Inside GNSS Media & Research

JUL-AUG 2018

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50 Inside GNSS J U L Y / A U G U S T 2 0 1 8 tion of IP core licensing and/or royalties for its use. e commercial implemen- tations will serve multiple applications such as multipath and spoofing evalu- ation, ionosphere scintillation, interfer- ence monitoring, etc. One such soware receiver version is capable of running in real time more than 300 channels in a general purpose computer with an Intel Core i7-7490k processor. The solution also provides a hardware front end for data collection and is integrated with the soware receiver provided (see Manu- facturers section near the end of this paper). Other options will provide free academic versions (normally introduced with a textbook) (I. Petrovski and T. Tsujii) and will then charge for a profes- sional version of the soware, including one which will also have the option of a custom Radio Frequency (RF) front end to interface with the soware receiver. e result of years of experimenta- tion with soware radios as applied to GNSS technologies is a well-established set of tools for the scientific commu- nity with plenty of options to pick from depending on end applications, budgets, and experience in the field of radio navi- gation. GNSS-SDR Development of the work presented here was accomplished with GNSS-SDR. is is an open source receiver developed in C++ that uses the GNURadio Applica- tion Programming Interface (API) to develop a real time software receiver. e high level of flexibility and re-con- figurability makes this implementation a very appealing solution to an ever increasing radio navigation signal envi- ronment. GNSS-SDR provides an interface to different suitable RF front-ends and implements the entire receiver chain from signal reception up to the naviga- tion solution. Its design allows any kind of customization, including interchange- ability of signal sources, signal processing algorithms, interoperability with other systems, and output formats, and offers interfaces to all the intermediate signals, parameters, and variables (C. Fernández- Prades et alia, 2011). GNSS-SDR runs on a personal computer or an embedded platform and provides interfaces through Universal Serial Bus (USB) and Ethernet buses to a variety of either commercially available or custom-made RF. As an object-oriented platform and with the idea of keeping a sense of abstraction in the blocks developed, GNSS-SDR is divided into several pro- cessing blocks that participate in the whole process of navigation for receiv- ers. ese blocks ( Figure 1 ) are part of the abstraction level in the soware and can be divided as follows: 1) S i g n a l S o u r c e B l o c k s : H ide t he complexity of accessing each spe- cific signal source, providing a single interface to a variety of different implementations. 2) Signal Conditioner Blocks: Adapt the sample bit depth to a data type trac- table at the host computer running the soware receiver, and optionally intermediate frequency to baseband conversion, resampling, and filtering. 3) Channel Blocks: Encapsulate all sig- nal processing devoted to a single sat- ellite. is is a large composite object which encapsulates the acquisition, tracking, and decoding modules. 1) Acquisition Block s: Prov ide a coarse estimation of two signal parameters: the frequency shift ( f d ) with respect to the nominal Intermediate Frequency (IF) fre- quency, and a code delay term (τ) of in view satellites relative to the shifted version of the local code replica. 2) Tracking Block: Aims to perform a local search for accurate esti- mates of code delay and carrier phase, with their eventual varia- tions based on the input provided by the acquisition block. 3) Telemetry Decoder Block: Detects and decodes the navigation mes- sage conta ining t he t i me t he message was transmitted, orbital parameters of satellites (ephem- eris), and an almanac. 4) O bs e r va b l e s B l o c k : C ol lec t s a l l the data provided by every tracked channel, aligns all received data into a coherent set, and computes the observables (pseudorange, carrier phase, etc.). 5) Position Velocity and Time (PV T) Block: Computes the position solu- tion of the receiver based on all the data generated by the previous block. Extending the Receiver to GLONASS Processing Due to its different layers of abstrac- tion, prototyping of the GLONASS L1 C/A signal was done rapidly in the code base. Figure 2 shows the blocks of code added to the system and its object ori- ented properties relationship with the core blocks of the receiver. Note that the figure only displays the blocks cre- ated or modified for GLONASS L1 C/A, and is by no means a detailed Unified Modeling Language (UML) diagram of the objects present in the platform. It is worth mentioning that the GNSS-SDR platform uses some of the key concept ideas of GNU Radio in the sense that it has an abstract block upon which all other interfaces and implementations WORKING PAPERS FIGURE 1 GNSS-SDR System Architecture; Modified from work presented by C. Fernández-Prades et alia, 2011

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