FT8 Time Sync: Keeping Your Computer Clock Accurate

FT8 is famous for decoding signals you cannot hear, but it has one demanding requirement that catches out new and experienced operators alike: accurate time. Every FT8 transmission starts at a fixed point in a 15-second cycle, and every station on the band assumes that everyone else is on the same clock. If your computer drifts, you stop decoding stations and they stop decoding you. Good FT8 time sync is therefore not a refinement; it is a basic requirement of the mode.

This guide explains why timing matters so much in FT8, how to read the DT value in WSJT-X and similar programs, and how to keep your clock accurate on Windows, Linux and macOS, including at portable sites without internet access. It focuses on methods and habits rather than particular version numbers, because operating systems and utilities change regularly.

Why FT8 Depends on Accurate Time

FT8 divides time into fixed transmit and receive periods of 15 seconds, aligned to UTC. Stations transmit in one period and listen in the next. The decoder looks for signals that start close to the beginning of each period, and it searches only a limited window around that expected start time.

If your clock is off, two things go wrong at once. Your receive window is misaligned, so signals from correctly timed stations appear early or late and some fall outside the range the decoder searches. And your own transmissions start at the wrong moment, so other stations see your signal offset and may fail to decode it. A small error reduces decodes gradually; a large one stops them entirely.

The decoder tolerates a limited offset, which is why a clock that is slightly wrong still seems to work. The danger is that a slowly drifting clock degrades your results long before it stops them completely. You lose the weakest decodes first, which are exactly the DX signals you most want.

Why computer clocks drift

A computer keeps time with a crystal oscillator that is accurate enough for everyday use but not for FT8 over days or weeks. The drift rate varies between machines and changes with temperature, so a shack computer that warms up during a long session can drift faster than expected. Laptops that sleep and wake, virtual machines and single-board computers without a battery-backed clock are particularly prone to timing errors.

Operating systems correct drift by synchronising with time servers over the internet using the Network Time Protocol, or NTP. The question is how often they do it and how precisely. Default settings designed for office computers are not always good enough for FT8.

Reading the DT Column

WSJT-X and related programs display a DT value for every decoded signal. DT is the measured time offset of that signal relative to where your software expected it, in seconds. It is the most useful timing diagnostic you have, because it compares your clock with the clocks of many other stations at once.

• Most DT values near zero: your clock is good. Individual stations may still show offsets, because their clocks may be wrong, but the overall picture is centred.
• Most DT values offset in the same direction: your clock is probably the one that is wrong. If nearly every station shows a similar positive or negative offset, the common factor is you.
• One station with a large DT: that station’s clock is probably wrong, or the signal arrived by an unusual path. It is not a reason to adjust your own clock.
• DT growing steadily over a session: your clock is drifting and is not being corrected often enough.

Some propagation effects add a small, genuine delay. Long-path signals and some EME paths show offsets that are real rather than clock errors. Those are exceptions; for ordinary HF work, a consistent offset across many stations points at your own clock. If you are still setting up the software itself, our guide to a WSJT-X FT8 setup that works first time covers audio levels, rig control and other basics that sit alongside timing.

Time Sync Options Compared

There are several ways to keep your clock accurate. The best one depends on your operating system, whether you have internet access at the operating position, and how much you want to configure.

Windows

Windows includes a time service that synchronises the clock with an internet time server. Microsoft documents it as the Windows Time service. On a typical desktop installation it is designed to keep office computers reasonably close to the correct time, and it may update less often than an FT8 operator would like.

A first step is simply to open the date and time settings and use the option to synchronise now, then watch your DT values. If they centre on zero and then slowly drift away over hours, the update interval is the problem.

Many FT8 operators install a dedicated time synchronisation utility instead. Some are full NTP implementations that run as a background service and continuously discipline the clock; others are lighter tools that query a time server at a short, configurable interval and correct the clock. Both approaches work. The important points are:

• Run it as a service, or at least start it automatically, so the clock is correct before you open WSJT-X.
• Use a pool of time servers rather than a single server, so one unreachable server does not stop synchronisation.
• Run only one time synchronisation tool at a time. Two programs fighting over the clock can make timing worse.
• Check that the tool has the permissions it needs to change the system clock; some require administrator rights.

Linux and Single-Board Computers

Most Linux distributions include an NTP client. Some use a lightweight time synchronisation service by default, while others use chrony or the classic ntpd. Chrony is a popular choice for amateur stations because it copes well with intermittent connections, machines that are switched on and off, and clocks that start far from the correct time.

On a Linux shack computer, make sure exactly one time service is enabled, that it starts at boot, and that it reports itself as synchronised. The status commands of the time service show the current offset and which servers are in use. A good habit is to check that status once after setting up the machine and once after any major system update.

