Matteo Sgammini

An architecture for an embedded antenna-array digital GNSS receiver using subspace-based methods for spatial filtering

This paper presents an architecture for an embedded multi-antenna digital GNSS receiver. A two-stage adaptive beamformer for interference suppression and Line-of-Sight (LoS) signal amplification is presented and analyzed w.r.t. to an efficient implementation on embedded receivers. Jammer signals are mitigated at pre-correlation stage whereas the LoS signals are amplified at post-correlation stage. The method is based on a subspace-based approach where filter coefficients are derived from the eigenvalues and -vectors of the covariance matrix. In the first stage, the covariance matrix is determined immediately from the digital antenna signals for interference mitigation and in the second stage, the matrix is computed based on the correlator outputs of each satellite in LoS. Dedicated buildingblocks for covariance matrix estimation and filtering are required for interference mitigation since this operation is computed on sampling rate. A fixed-point VHDL implementation and related costs in terms of logic-cell requirements on an FPGA are provided for both blocks. Eigendecomposition is computed on an embedded processor. The implementation of two decomposition algorithms (one for interference mitigation and the other one for LoS-signal amplification) are presented. Optimizations and costs in terms of processing-cycles on an embedded processor are provided.

Ionospheric threat simulation for GNSS using the Spirent hardware signal simulator

Abstract Ionospheric disturbances present a considerable hazard to single-frequency satellite navigation systems for airborne users. We discuss our implementation of three ionospheric threat models in the DLR “multi-output advanced signal test environment for receivers” global navigation satellite system simulator, which is based on Spirent GSS 7780/7790 signal generator. These threat models include the standard front-based threat model developed for the integrity assessment of ground-based augmentation systems (GBAS), a simplified plasma bubble model, and ionospheric scintillation, which can be combined with either of the two previously mentioned models. These effects can now straightforwardly be simulated at the German Aerospace Center’s research facilities. As an example, we simulate a GBAS ground facility with code–carrier divergence monitoring, affected by an ionospheric front, and we show the results of a simulation with coincidental occurrence of a plasma bubble and scintillation with an S4 index of 0.4.

Noise characterization of a multi-channel receiver using a small antenna array with full diversity for robust satellite navigation

The employment of a DMN in small antenna arrays results in an increased antenna noise temperature due to increased ohmic losses. On the other hand, it minimizes the amplifier noise contribution considerably, thus reducing the equivalent system noise temperature. Therefore, the use of a DMN for small antenna arrays displaying full diversity is not only beneficial but necessary for optimizing receiver performance. The analysis sketched here will be extended to null-steering or interferer-cancellation scenarios.

ADIBEAM: Design and experimental validation of a robust beamforming platform for Galileo reference ground stations

The increase in navigation accuracy demanded by EGNOS and Galileo and their future evolution encourages the study and design of advance receiver architectures. In that direction, the ADIBEAM project focuses on the design of high accuracy ground stations, specifically GNSS Reference Stations, in order to improve its robustness in front of multipath and interference errors with the goal of achieving centimeter level tracking accuracy. The adoption of antenna arrays and digital beamforming at the ground reference station receivers is one of the most promising approaches to cope with errors induced by multipath and interference. This paper proposes an innovative design of a ground based tracking station. In particular, the architecture proposed is based on the use of an antenna array and digital beamforming techniques, considering both adaptive and deterministic methods. The work carried out here considers the ground based tracking system, including the characterization of the system components (antennas, RF chains, calibration techniques, GNSS software receiver and digital beamforming), and the development of a software based experimentation platform representative of the proposed design for a hardware prototype. The sensitivity to perturbations and the extreme difficulty to perfectly control and calibrate all the components of the system, especially regarding the antenna array implementation, requires that the proposed design take into consideration not only the benefits of the possible solutions but also their feasibility for a real implementation. In summary, this paper will present the design proposed and the Experimentation Platform used for its validation.

Navigating in the Galileo test environment with the first GPS/Galileo multi-antenna-receiver

Array processing is a very promising technology for mitigation and detection of radio interference in receivers of satellite navigation systems. This paper presents early results of a multi-antenna receiver measurement campaign in Berchtesgaden GATE.

Ionospheric deformation of broadband GNSS signals and its analysis with a high gain antenna

The ionospheric delay of global navigation satellite systems (GNSS) signals typically is compensated by adding a correction value to the pseudorange measurement. We examine the ionospheric signal distortion beyond a constant delay. These effects become increasingly significant with increasing signal bandwidth and hence more critical for the new broadband navigation signals. By simulation, we first demonstrate that the signal modulation constellation diagram is particularly susceptible to the influence of the ionosphere already at moderate electron content. Using high gain antenna measurements of the Galileo E5 signal, we then verify that the expected influence can indeed be observed and compensated. A new method based on a binned maximum likelihood estimator is derived to estimate the total electron content (TEC) from a single frequency high gain antenna measurement of a broadband GNSS signal. Results of the estimation process are presented and discussed comparing to common TEC products such as TEC maps and dual-frequency receiver estimates.

Dual polarization beamforming algorithm for multipath mitigation in GNSS

A multipath mitigation technique for GNSS is proposed.The technique exploites the change of polarization when a signal is reflected.A novel statistic based on the outputs of a correlator bank is introduced to estimate the multipath DoA and polarization.Applying the technique increases the estimation performance in the case of highly temporally or spatially correlated multipath and line-of-sight signal. This paper treats the problem of line-of-sight (LOS) parameter estimation in strong multipath environments. In the case of highly temporally and spatially correlated LOS and multipath signals, such as urban canyons, common multipath mitigation methods are highly degraded, as signal separation cannot be performed in the spatio temporal domain. In this case, we exploit the LOS and multipath polarization diversity to decouple the signals using an antenna array with right-hand-circular polarization (RHCP) and left-hand-circular polarization (LHCP) feeds. The multipath direction- of-arrival (DOA) and polarization coefficients can effectively be estimated from the LHCP spatial covariance matrix. The LOS DOA can be estimated from the RHCP spatial covariance matrix. The spatial covariance matrices are calculated from the outputs of a matched correlator bank. The DOA and polarization estimates are used to implement a dual polarization beamformer, which maximizes the LOS energy and suppresses multipath energy over both polarizations. The LOS time-delay is estimated from the beamformer output with a maximum-likelihood estimator with a significantly reduced number of parameters and computational complexity in comparison to a full model estimator. Simulation results for GPS show that the proposed dual polarization beamforming algorithm performs better than an equivalent single-polarization beamformer in a dense multipath environment.