OpenPlotter – Marine HF Weather Fax – Telecommunications, Navigation & Electronics

Introduction

Last August I discussed using the RTL-SDR for reception of HF Weather Fax information using a setup of Windows10/SDR#/FLDIGI (Ref.1). Recently I was asked whether OpenPlotter could be used to do the same thing. OpenPlotter has an equivalent program to SDR# called Gqrx which can be installed and used with the RTL-SDR (Ref.2). FLDIGI can be installed separately using the Raspberry Pi Add/Remove software tool. In order for FLDIGI to monitor the output of the Gqrx and decode the information, Pulse Audio Volume Control can be installed. This serves the same purpose as a Sound Mixer in Windows or a Virtual Cable. Gqrx pushes the capacity of the RPi3B which I used for the test, so RPi4 is recommended. In order to prevent cutting in/out, I lowered the Gqrx sampling rate to 1.2Msps.

Reception Preparation

In my setup, I use the RTL-SDR with a Nano Upconverter. There are versions of the RTL that cover the HF band as well with direct sampling, but I haven’t tried one of those yet. I run OpenPlotter on a RPi3B+ with one RTL. In order to demodulate the HF Wx Fax signal we need to install FLDIGI. Use the RPi Add/Remove software menu to search for and install FLDIGI “fldigi-4.xy”. The installation will install the two items as shown in Figure 1. Using Add/Remove software again, search for and install the Pulse Audio Volume Control “pavucontrol-3xy” as shown in Figure 2.

Fig.1 Install fldigi-4.1xy Digital Modem Program

Fig.2 Install pavucontrol-3xy Pulse Audio Volume Control

Now we can start reception. Since I normally use my RTL-SDR for AIS reception, I first have to stop this process. In SDR VHF, select the AIS process and stop it as shown in Figure 3. Now start Gqrx and ensure you select the correct input receiver, in my case Realtek RTL2838UHIDIR as shown in Figure 4. On the RPi3B+ the Gqrx tends to cut in/out, so I reduced the sample rate from 1.8Msps to 1.2Msps. Next open FLDIGI, and in the Configuration menu for Audio, select Pulse Audio. Finally, open pavucontrol, and ensure the FLDIGI is set to monitor the output of Gqrx as shown in Figure 5.

Fig.3 Stop AIS Process using the RTL-SDR if necessary

Fig.4 Gqrx Select Rx Device & Reduce Fs=1.2Msps

Fig.5 Gqrx as Playback Device & FLDIGI Monitoring Playback

Reception

The latest HF WEFAX Frequency Schedules (Jan14th_2021) for North America East Coast are shown in Figure 6 (Ref.2).

Fig.6 NOAA Frequency Schedules North America East Coast Jan14_2021

Figure 7 shows a HF WEFAX signal being received from Boston at 6340.5KHz. The Gqrx carrier is set at 6338.0KHz/USB so that the strong carrier is aligned with the right FLDIGI marker edge. Figure 8 shows an image being decoded.