Tutorial VDES

Tutorial: VDES

What it is. VDES, the VHF Data Exchange System, is the planned successor and extension of AIS. Besides the old AIS channels it adds faster terrestrial data channels and satellite links. OnAir decodes the application-specific messages (ASM) on the AIS channels and detects, but does not decode, the high-rate terrestrial bursts. It is mainly interesting for people who want to see whether VDES traffic is already on the air near them.

What you need

• Radio. Any radio that tunes about 150 to 165 MHz. The mode asks for 1 Msps; the lowest rate that works is 400 ksps. An RTL-SDR does it.
• Antenna. A vertical VHF antenna with a clear view of the water, as for Tutorial-AIS. A filter for the 156 to 163 MHz band and a low-noise amplifier help.
• Frequencies. The repository's frequency database lists the ASM channels 1 and 2 at 161.950 and 162.050 MHz (25 kHz wide). The mode's own information adds the ASM channels at 161.975 / 162.025 MHz (the AIS channels) and the 100 kHz VDE-TER channels at about 157.2 and 161.8 MHz. The default start frequency is 161.9 MHz.

Before you start

• At 1 Msps the capture covers about 0.5 MHz either side of the centre. Tune to 161.9 MHz to cover the ASM channels and the 161.8 MHz VDE-TER channel together. Tune near 157.2 MHz to see the other VDE-TER band.
• Do not tune exactly on a channel: keep the DC spike of the radio away from it.
• Gain: moderate. Strong paging and broadcast signals are the usual problem in this band.

Step by step

1. Mode list, group Maritime, entry Maritime, VHF.
2. Check the frequency (161.9 MHz) and the rate (1 Msps).
3. Press Start.
4. Open the tabs ASM messages and VDE-TER bursts.
5. Let it run for several minutes. VDES traffic is still rare in most areas; a quiet screen is normal.

What you should see

On the ASM messages tab, rows with Time, Channel, Bits, SNR, AIS type and Payload (in hex). On the VDE-TER bursts tab, rows with MHz, Width, Modulation, Level, Length and Offset. The status line counts frames good and bursts failed.

Reading the screen

• ASM messages. GMSK frames at 9.6 kbit/s with AIS framing and a CRC-16 check. The payload is shown as it was sent; it is read as an AIS message only when its first bits look like one, and the AIS type column then names it. hex switches the payload display, Copy copies a row, Clear empties the list.
• VDE-TER bursts. The 100 kHz channels at 157.2 and 161.8 MHz carry bursts of 25, 50 or 100 kHz. OnAir logs each burst with band, bandwidth, level and a modulation class (pi/4-QPSK, 8PSK, 16QAM) estimated from the symbol statistics, assuming root-raised-cosine 0.3 at 19.2 kbaud per 25 kHz. The content is not decoded.
• The channel table shows each channel's state and whether it is inside the current capture ("not in band").

Options and settings

There is a simulated signal for trying the screens without a radio. Frequency and rate can be changed while running.

Troubleshooting

• Empty ASM list. ASM traffic is rare. Check Tutorial-AIS first: if AIS works on the same antenna, the radio side is fine.
• Many VDE-TER bursts but no content. Expected: they are only detected and classified.
• Bursts at wrong bandwidth. Weak bursts are misjudged; look at Level.
• Channel says "not in band". Retune so that the channel is inside the sampled spectrum, or raise the rate.
• Crashes or dropouts at high rates. Use 1 Msps; there is nothing to gain from more.

Tips

Compare the ASM channels with the AIS mode running on the same radio: ASM frames sit on the same two frequencies.

Legal and privacy notes

These are public maritime broadcasts. Receiving is legal in many countries; check yours. See Legal.

Under the hood

VDES is defined in ITU-R M.2092. ASM uses the AIS physical layer (GMSK 9600 bit/s, HDLC, CRC-16). VDE-TER uses pi/4-QPSK, 8PSK and 16QAM in 25 to 100 kHz channels; satellite links use other bands and are not covered here.

Related

Mode-VDES, Tutorial-AIS, Tutorial-DSC, Tutorial-NAVTEX, Troubleshooting