Critical government and industry sectors (such as law enforcement, transportation, communication, and finance) are growing increasingly dependent on Global Navigation Satellite Systems (GNSS) for positioning, navigation, and timing. At the same time, the availability of low-cost GNSS jamming devices are presenting a serious threat to GNSS and increasing the likelihood of outages to infrastructures relying on GNSS. The attacks range from malicious parties intentionally jamming GNSS signals within a targeted geographical region to uninformed users causing accidental interference. This paper is an overview of different approaches adopted to date to mitigate GNSS disruption caused by intentional and unintentional jamming. The first approach outlined in this paper is the use of inertial systems to aid GNSS. The second and third approaches are the filtering of jamming/interference in the spatial and time-frequency domains, respectively. The fourth approach is vector tracking of GNSS signals in the receiver.

Protecting GNSS Receivers From Jamming and Interference

In most wired and wireless systems, carrier tracking is an essential task that allows the receiver to precisely synchronize with the carrier of the incoming signal. Stringent carrier tracking requirements are imposed in systems that are sensitive to carrier mismatches, such as orthogonal frequency division multiplexing (OFDM), digital communication receivers employing high-order constellations, and terrestrial- or satellite-based positioning systems, just to mention a few. In the recent years, even more critical requirements are being imposed due to the emergence of new applications and services that are pushing traditional systems to operate in much more challenging conditions than the ones for which they were originally designed. The presence of severe fading, signal outages, abrupt phase changes and high user dynamics, are currently compromising the validity of well-known and long-established traditional carrier tracking techniques, thus calling for the development of new robust carrier tracking algorithms. In this paper, we provide a detailed survey on the five main strategies that can be adopted to cope with the technical challenges of robust carrier tracking. These strategies range from some basic optimizations of current tracking loops, to the use of Kalman filter-based architectures, or the application of innovative carrier tracking techniques based on particle filters or compressive sensing. We will also review some open-loop techniques, which are widely adopted in burst-mode communications receivers, as an alternative and potential candidate solution for robust carrier tracking in harsh conditions.

/pdf/survey-on-robust-carrier-tracking-techniques-3xonm4udv9.pdf

Survey on Robust Carrier Tracking Techniques

This article discusses theoretical and practical aspects of direct position estimation (DPE) of global navigation satellite system (GNSS) receivers. DPE is an alternative approach to the well-established positioning method, where time delays and Doppler shifts to the visible satellites are estimated to solve for a geometrical problem to compute the receiver's position. In contrast, DPE infers position directly from the sampled data without intermediate steps. This approach has been seen, both theoretically and experimentally, to substantially increase the receiver's accuracy and reliability.

Direct Position Estimation of GNSS Receivers: Analyzing main results, architectures, enhancements, and challenges

In recent years, Kalman filter (KF)-based tracking loop architectures have gained much attention in the Global Navigation Satellite System field and have been widely investigated due to its robust and better performance compared with traditional architectures. However, less attention has been paid to the in-depth theoretical analysis of the tracking structure and to the effects of Kalman tuning. A new approach is proposed to analyze the KF-based tracking loop. A control system model is derived according to the mathematical expression of the Kalman system. Based on this model, the influence of the choice of the setting parameters on the temporal evolution of the system response is discussed from the perspective of a control system. As a result, a reasoned and complete suite of criteria to tune the initial error covariance as well as the process and measurements noise covariances is demonstrated. Furthermore, a strategy is presented to make the system more robust in higher order dynamics without degrading the accuracy of carrier phase and Doppler frequency estimates.

Theoretical analysis and tuning criteria of the Kalman filter-based tracking loop

This paper presents a generalized theoretical framework for carrier tracking of global navigation satellite systems signals by constructing a state space representation for the carrier tracking loop and applying control system design techniques to derive state feedback and state estimator gain matrices. Both phase-locked loop and frequency-locked loop are studied using this approach. Their performances are evaluated using the closed-form expressions in the presence of thermal noise, oscillator noise, and receiver platform dynamics. The proposed approach unified the phase tracking and frequency tracking within the same theoretical framework, thereby facilitating a systematic analysis of the effects of tracking loop design parameters under weak signal and highly dynamic signal environments.

/pdf/generalized-gnss-signal-carrier-tracking-part-i-modeling-and-3qtx2hmfdh.pdf

Generalized GNSS Signal Carrier Tracking: Part I—Modeling and Analysis

In this paper we discuss several different Kalman filter
implementation methods to estimate signal parameters in
a GNSS receiver’s signal tracking loop with a special
emphasis given to how to construct the measurement
residuals and the relevant design matrix. They are three
variations of error state Kalman filter for a loop filter, and
a direct state Kalman filter for the entire signal tracking
loop. Detailed signal models for a local channel signal
tracking loop are presented. Based on this, four different
Kalman filters for code and carrier trackings are designed
compared with the traditional signal tracking loop.
Especially, the block diagram implementation of the
direct state Kalman filter for the entire signal tracking
loop is proposed by using the analogy relationship with
the traditional method in a scalar tracking mode and a
vector tracking mode. Numerical simulation results show
that the Kalman filter-based signal tracking loops provide
2 dB carrier-to-noise ratio gain with 25 percent of
accuracy improvement in the carrier phase tracking, and 3
dB carrier-to-noise ratio gain with 30 percent of accuracy
improvement in the Doppler tracking compared to the
traditional methods.

Performance Comparison of Different Forms of Kalman Filter Approach for a Vector-Based GNSS Signal Tracking Loop

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Performance Comparison of Different Forms of Kalman Filter Approach for a Vector-Based GNSS Signal Tracking Loop