{"id":7935,"date":"2023-02-21T14:42:13","date_gmt":"2023-02-21T19:42:13","guid":{"rendered":"https:\/\/jeremyclark.ca\/wp\/?p=7935"},"modified":"2025-12-17T09:49:51","modified_gmt":"2025-12-17T14:49:51","slug":"rtl-sdr-for-marine-gmdss-dsc-on-multipsk","status":"publish","type":"post","link":"https:\/\/jeremyclark.ca\/wp\/telecom\/rtl-sdr-for-marine-gmdss-dsc-on-multipsk\/","title":{"rendered":"RTL-SDR for Marine GMDSS\/DSC on MULTIPSK"},"content":{"rendered":"\n

Introduction<\/h2>\n\n\n\n

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<\/a>\/2<\/a>). 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. <\/p>\n\n\n\n

Parameter<\/strong><\/td>MF\/HF<\/strong><\/td>VHF<\/strong><\/td><\/tr>
Frequency<\/td>2187.5, 4207.5, 6312.0, 8414.5, 12577.0, 16804.5 KHz<\/td>156.525MHz<\/td><\/tr>
Baud Rate<\/td>100<\/td>1200<\/td><\/tr>
Character<\/td>10bits (#s 00-99)
Error Correcting Code
b1-7 info b8-10 parity<\/td>		10bits (#s 00-99)
Error Correcting Code
b1-7 info b8-10 parity<\/td><\/tr>
Frame<\/td>6-7secs<\/td>0.5secs<\/td><\/tr>
Modulation<\/td>J3E
USB(300,2700)
nom 1700Hz\/+-85Hz
d_Mark\/Space=170Hz<\/td>		F3E\/G3E
NBFM
1700Hz\/+-400Hz
d_Mark\/Space=800Hz<\/td><\/tr>
BW<\/td>800Hz<\/td>3500Hz<\/td><\/tr><\/tbody><\/table>
Fig1. DSC HF\/VHF Parameters<\/figcaption><\/figure>\n\n\n\n
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Fig.2 DSC Frame<\/figcaption><\/figure>\n\n\n\n

DSC HF\/VHF Test Transmissions Audacity – Multipsk<\/h2>\n\n\n\n
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Fig.3 Audacity DSC Test File to Multipsk Decoder<\/figcaption><\/figure>\n\n\n\n
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Fig.4 DSC HF Test Message (Ref.3<\/a>)<\/figcaption><\/figure>\n\n\n\n
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Fig.5 DSC VHF Test Message (Ref.3<\/a>)<\/figcaption><\/figure>\n\n\n\n
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Fig.6 Multipsk HF DSC Decode<\/figcaption><\/figure>\n\n\n\n
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Fig.7 Multipsk VHF DSC Decode<\/figcaption><\/figure>\n\n\n\n

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>). A simple test is shown in Figure 3. The HF\/VHF wav files are played in Audacity (Ref.4<\/a>) and fed by VB cable to Multipsk (Ref.5<\/a>) 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. <\/p>\n\n\n\n

RTL-SDR Reception<\/h2>\n\n\n
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Fig.8 VHF DSC Tx on PlutoSDR and Rx on RTL-SDR<\/figcaption><\/figure>\n<\/div>\n\n\n
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Fig.9 GNU Radio NBFM VHF DSC Tx Schematic<\/figcaption><\/figure>\n\n\n\n
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Fig.10 VHF DSC Tx Spectrum & Rx Spectrum<\/figcaption><\/figure>\n\n\n\n
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Fig.11 Multipsk VHF DSC Decode<\/figcaption><\/figure>\n\n\n\n

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. <\/p>\n\n\n\n

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