For decades, GPS was the only global satellite positioning system available. In 2011, the Russian radio navigation system GLONASS started to provide worldwide coverage. China and Europe are also deploying their own satellite positioning systems (the Compass and Galileo, respectively), and two additional Galileo satellites were launched on October 12, 2012.
Multiconstellation, Multifrequency for Improved Performance
Because of the growing number of radio navigation systems, the number of available satellites as well as the number of frequencies used has increased. From a user perspective, this offers a significant improvement in positioning-service accuracy and availability.
New Technological Challenges
The availability of these GNSS systems working side by side offers unquestionable improvement in positioning performance. It also adds new technological challenges, in both the design of hardware receivers such as multifrequency, front end, and bandwidth as well as the signal processing algorithms (for example, acquisition or tracking). In addition, GNSS systems are more prominent in our daily lives.
Understanding and Training
Based on these new challenges, the need for GNSS education in Europe is high. This led ENAC, in partnership with Universitaet der Bundeswehr Muenchen, Politecnico di Torino, and others, to set up a GNSS European master. This master is supported by the European Union through the G-Train project (7th Framework Programme, grant agreement No. 248016). Thus, ENAC and M3 Systems decided to collaborate on the development of an educational tool for students and researchers so that users can dive into the details of the GNSS processing algorithms. The tool is an open multiconstellation and multifrequency software receiver. It is fully compatible with most existing GNSS systems (and their respective frequencies), and it provides complete freedom with respect to the algorithms’ parameterization. Compared to a usual hardware receiver, this software can implement complex signal processing algorithms, and it offers complete flexibility to users.
The RF Front End, Acquisition, and Tracking
Usually, GNSS receiver architectures are composed of three functional blocks. The developed solution focuses on the two blocks performing the signal processing—the RF front end, and the acquisition and tracking functions. The RF front end filters and amplifies the high-frequency signal. Then, the signal is downconverted to an intermediate frequency, and finally, it is sampled. The acquisition and tracking functions ensure the phase and range measurement extraction and the demodulation of the navigation messages.
A Software Defined Radio Using an NI USRP‐2920 as an RF Front End
Because of its large range of adjustable centre frequencies (50 MHz to 2.2 GHz) and its bandwidth (up to 20 MHz), the NI USRP-2920 offers the perfect balance of performance, flexibility, and cost for an educational GNSS application. Once digitized, I/Q data is processed by the software receiver. This open receiver is almost completely configurable. With such a tool, users can access data all along the processing chain. This data is displayed using an NI LabVIEW human machine interface (HMI) application that controls the systems, in which users can modify its parameters. Using the NI USRP-2920 as a front end for the GNSS signals has only two constraints. First, the power of GNSS signals is 45 times lower than that of the thermal noise. Despite the internal gain of the NI USRP™ (up to 25 dB) and the use of an active antenna (such as a Ublox ANN-MS-0-005 with a gain of about 27 dB, or a Septentrio PolaNt MC with a gain of 39 dB), it requires the addition of a preamplifier (in this case, an 18 dB gain ZX60-33LN-S+).
Second, it is necessary to increase the frequency and phase stability of the USRP clock using an external reference. We used a reference time provided by an external GPS to synchronize the USRP. It is important to note that any high-quality 10 MHz frequency generator (such as an oven-controlled crystal oscillator with a thermal stability of about 0.005 ppm) could work.
An NI USRP, ORUS, and LabVIEW Solution
The RF function consists of an active antenna, a preamplifier, and the NI USRP-2920 software defined radio synchronized to an external time reference. The acquisition and tracking block is ensured by a fully configurable open software receiver (ORUS) developed by M3 Systems. A command and control LabVIEW application controls the entire system and displays the process data. The acquisition and tracking of both the GPS and Galileo signals were successfully performed with the developed solution. Using the available data, it is possible to analyze I and Q correlation outputs, discriminator outputs, phase, and Doppler.
Future FPGA Implementation
The next step is to add the navigation functional block to the solution so that position, velocity, and time can be computed. Then, a reciprocal system for multiconstellation, multifrequency GNSS signal generation will be developed using the NI USRP to provide a complete open receiver and generator tool for educational and research activities. As a second step, we intend to implement the software receiver in the LabVIEW FPGA development environment to obtain a real-time, open GNSS solution that can be used on various NI platforms such as NI FlexRIO or the vector signal transceiver.
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