GNSS-SDR is an open source Global Navigation Satellite Systems software receiver, written in C++, that is able to work either from raw signal samples stored in a file, or in real-time with a radio-frequency front-end as signal source, and performs all the signal processing until the observable computation and Position-Velocity-Time solution. Its modularity allows users to populate the framework with their own algorithms, allowing to put the focus on the signal processing implementation without worrying about how to embed that algorithm in a whole GNSS receiver. It also makes possible fair performance benchmarks using real GNSS signals, and its open source license allows free downloading, use and code inspection. The proposed software receiver targets multi-constellation/multi-frequency architectures, pursuing the goals of efficiency, modularity, interoperability, and flexibility demanded by user domains that require non-standard features, such as earth observers or geodesists, and allowing applications such as the observation of the ionosphere, GNSS reflectometry, signal quality monitoring, space weather, and high-accuracy positioning based on carrier-phase navigation techniques. In this project, we focus on signal processing, understood as the process between the ADC and the computation of code and phase observables, including the demodulation of the navigation message. We purposely omit data processing, understood as the computation of the navigation solution from the observables and the navigation message, since there are a number of well-established libraries and applications for that (also in the open source side, such as GPSTk or RTKLIB).