OpenPlotter (Ref.1) is a Linux based Operating System that runs on the Raspberry Pi computer. The Raspberry Pi is extremely popular in the education field. It is well documented, compact and requires very little power. This makes it ideal to form the basis of an electronics navigation system for a small craft. In the previous three posts (Ref.2, Ref.3, Ref.4) we looked at how to install OpenPlotter on the RPi and remote it from your laptop, how to connect GPS and finally how to connect and view AIS on OpenCPN. In this post we will look at how to use Gqrx (part of SDRVHF) to receive Marine/Air frequencies.
Gqrx Installation & Configuration
In the previous post, we installed the SDR VHF application under OpenPlotter. Then we installed and activated the AIS option. When you open SDR VHF, you see the menu as shown in Fig.1. Highlight the GQRX option and install it. This may take several minutes. Once it is installed, go under Processes and uncheck/stop the AIS process and then activate and start the GQRX process. The reason we do this is that only one process can use the RTL-SDR at one time. You can check whether the RTL-SDR is properly detected by using the following command in Terminal:
>dmesg | grep rtl
You can now launch GQRX from Processes by clicking on Show, or under the main Pi menu.
Marine/Air VHF Reception using Gqrx
Once Gqrx opens, click on the icon under Tools and ensure that you have selected the correct RTL-SDR that you detected in Terminal. Gqrx works with several different types of receivers, so ensure you select the correct one. Once you have done this, hit the start icon under File to start the receiver. There are various menus where you can adjust the receiver gain and also the PPM error correction. I noticed that when receiving WBFM signals, the reception breaks up a bit due to the large processor requirement put on the RPi3B that I was using. This should not be a problem on the RPi4. You can check processor loading using Terminal:
Another thing to be careful of is the Gain setting. Only use enough gain for proper receiver reception, cranking up the gain may actually bury your signal in intermod, so only use what you have to. Start at 0dB and slowly work your way up. Fig.2 YouTube Video shows the complete process.
Please send your comments, questions and suggestions to:
#2. – “OpenPlotter AIS Connection”
#3. – “OpenPlotter – GPS Connection”
#4. – “OpenPlotter – Navigation System on RPi”
#5. – “RTL-SDR for AIS”
#6. – “AIS GMSK Modulator – Scicos Simulation”