GNSS vulnerabilities have become evident in the last decade. Authentication of the GNSS signals and data can be an important building block contributing to mitigating these vulnerabilities. This paper presents a Navigation Message Authentication (NMA) scheme based on the Timed Efficient Stream Loss‐tolerant Authentication (TESLA) protocol and a novel concept based on a single one‐way chain for all senders and cross‐authentication. The paper presents an NMA implementation in the Galileo Open Service (OS) navigation message that should provide similar navigation performance to data‐authenticated users and standard non‐authenticated users in terms of time to first fix, accuracy, and availability even in difficult reception conditions. The proposal also maintains a high level of signal unpredictability to help receivers protect against replay attacks. The scheme and implementation proposed yield significant improvements compared to the state of the art, offering the opportunity for Galileo to become the reference GNSS in civil navigation authentication. Copyright © 2016 Institute of Navigation

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Snapshot techniques are based on computing a position using only a set of digital signal samples captured over some milliseconds. Existing techniques require, in addition to the satellite ephemerides, a rough knowledge of the position and/or time at which the snapshot was captured. We propose a new method to instantaneously compute a snapshot position and time solution without any reference time or position. The method is based on the addition of a fifth unknown to the instantaneous Doppler equations, which accounts for a time difference between the reference time and the measurement time. Using this new system of equations at different initialization times separated by some hours, time uncertainties of days or weeks can be solved. The algorithm has been implemented in a snapshot GPS software receiver in MATLAB, proving that position accuracies of a few meters with time uncertainties of several weeks can be obtained in a few seconds.