Software
3.4 Electrical Ground Support Equipment (EGSE)
The EGSE will use a software defined radio - SDR, as its radio via software. Traditionally, these physical layer functions are implemented with hardware components (e.g., filters, amplifiers, modulators/demodulators, detectors, etc.). The hardware redesign is expensive and time consuming, while software code redesign requires less effort in terms of money and time. One of the most important advantages of SDR over the traditional radio communication systems is the flexibility. SDR offers the ability to transmit and receive different radio protocols or waveforms changing software and without modifying the SDR platform (the combination of hardware and operating environment where the application is running). Furthermore, data processing may be performed with any general-purpose computer rather than using specialized hardware. In the field of ground station technologies, this gives the possibility to easily modify the design to communicate with satellites using different modulation schemes and protocols.
Among the several available models of SDR, we selected the universal software radio peripheral (USRP) B210 (Figure 2.2) [1], by Ettus Research Ltd. The USRP is designed to allow general-purpose computers to function as high bandwidth SDR. In essence, it serves as a digital baseband and IF section of a radio communication system. All high-speed general purpose operations like digital up and down conversion, decimation and interpolation are done on the field programmable gate array (FPGA). The powerful combination of flexible hardware, open source software and a community of experienced users make it the ideal platform for a SDR development. It is a wide bandwidth transceiver with a 8 dB noise figure providing 56 MHz of bandwidth in the frequency range from 70 MHz to 6 GHz and working in half or full duplex mode on different transmit and receive frequencies, thanks to its independent receive and transmit local oscillators and synthesizers.
The Ettus Research has developed a free, open-source and multi-platform software compatible with all USRP devices named USRP Hardware Driver (UHD). It can be used standalone or with different third-party applications, for developing SDR platforms, such as LabVIEW, MATLAB/Simulink and GnuRadio. The first two software are commercial while the latter is a free, open-source and multi-platform software, even if it runs better in Linux since it is Linux native. GnuRadio uses a two-tier structure: the computationally intensive processing functions are implemented in C++ while application-defined control and coordination of blocks are developed in Python. GnuRadio comes with a graphical programming interface implemented in XML language named GnuRadio Companion (GRC) that allows generating flowgraphs by dragging and dropping the blocks from a list without writing the Python code.
A drawback of using GnuRadio is that documentation is spread through the GnuRadio website, forums, presentations, papers and thesis works, making the learning curve quite steep but the users’ community is very active and increasing year by year.