Tutorial DSC

Tutorial: DSC

What it is. Digital Selective Calling (DSC) is the digital part of the maritime distress system. A ship or coast station sends a short burst that names who is calling, who is being called, the category (routine, safety, urgency, distress) and sometimes a position. Distress alerts on 2187.5 kHz, 4207.5 kHz and the other HF channels are the best known. OnAir shows the public call data in a list. It decodes nothing but the call burst itself; voice traffic that follows is not part of this mode.

DSC, NAVTEX and weather fax share one mode panel (group Maritime), so some controls below are the same as in Tutorial-NAVTEX and Tutorial-Weather-fax.

What you need

• Radio. DSC on MF and HF needs a radio that tunes 2 to 18 MHz: an HF-capable SDR, a receiver such as an RX-888 or SDRplay, or a HackRF or RTL-SDR with an upconverter or direct-sampling mode. The mode asks for 2 Msps; 250 ksps is the lowest working rate. See Supported-radios and the radio pages.
• Antenna. A long wire, an active loop or a whip, as far from noise sources as possible. For 2187.5 kHz a long wire is best; the signal is mostly local ground wave, so coastal listening works best.
• Frequencies from the repository's frequency database. DSC distress channels: 2187.5 kHz (MF), 4207.5 kHz, 6312 kHz, 8414.5 kHz, 12577 kHz and 16804.5 kHz (HF), and VHF channel 70 at 156.525 MHz (1200 baud, ITU-R M.493). The mode's default is 2.1875 MHz.
• Converter note. If you use an upconverter, enter the frequency you would see on a normal radio; the converter offset is set in the radio settings.

Before you start

• HF DSC is 100 baud FSK with a 170 Hz shift, sent as two tones 85 Hz either side of the dial frequency. Tune the dial to the channel frequency exactly (for example 2187.5 kHz); the Tuning view shows where the tones really sit.
• Gain: low enough that atmospheric noise does not overload the radio, high enough that the noise floor lifts. Turn off AGC-like auto-gain if available.
• Avoid putting the signal on the DC spike. An offset of a few kHz is fine for the decoder.

Step by step

1. Mode list, group Maritime, entry Ship calls, 2187.5 kHz.
2. Check Service: auto chooses by frequency; select DSC to force it.
3. Press Start. The sub-views along the top are NAVTEX, DSC, Fax, Tuning and Fax stations; select DSC. With Service on auto the panel can follow the service it detects.
4. Use Tuning to check the tones. You should see two peaks 170 Hz apart centred on the dial frequency.
5. Wait. DSC traffic on 2187.5 kHz is mostly routine tests and calls; true distress calls are rare.

What you should see

In the DSC list: a row per received call with UTC, Call, Category, From, To, Position and Check. Click a row to see the details on the right: format, category, sender, addressee, the ship in distress, nature of distress, position and time, telecommand, working frequencies, and a line such as "check ok, 0 symbols lost, MF/HF" (or "VHF ch 70"). A call with a position also gets a small map. The list header counts NAVTEX and DSC messages ("n NAVTEX, n DSC").

Reading the screen

• DSC list. One row per call. Call gives the format (individual, group, all ships, distress alert, etc.). Category shows routine, safety, urgency or distress. From and To are the sender and addressee (MMSI). Position is present in distress alerts and some calls. Check is the error-check result ("check ok" or "check failed"). Distress calls are shown in red.
• Tuning. A spectrum of the audio relative to the dial: markers show the tones each service expects (FSK 85 Hz either side of the dial for NAVTEX and DSC; 1500 and 2300 Hz for fax black and white). A line such as "FSK tones -85 / +85 Hz, shift 170 Hz (170 expected), 100.0 baud" confirms a good lock.
• Listen. A Listen control plays the audio so you can hear the tones; decoding is the same either way.

Options and settings

• Service: auto, NAVTEX, DSC or weather fax. With the test signal or a recording every decoder runs.
• clear empties the lists.

Troubleshooting

• No FSK signal found. The mode reports "NAVTEX and HF DSC sit on the dial frequency, 85 Hz either side". Retune precisely, or check sideband.
• VHF channel 70 shows nothing in Tuning. VHF DSC is FM with audio tones of 1300 and 2100 Hz, so it does not show in the FSK tuning view. The decoder has a VHF variant (calls are marked "VHF ch 70"); use an FM-capable tuning at 156.525 MHz.
• Check fails often. Noise burst or fading. Move the antenna away from computers and chargers.
• Nothing for hours. Normal on HF; try 2187.5 kHz near a coast, or compare with Tutorial-NAVTEX to confirm the setup.
• Overload or birdies. Reduce the gain; add a band-pass filter.

Tips

DSC calls repeat: a call is often sent several times. If the mode misses one, the next copy may succeed.

Legal and privacy notes

DSC calls are public radio signals, but the identity and position of a vessel are personal data in some places. Receiving is generally allowed; publishing identities may not be. Never use a received distress call as anything but a prompt to alert the proper authority. OnAir is a receiver: you must not transmit DSC without the proper certificate and licence. See Legal.

Under the hood

HF/MF DSC: 100 baud FSK, 170 Hz shift, 10-bit symbols with error-detection and time diversity, per ITU-R M.493 (system) and M.541 (operational procedures). VHF DSC: 1200 baud FSK with 1300/2100 Hz tones on channel 70. Positions come in the distress alert and "position update" formats.

Related

Mode-DSC, Tutorial-NAVTEX, Tutorial-Weather-fax, Tutorial-AIS, Troubleshooting, Legal