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- 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.
- 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.
- 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.
- Signal quality monitoring aspects in GNSS signals affected by evil waveformsPublication . Sousa, Fernando; Nunes, Fernando; Negrinho, António; Fernandes, Pedro; Boto, PedroEvil waveforms (EWF) are anomalies in the GNSS transmitted signals that can degrade significantly the accuracy of the PVT solution. The cross-correlation function of the incoming signal disturbed by EWF distortion and the locally-generated code signal is obtained analytically for threat models TM-A, TM-B and TM-C. These results are useful to evaluate efficiently the performance of EWF detectors, namely the detectability and hazard regions.
- State-space estimation of ionospheric scintillation processes with a static GNSS receiverPublication . Sousa, Fernando; Nunes, FernandoThe determination of the ionospheric scintillation processes that affect the reception of Global Navigation Satellite System (GNSS) signals is an important issue as it permits to monitor the ionosphere for practical and scientific applications and optimize the receiver performance in scintillation-prone scenarios using, for instance, a differential GNSS configuration. We propose an hybrid architecture for the receiver being constituted by a vector tracking block that estimates the receiver's clock drift coupled with an ensemble of extended Kalman filters driven by the prompt correlators that estimate the amplitude and phase of the fading processes. Those processes include as an important case the ionospheric scintillation.
- 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.
- 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.
- Generalized gamma fading simulation of scintillation disturbed GNSS signalsPublication . Nunes, Fernando; Sousa, FernandoThe generalized gamma distribution includes as particular cases different well-known distributions, such as the Nakagami-m. We propose a computationally efficient technique to generate correlated time-series with generalized gamma distribution. This method is useful in the simulation of GNSS signals disturbed by ionospheric scintillation. The technique requires only knowledge of two parameters, besides the definition of a model for the amplitude power spectrum.
- Statistical characterization of ionospheric scintillation using a phase-screen modelPublication . Nunes, Fernando; Sousa, Fernando; Marçal, José M. V.A fast technique to simulate the effect of equatorial ionospheric scintillation on GNSS signals was proposed by the authors in a previous work. The algorithm uses a single-layer phase-screen model of the ionosphere and the scintillation is expressed by the Huygens-Fresnel (HF) integral which can be determined analytically. Herein, we characterize statistically the amplitude and phase obtained with the HF integral for different values of the scintillation index S4. A phase unwrapper is also developed to determine the phase from the modulo two-pi version provided by the argument of the HF integral.