Amateur radio astronomy experiments based on software defined radio have increased in the past few years due to an incredible flexibility of the software defined radio device (Laufer, 2015). Software defined radio-devices save money from buying a number of different items of hardware, that are used in the transfer of the signal from hardware into software for further processing (Marcus, 2012). The GNU-radio is a free software that is considered to be a sig- nal processing framework, which depends on software defined radio devices (Gnuradio.org, 2016). This signal processing frame work is used to control the software defined radio hard- ware used in the development of the telescope, by using the GNU-radio companion block called Osmocom block. The Osmocom block is one of the most powerful block of the GNU- radio companion, which is implemented on the computer as a software. Its main function is to connect the software defined radio-hardware with other GNU-radio software applica- tions (Seeber, 2014). Without having the Osmocom block installed on a computer, radio astronomy experiments based on the SDR-device with GNU-radio applications could not be possible. Therefore, it is important to have it installed first on a computer before considering the use of any GNU-radio application. The guidelines on the installation and the use of the Osmocom block can be found at http://sdr.osmocom.org/trac/wiki/GrOsmoSDR. Lately, the use of the GNU-radio applications in conducting cheap radio astronomy experiments based on software-defined radio have also increased. This is because the GNU radio applica- tions are easy to use and have a collection of signal processing blocks that are useful in the building of radio receivers within the GNU-radio companion. Furthermore, the GNU radio software provides hands on experience on the processing of the received digital signals from the SDR-device (Airspy) by using one of the radio receivers compatible with the GNU-radio software such as the GQRX receiver and the simple ra receiver.
4.2.1
Digital signal processing and recording with GNU-Radio
Companion
The GNU-radio companion is one of the most important and powerful component of the GNU-radio software (Gnuradio.org, 2016). It allow the user to graphically program GNU- radio applications such as simple ra receiver or real time spectrum analyser that provides features like frequency spectrum plot, spectrum waterfall plot, constellation plot and scope plot (Seeber, 2014). Graphically programming in GNU-radio involves the connecting of the GNU-radio blocks found within the GNU- radio companion as illustrated in figure (4.3), which is achieved by having the Osmocom block as the key component. More information on how to install and use GNU-radio companion on Linux and other operating systems can be found at http://www.gnuradio.org/. The following are some of the main features of a real time spectrum analyser developed in GNU-radio companion.
1. The frequency spectrum plot measures the signal amplitude in dB against frequency in MHz. Therefore, it is sometimes referred to as the frequency domain spectrum analyser (Seeber, 2014). With the real time spectrum analyser shown in figure (4.4), an observed source of our interest is found in the middle of the spectrum analyser, while the left and right hand side of the spectrum analyser represent the lowest and the highest frequency respectively. This means that the frequency at which you are observing the source is found in the middle of the spectrum analyser.
Figure 4.3: Image of the graphically programmed GNU-radio application of the simple ra https://www.google.com/amp/slideplayer.com/amp/7446123/.
Figure 4.4: The frequency spectrum plot showing the measured signal amplitude in dB against frequency in MHz
(4.5), and it is referred to as the waterfall plot window. The waterfall plot is sometimes called the frequency domain analysis. With these two real time spectrum analysers, one can visualize the strength and the components of the detected signal in both plots as shown in figure (4.6). In radio astronomy, waterfall plots are very useful in the study of various objects such as Fast Radio Bursts (FRBs), Pulsars, Rotating Radio Transients etc.
Figure 4.5: Shows the waterfall plot window with a strong detected signal in the middle
4.2.2
GQRX Receiver
The GQRX is an open source software based on software defined radio device developed by Alexandru Csete. It is also implemented on a computer as a software receiver that works with GNU-radio and Qt tool kit (Alexandru, 2016). Additionally, the GQRX software has been proven to be compatible with different software defined radio devices such as Funcube Dongle, RSTL-SDR, Airspy, HarckRF, BladeRF, RFSpace, USRP and SoapySDR (Alexandru, 2016). The GQRX receiver provided by the GQRX - software has similar features of that of the Airspy-receiver on a machine running on Windows OS, the only difference is that it allow the input of the Local Oscillator frequency of the LNB used as the front-end receiver of the telescope. This receiver can also receive signals in Amplitude
Figure 4.6: Shows a real time spectrum analyser of the GQRX receiver during the Solar observation with the 1 m off-the shelf UH telescope
Modulation (AM), Frequency Modulation (FM) and Single Side Band (SSB) with an audio output (Laufer, 2015). However, the GQRX software with a software defined radio-hardware works well on Linux and Mac system supported by the Osmocom block, which has been discussed earlier in section (4.2) of this chapter. In this study, we have experimented with simple radio astronomy observations with the GQRX-receiver and the Airspy-receiver, and one of them is shown in figure (4.6) that shows the solar observation in real with the GQRX receiver. One of the advantages of using the receiver provided by the GQRX-software is that, you are able to visualize the detected signal in both the FFT and the Waterfall display in real time. The observed signal by the telescope is recorded in WAV format and can be replayed later with the GQRX receiver. For more information about the GQRX-software follow http://gqrx.dk