Why FT8 needs an accurate clock
FT8 transmissions start every 15 seconds, at 0, 15, 30 and 45 seconds past each minute UTC, and FT4 every 7.5 seconds. Every station transmits and listens in step. WSJT-X can only decode signals that start within about ±2 seconds of where it expects, and decoding gets less reliable well before that. The DT column in WSJT-X shows each station's timing error, including yours. Aim to keep your clock within 0.3 seconds.
Fixing your clock
- Windows: Windows' own time sync only runs occasionally and can drift by seconds. Use a time-sync program such as Meinberg NTP, Dimension 4 or NetTime, or the Time Sync option in some logging programs.
- Mac: turn on "Set time and date automatically" in Date & Time settings. It keeps within a fraction of a second.
- Linux and Raspberry Pi: use chrony or systemd-timesyncd, both usually on by default.
- Portable with no internet: a USB GPS receiver with suitable software keeps the clock accurate anywhere.
How the check works
The page asks a time server for the time several times, measures how long each request takes, and uses the fastest to work out the difference from your clock, much as NTP does. The result is usually accurate to within 10 to 50 ms. The bars show where you are in the current FT8 and FT4 periods, using the corrected time. For band and frequency details, see the digital mode frequencies page.
Questions
How accurate does my clock need to be for FT8?
Within about 1 second to decode reliably, and ideally within 0.3 seconds. WSJT-X can tolerate up to about 2 seconds, but decodes drop off.
Why does WSJT-X show a large DT?
Usually because your computer's clock is wrong. If every station shows a similar DT, your clock is out.
Is Windows time sync good enough for FT8?
Not always. Windows syncs infrequently and can drift by seconds. A dedicated time-sync program keeps it accurate.
When do FT8 periods start?
At 0, 15, 30 and 45 seconds past every minute, UTC.