OpenPlotter3 – Marine HF Weather Fax


Fig1. OpenPlotter3 Marine HF WEFAX Equipment Setup

Recently I revisited OpenPlotter after a two year interval to see what had changed in the latest version 3 (Ref.1/2). I tested the Starter and Headless images and setup AIS & GPS on OpenCPN. In 2021 I had looked at various aspects of version 2 (Ref.3/13). In this post I will look at the RPi4 OpenPlotter3 Starter 64bit image and connection of an RTL-SDR V3 for reception of Marine HF Weather Fax on Gqrx & Fldigi as shown in Figure 1.

RPi4 OP3 Starter Image Setup

Fig.2 OpenPlotter3 Opening Screen
Fig.3 RaspberryPi4 Configuration
Fig.4 OpenPlotter3 Settings
Fig.5 OpenPlotter3 Settings Cont’d
Fig.6 SDR VHF Package Installation GQRX

I first downloaded the RPi4 OpenPlotter3 Starting 64bit image 23_08_04 (Ref.14). Following the installation instructions, I used the Raspberry Pi Imager to write the img_zip to a 32GB SD card. I then inserted the SD card and connected the RPi4 to a mouse/keyboard/hdmi_tv. The boot up process went smoothly into the OpenPlotter3 Desktop as in Figure 2. After this, I used the RPi4 Configuration Menu to enable SSH/VNC/I2C in order to control the RPi4 over a LAN using VNC as in Figure 3.

I connected the RPi4 over the LAN and logged in with VNC viewer. I then changed the default password. After this, I checked the OpenPlotter3 Settings Menu as in Figure 5/6. Only two packages needed updating Documentation/Dashboards. I then installed the SDR VHF package. Once installed, I opened the SDR VHF package and installed the GQRX receiver program.

GQRX Setup

Fig.7 GQRX Parameters for RTL-SDR V3

In order to receive the HF WEFAX stations, an RTL-SDR V3 can be used with direct sampling. Figure 7 shows the parameters that have to be entered into the GQRX I/O Device configuration in order to use this direct sampling. Direct sampling is not used for regular VHF/UHF operation.

FLDIGI Configuration

Fig.8 Installation of FLDIGI
Fig.9 FLDIGI Audio Configuration Screen

In order to decode the HF WEFAX signal, FLDIGI is used. This is a popular amateur radio decoding program and has an option for WEFAX. Figure 8 shows the installation using the RPi4 Add/Remove Software menu. Figure 9 shows the Audio configuration screen for PulseAudio.

PulseAudio Volume Control

Fig.10 Installation of Pulse Audio
Fig.11 Configuration of Pulse Audio
Fig.12 Configuration of Pulse Audio Cont’d

In order to take the audio output of GQRX and feed this to the input of FLDIGI, PulseAudio is used. Figure 10 shows the installation from the RPi4 Add/Remove Software menu. Figures 11/12 show the various volume adjustments. I also use a small USB speaker to monitor the GQRX output.

HF WEFAX Frequencies

Fig.13 Frequency Schedule for NOAA HF WEFAX North America East Coast

Figure 13 shows the HF WEFAX frequencies for North America East Coast. In my location the two frequencies of interest are 6340.5/9110KHz from Boston. The complete reference is given in Ref.15.

Reception Results

Fig.14 GQRX Reception of WEFAX 9110KHz CNR <= 10dB
Fig.15 FLDIGI Received WEFAX Image CNR <= 10dB
Fig.16 GQRX Reception of WEFAX 9110KHz CNR >= 20dB
Fig.17 FLDIGI Received WEFAX Image CNR >= 20dB
Fig.18 FLDIGI Received WEFAX Image CNR >= 20dB

For good reception, the CNR should be >= 20dB. Figure 14 shows GQRX reception on May 6th when the CNR <= 10dB. The received WEFAX image was very grainy as shown in Figure 15. Figure 16 shows GQRX reception on May 7th when the CNR >= 20dB. The received images were excellent as shown in Figures 17/18.

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#1. – “OpenPlotter3 – Sensors_a”

#2. – “OpenPlotter3 Headless for GPS and RTL-SDR AIS on RPi4”

#3. – “OpenPlotter – RRi4 – Headless_b”

#4. – “OpenPlotter – Rpi4 – USBpwr”

#5. “Open Plotter – RPi4 Headless_a”

#6. “OpenPlotter – GRIB Files”

#7. – “OpenPlotter – Sensors_b”

#8. – “OpenPlotter – AIS+”

#9. – “OpenPlotter – Sensors_a”

#10. – “OpenPlotter – RTL-SDR for VHF/AIS”

#11. – “OpenPlotter – AIS Connection”

#12. – “OpenPlotter – GPS Connection”

#13. -“OpenPlotter – Naviagtion System on RPi”

#14. – “OpenPlotter Images”

#15. – “Worldwide Marine Radiofacsimile Broadcast Schedules NOAA”

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.