Introduction
In previous posts I looked at decoding GMDSS/DSC signals using Multipsk (Refs.1/4). Recently I have been able to use my balcony again to receive HF & VHF signals. I noticed that I was able to decode VHF DSC using the USB mode, which I had previously set for HF, instead of NBFM. So I thought I would review the various modulation methods and what the specs say that I should be seeing on receive.
GMDSS/DSC VHF Reception
| Parameter | MF/HF | VHF |
| Frequency | 2187.5, 4207.5, 6312.0, 8414.5, 12577.0, 16804.5 KHz | 156.525MHz |
| Baud Rate | 100 | 1200 |
| Character | 10bits (#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 |
| Frame | 6-7secs | 0.5secs |
| Modulation | J3E USB(300,2700) nom 1700Hz/+-85Hz d_Mark/Space=170Hz | F3E/G3E NBFM 1700Hz/+-400Hz d_Mark/Space=800Hz |
| BW | 800Hz | 3500Hz |
Figure 1 shows a summary of the HF/VHF DSC parameters. In VHF mode, frequency modulation of the carrier is used as opposed to upper side band for HF. Figure 2 shows the receive block diagram. Previously, I had used signal captures from a signal database (Ref.8), but this time I received a local coastal station. Reception was done with SDR# and the baseband recorder to capture IQ samples around a known transmission time. Figures 3 & 4 show the playback with NBFM and USB decoding. Figures 5 & 6 show the Multipsk decodes.
Frequency modulation is a non linear process, so that the output spectrum contains an infinite number of tone sidebands given by the Bessel function expansion. For a low modulation index, the output spectrum will mainly contain the primary harmonics. Figure 6 clearly shows the extra harmonic content. Although usb mode managed to decode, clearly NBFM is the right mode.
GMDSS/DSC VHF Scicos Simulation
Figure 7 shows a Scicos simulation of the VHF DSC CH70 signal. Data is generated in the Scicoslab Editor at 1200baud and Figure 8 shows the FM VCO spectrum output.
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References
#1. – “RTL-SDR for VHF Marine GMDSS/DSC”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-vhf-marine-gmdss-dsc/
#2. – “RTL-SDR for HF Marine GMDSS/DSC”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-hf-marine-gmdss-dsc/
#3. – “RTL-SDR for Marine GMDSS/DSC on RaspberryPi4”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-marine-gmdss-dsc-on-raspberrypi4/
#4. – “RTL-SDR for Marine GMDSS/DSC on Multipsk”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-marine-gmdss-dsc-on-multipsk/
#5. – “GMDSS A User’s Handbook”, Denise Brehaut, 6th Edition, Adlard Coles
https://www.amazon.ca/dp/B071JXWBWC/
#6.- “GMDSS Global Maritime Distress and Safety System”
https://en.wikipedia.org/wiki/Global_Maritime_Distress_and_Safety_System
#7. – “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
#8. – “GMDSS HF & VHF Wav Files”
https://www.sigidwiki.com/wiki/GMDSS_Digital_Selective_Calling
#9. – “Audacity”
https://www.audacityteam.org/
#10.- “MULTIPSK”, Ham Radio Software, F6CTE
http://multipsk.fr/index_anglais.html
#11. – “SDR#”
https://airspy.com/download/