Single-board computers deserve special attention. Many have no battery-backed real-time clock, so after a power cut they boot with the wrong time until the network is available. If you use one as a digital-mode station, either add a real-time clock module, give it a reliable network connection, or provide a GPS time source. Starting FT8 before the clock has synchronised is a common reason for a session that decodes nothing at all.

macOS

macOS synchronises the clock automatically when the option to set the date and time automatically is enabled. For most home stations with a stable internet connection this is adequate. As on other systems, watch the DT values during long sessions. If you see a steady drift, consider a third-party time tool that updates more frequently, but avoid running it alongside another tool that also changes the clock.

Portable Operation Without Internet

Portable FT8 operation, for example from parks, summits or a field day site, often has no reliable internet connection. Mobile data may work at some sites and not others. You need a plan for time that does not depend on it.

GPS time

A small USB GPS receiver can act as a local time source. Software on the computer reads the time from the GPS and sets the system clock, or a full NTP service uses the GPS as a reference. GPS time is accurate, but the receiver needs a view of the sky and a short time to acquire satellites. Set it up at home first, confirm that it actually corrects the clock, and only then rely on it in the field.

Synchronise before you leave

If you cannot use GPS, synchronise the clock immediately before you leave home and keep the computer from sleeping. A fresh sync gives you a period of acceptable accuracy, although the length of that period depends on how fast your machine drifts. Measure it once: synchronise, run WSJT-X for an afternoon, and watch how quickly DT moves away from zero.

Time from decoded signals

Some utilities can estimate the clock error from the DT values of decoded FT8 signals and correct the clock accordingly. This is a clever fallback when nothing else is available, because it effectively averages the clocks of many other stations. It only works when you are already decoding enough signals, and it assumes most of those stations have accurate time, so treat it as a backup rather than the primary method. If you operate portable regularly, our guides to Parks on the Air and battery power for portable operation cover the rest of the field kit.

Troubleshooting Timing Problems

When FT8 decoding suddenly gets worse, timing is one of the first things to check, but it is not the only possibility. Work through the symptoms methodically.

• No decodes at all: check the clock first, then audio input levels and the selected sound device. A clock that is far off can stop decoding completely.
• Decodes, but every DT is offset: your clock is off. Synchronise it and watch whether the offset returns to zero.
• Others do not answer your calls: your transmissions may be starting late or early. Check DT on received signals; if it is offset, your transmissions are offset by a similar amount.
• Timing fine at the start, poor later: drift. Shorten the synchronisation interval or switch to a continuous NTP service.
• Timing jumps after waking the computer: sleep and hibernation can leave the clock wrong until the next sync. Disable sleep during operating sessions.
• Timing inconsistent on a virtual machine: virtual machine clocks can drift or jump. Run the time service inside the guest system, or operate FT8 on the host instead.

If timing looks fine and decoding is still poor, the cause may be elsewhere in the station. A program such as PSK Reporter shows who is decoding your transmissions, which helps you separate a transmit problem from a receive problem.

Good Habits for Reliable FT8 Timing

Timing problems are easiest to prevent with a few routine habits that take almost no time once they are part of your setup.

• Install one reliable time synchronisation method on every computer you use for FT8, and leave it running permanently.
• Glance at the DT column at the start of every session. It takes seconds and confirms your clock against the band.
• Never adjust your clock to match a single station with a strange DT. Look for the consensus across many stations.
• Test your portable time solution at home before you depend on it in the field.
• After operating system updates, confirm that the time service is still enabled and synchronised.

The Bottom Line

FT8 depends on every station sharing the same clock, aligned to UTC within a limited tolerance. Your computer clock drifts, and default synchronisation settings are not always frequent enough for digital modes. Use the DT column to check your timing against the band, run one continuous time synchronisation method on each operating computer, and plan a GPS or pre-departure sync for portable operation. With FT8 time sync handled properly, you will decode more weak signals and more of your calls will be answered.

Frequently Asked Questions

Why does FT8 need accurate time?

FT8 transmissions start at fixed points in 15-second periods aligned to UTC, and the decoder only searches a limited window around the expected start time. If your clock is off, you decode fewer signals and other stations decode you less reliably.

What does DT mean in WSJT-X?

DT is the measured time offset of a decoded signal relative to where your software expected it, in seconds. If most stations show a similar offset, your own clock is probably wrong; a single odd value usually means that station’s clock is off.

Is the Windows built-in time sync good enough for FT8?

Sometimes. It keeps the clock roughly correct, but its default update interval may let the clock drift during long sessions. Many FT8 operators use a dedicated time synchronisation tool that updates more often and runs as a service.

How do I keep accurate time for FT8 without internet?

Use a USB GPS receiver with software that sets the system clock from GPS time, or synchronise immediately before leaving home and keep the computer awake. Tools that estimate clock error from decoded signals are a useful last resort.

Can I run two time sync programs at once?

It is best not to. Two programs adjusting the same clock can work against each other and make timing less stable. Choose one method per computer, configure it properly, and disable any others.

Should I adjust my clock to match a station with a large DT?

No. A single station with a large DT usually has a wrong clock itself. Only adjust your clock when most decoded stations show a similar offset, which indicates that your own clock is the one that has drifted.