RTL-SDR for Marine GMDSS/DSC on MULTIPSK

Introduction

Recently I received an interesting question from Scott in Vancouver about Marine Channel 70 and DSC Digital Selective Calling. DSC is a part of the GMDSS Global Maritime Distress and Safety System (Ref.1/2). It can be used by ships on the MF/HF bands or in the VHF band. The ship MMSI identifier can be linked with GPS to include the ship position making for very fast emergency calling. Figure 1 lists the DSC parameters and Figure 2 shows the frame structure.

ParameterMF/HFVHF
Frequency2187.5, 4207.5, 6312.0, 8414.5, 12577.0, 16804.5 KHz156.525MHz
Baud Rate1001200
Character10bits (#s 00-99)
Error Correcting Code
b1-7 info b8-10 parity		10bits (#s 00-99)
Error Correcting Code
b1-7 info b8-10 parity
Frame6-7secs0.5secs
ModulationJ3E
USB(300,2700)
nom 1700Hz/+-85Hz
d_Mark/Space=170Hz		F3E/G3E
NBFM
1700Hz/+-400Hz
d_Mark/Space=800Hz
BW800Hz3500Hz
Fig1. DSC HF/VHF Parameters
Fig.2 DSC Frame

DSC HF/VHF Test Transmissions Audacity – Multipsk

Fig.3 Audacity DSC Test File to Multipsk Decoder
Fig.4 DSC HF Test Message (Ref.3)
Fig.5 DSC VHF Test Message (Ref.3)
Fig.6 Multipsk HF DSC Decode
Fig.7 Multipsk VHF DSC Decode

Due to my location in Toronto, it is unlikely that I will receive an HF DSC signal, and hopefully I won’t receive a VHF distress signal from Lake Ontario. So in order to test reception, I located a test wav file from SigWiki a repository of telecom signal captures (Ref.3). A simple test is shown in Figure 3. The HF/VHF wav files are played in Audacity (Ref.4) and fed by VB cable to Multipsk (Ref.5) and decoded as shown in Figures 4 – 7. The HF wav file is longer with slower baud rate and the FSK is audible. The VHF wav is much shorter.

RTL-SDR Reception

Fig.8 VHF DSC Tx on PlutoSDR and Rx on RTL-SDR
Fig.9 GNU Radio NBFM VHF DSC Tx Schematic
Fig.10 VHF DSC Tx Spectrum & Rx Spectrum
Fig.11 Multipsk VHF DSC Decode

Figure 8 shows reception of VHF DSC on the RTL-SDR. The VHF DSC wav file is transmitted on VHF CH70 using the PlutoSDR and received using Multipsk and decoding. The PlutoSDR can be controlled either by SDRangel or GNU Radio using an NBFM transmitter. This is shown in Figures 9. Figure 10 shows the Tx & Rx spectrum and Figure 11 shows the Multipsk decode.

Fig.12 YouTube Video RTL-SDR for Marine GMDSS/DSC on Multipsk

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References

#1.- “GMDSS Global Maritime Distress and Safety System”
https://en.wikipedia.org/wiki/Global_Maritime_Distress_and_Safety_System

#2. – “ITU-R M.493-15 Digital Selective Calling System for use in Maritime Mobile Service”
https://www.itu.int/rec/R-REC-M.493-15-201901-I/en

#3. – “GMDSS HF & VHF Wav Files”
https://www.sigidwiki.com/wiki/GMDSS_Digital_Selective_Calling

#4. – “Audacity”
https://www.audacityteam.org/

#5.- “MULTIPSK”, Ham Radio Software, F6CTE
http://multipsk.fr/index_anglais.html

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.