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- Jamming detection in GNSS signals using the sample covariance matrix.Publication . Nunes, Fernando; Sousa, FernandoWe propose a blind method to detect interference in GNSS signals whereby the algorithms do not require knowledge of the interference or channel noise features. A sample covariance matrix is constructed from the received signal and its eigenvalues are computed. The generalized likelihood ratio test (GLRT) and the condition number test (CNT) are developed and compared in the detection of sinusoidal and chirp jamming signals. A computationally-efficient decision threshold was proposed for the CNT.
- Robust vector tracking for carrier phase signals with separate estimation of common and single channel errorsPublication . Marçal, J.; Nunes, F.; Sousa, FernandoA robust implementation of a partitioned vector tracking for GNSS carrier phase signals is presented, with separate estimation of common and individual satellite errors, based on a combination of vector and scalar loop estimates. The partitioned vector tracking is a hybrid implementation of a phase tracking loop with contributions from a vector based implementation (where information of all satellites may be relevant), but also taking into account individual satellite errors, such as ionospheric and tropospheric effects, and can also include scintillation or shadowing. The goal is to have a robust implementation, with low complexity, where common mode errors can be quickly estimated, and individual errors can be estimated apart, with a prompt response, leading to an efficient approach that can cope with disturbances in the nominal operation of the receiver. This prompt estimation of the common and differential errors can be used for quality monitoring, detecting events by comparing with the traditional Kalman filter solution, and can be used in a predictive model to aid the tracking.
- Generation of Nakagami correlated fading in GNSS signals affected by ionospheric scintillationPublication . Nunes, Fernando; Sousa, FernandoWe propose a low complexity technique to generate amplitude correlated time-series with Nakagami-m distribution and phase correlated Gaussian-distributed time-series, which is useful for the simulation of ionospheric scintillation effects in GNSS signals. To generate a complex scintillation process, the technique requires solely the knowledge of parameters Sa (scintillation index) and σφ (phase standard deviation) besides the definition of models for the amplitude and phase power spectra. The concatenation of two nonlinear memoryless transformations is used to produce a Nakagami-distributed amplitude signal from a Gaussian autoregressive process.
- Performance of an adaptive partitioned vector tracking algorithm with real scintillation dataPublication . Marçal, José M. S. V.; Nunes, Fernando; Sousa, FernandoVector tracking algorithms have been used in both frequency and code tracking of GNSS signals in the last years with success. Vector phase tracking algorithms still present a challenge due to the order of magnitude of the errors when compared to the wavelength of the carrier wave, especially in situations where strong disturbances are present in one or more satellites. Partitioned vector tracking for carrier phase signals has been used in simulation scenarios with good results. In this work the performance with real scintillation data is shown. Furthermore, an adaptive method with dynamic optimization of the weights in the partitioned vector tracking is included, in order to adjust the tracking structure to the variation of environmental parameters.
- Characterization and performance analysis of a VDFLL GNSS receiver architecturePublication . Sousa, Fernando; Nunes, FernandoA non-coherent vector delay/frequency-locked loop architecture for GNSS receivers is proposed. Two dynamics models are considered: PV (position and velocity) and PVA (position, velocity, and acceleration). In contrast with other vector architectures, the proposed approach does not require the estimation of signals amplitudes. Only coarse estimates of the carrier-to-noise ratios are necessary.
- Performance of a scintillation robust vector tracking for GNSS carrier phase signalsPublication . Marçal, José; Nunes, Fernando; Sousa, FernandoAn implementation of a robust vector phase-locked loop (VPLL) for joint tracking of carrier phase from the navigation satellites in the event of ionospheric scintillation is presented. This partitioned tracking method leads to an increased tolerance to ionospheric disturbances due to scintillation in the carrier phase tracking, providing a trade off between the pure VPLL and the classic scalar tracking loops. This method also allows for smoother transitions by using a weighted combination of scalar and vector tracking operation modes, which can be of importance when the environmental variables change and the state estimation may not cope with the fast changing dynamics, leading to harsh transitions and possibly to phase lock loss or inaccurate results.
- Performance of a VDFLL GNSS receiver architecture in presence of interferencePublication . Sousa, Fernando; Nunes, FernandoWe discuss the advantages of using a vector delay/frequency-locked loop (VDFLL) architecture for GNSS receivers operating in scenarios disturbed by radio-frequency interference. Narrowband (sinusoidal) and wideband (chirp) interference signals are considered in the analysis. The VDFLL is constituted by a bank of code and frequency discriminators feeding a central extended Kalman filter that estimates the receiver's position, velocity, and clock bias. Simulation results are obtained using sine-phase binary offset carrier BOCs(1,1) modulation. The results are compared with the theoretical performance determined by the effective signal power to noise density ratio.
- Gnss blind interference detection based on fourth-order autocumulantsPublication . Nunes, Fernando; Sousa, FernandoGNSS signals affected by additive channel noise are approximately Gaussian, but often the presence of interference may change that distribution.We propose a blind detection method based on the Giannakis-Tsatsanis Gaussianity test to evaluate the presence of narrowband and wideband non-Gaussian interference. This method, resorting to the sample fourth-order autocumulants, is compared to other Gaussianity tests. The asymptotic covariance matrix of the estimated autocumulant vector is analytically determined, thus reducing the complexity of the proposed technique.
- Practical simulation of GNSS signals in the presence of ionospheric scintillatonPublication . Nunes, Fernando; Sousa, FernandoWe propose a low complexity technique to generate amplitude correlated time-series with Nakagami-m distribution and phase correlated Gaussian-distributed time-series, which is useful in the simulation of ionospheric scintillation effects during the transmission of GNSS signals. The method requires only the knowledge of parameters S4 (scintillation index) and σΦ (phase standard deviation) besides the definition of models for the amplitude and phase power spectra. The Zhang algorithm is used to produce Nakagami-distributed signals from a set of Gaussian autoregressive processes.
- Characterization and performance analysis of generalized BOC modulations for GNSSPublication . Nunes, Fernando; Sousa, Fernando; Leitão, JoséNovel modulations, based on the binary chirp waveform, are proposed as generalizations of binary offset carrier (BOC) schemes. They allow enhancement of the performance of Global Navigation Satellite System (GNSS) receivers affected by Gaussian noise, multipath, and interference. Adequate selection of parameters enables efficient use of the channel bandwidth to enlarge the Gabor bandwidth. Schemes characterized by autocorrelations with small secondary peaks are obtainable, permitting minimization of the probability of code acquisition with incorrect code phase and reducing the probability of false code-locks in tracking mode. Increased robustness to multipath and interference also occurs. These advantages make the proposed modulation schemes promising candidates for future navigation signals.
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