalman Filter-Based Tracking Loop in GNSS Receivers

The next phase of satellite technology is being characterized by a new evolution in non-geostationary orbit (NGSO) satellites, which conveys exciting new communication capabilities to provide non-terrestrial connectivity solutions and to support a wide range of digital technologies from various industries. NGSO communication systems are known for a number of key features such as lower propagation delay, smaller size, and lower signal losses in comparison to the conventional geostationary orbit (GSO) satellites, which can potentially enable latency-critical applications to be provided through satellites. NGSO promises a substantial boost in communication speed and energy efficiency, and thus, tackling the main inhibiting factors of commercializing GSO satellites for broader utilization. The promised improvements of NGSO systems have motivated this paper to provide a comprehensive survey of the state-of-the-art NGSO research focusing on the communication prospects, including physical layer and radio access technologies along with the networking aspects and the overall system features and architectures. Beyond this, there are still many NGSO deployment challenges to be addressed to ensure seamless integration not only with GSO systems but also with terrestrial networks. These unprecedented challenges are also discussed in this paper, including coexistence with GSO systems in terms of spectrum access and regulatory issues, satellite constellation and architecture designs, resource management problems, and user equipment requirements. Finally, we outline a set of innovative research directions and new opportunities for future NGSO research. 

/pdf/a-survey-on-nongeostationary-satellite-systems-the-1d8p2h9a.pdf

A Survey on Nongeostationary Satellite Systems: The Communication Perspective

A scheme for deception jamming detection based on beam scanning is proposed in this letter. This scheme detects the authenticity of the corresponding signal by forming a null in the direction of each visible satellite in turn. At the same time, the null direction is adjusted based on the attitude angle measured by the inertial unit to support the applicability of the scheme in scenarios where the attitude of the array changes continuously. Then, from an engineering point of view, an equalization scheme for arrays and radio frequency (RF) channels is proposed to improve the accuracy of the null direction. Finally, the effectiveness of the proposed scheme is verified by simulation.

Deception Jamming Detection Based on Beam Scanning for Satellite Navigation Systems

In this paper an adaptive narrow-band interference (NBI) suppression scheme based on a code-aided technique in the global navigation satellite system inter-satellite links (ISLs) is proposed. Unlike the commonly used infinte impulse response (IIR) notch filter and frequency domain filtering, the code-aided technique does not require interference detection or fast Fourier transforms, and the adaptive performance is better. We introduce the principle and implementation of code-aided technique, and the concept of interference influence coefficient is proposed to evaluate the influence of residual interference on the carrier-to-noise ratio (CNR) at the receiving end. Through simulation and by combining the tracking CNR after antijamming, the optimal antijamming parameters and scheme are obtained. Meanwhile, we analyze the relationship between the loss of CNR caused by residual interference and the interference influence coefficient. The final results of the bit error rate (BER) and ranging error in ISLs indicate that the adaptive interference suppression scheme based on the code-aided technique can effectively suppress NBI and ensure the reliability of ISL communications. In the case where the pseudo code rate is 10.23 MHz and the BER does not exceed $1 \times {10^{ - 5}}$ , for the NBI with a bandwidth of 2 MHz, the maximum interference-to-signal ratio (ISR) that can be suppressed by the proposed scheme is 40 dB. For the NBI with a bandwidth of 1 MHz, the maximum ISR that can be suppressed is 55 dB, and for single frequency interference, the maximum ISR that can be suppressed is 75 dB.

Adaptive Narrow-Band Interference Suppression and Performance Evaluation Based on Code-Aided in GNSS Inter-Satellite Links

In the estimation of the direction of arrival (DOA) for interference signals, the direction-finding error of the multiple signal classification (MUSIC) algorithm will increase in the case of multiple interferences or when the interfering signal power is weak. In this paper, a space-time conversion MUSIC (STC-MUSIC) algorithm is proposed, and the concept of a focusing parameter is introduced to improve the performance of the DOA estimation. Meanwhile, a method of variable step size peak search is proposed to reduce the amount of calculation of the STC-MUSIC algorithm. The final simulation and experimental results show that the STC-MUSIC algorithm improves the purity of the noise subspace effectively, thus improving the precision and robustness of the DOA estimation for interference signals significantly. In comparison to traditional algorithms, the convergence, stability, root mean square error (RMSE) and other performance characteristics are improved greatly.

Estimation of Interference Arrival Direction Based on a Novel Space-Time Conversion MUSIC Algorithm for GNSS Receivers.

For global navigation satellite system receivers, Kalman filter (KF)-based tracking loops show remarkable advantages in terms of tracking sensitivity and robustness compared with conventional tracking loops. However, to improve the tracking sensitivity further, increasing the coherent integration time is necessary, but it is typically limited by the navigation data bit sign transition. Moreover, for standard KF-based tracking receivers, the KF parameters are initialized by the acquired results. However, especially under weak signal conditions, the acquired results have frequency errors that are too large for KF-based tracking to converge rapidly to a steady state. To solve these problems, a two-stage KF-based tracking architecture is proposed to track weaker signals and achieve faster convergence. In the first stage, coarse tracking refines the acquired results and achieves bit synchronization. Then, in the second stage, fine tracking initializes the KF-based tracking by using the coarse tracking results and extends the coherent integration time without the bit sign transition limitation. This architecture not only utilizes the self-tuning technique of the KF to improve the tracking sensitivity, but also adopts the two-stage to reduce the convergence time of the KF-based tracking. Simulation results demonstrate that the proposed method outperforms conventional tracking techniques in terms of tracking sensitivity. Furthermore, the proposed method is compared with the standard KF-based tracking approach, proving that the proposed method converges more rapidly.

https://www.mdpi.com/1424-8220/19/6/1369/pdf

A Two-Stage Kalman Filter-Based Carrier Tracking Loop for Weak GNSS Signals

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Adaptive Narrow-Band Interference Suppression and Performance Evaluation Based on Code-Aided in GNSS Inter-Satellite Links

Estimation of Interference Arrival Direction Based on a Novel Space-Time Conversion MUSIC Algorithm for GNSS Receivers.

A Two-Stage Kalman Filter-Based Carrier Tracking Loop for Weak GNSS Signals