Guides Time And Gps

Time and position

Two things a station needs from its surroundings: the right time and where it is. With a network both are automatic. This page is about the day the network is not there, which is the day an EMCOMM station exists for.

Why the clock matters

FT8, FT4, JS8 and WSPR transmit in fixed time slots, and a decoder only looks for a signal where its clock says the slot is. A clock more than about a second off decodes nothing, with no error, on a band full of signals. A laptop's own clock drifts that far in days without correction.

Check it now:

timedatectl

System clock synchronized: yes means something is keeping it right. With a network, that something is the distribution's time service, and there is nothing else to do.

With no network: a GPS keeps the clock

The station profile installs gpsd, which reads a GPS receiver and shares it with every program. Handing its time to the system clock is the job of the machine's time daemon, and which one you have decides the route. Only one can be installed at a time: chrony, ntpsec and systemd-timesyncd each declare themselves the machine's time-daemon and conflict with the other two, so installing one removes the other (measured: Time daemons).

dpkg -l chrony ntpsec systemd-timesyncd 2>/dev/null | grep '^ii'

You have	Usually on	The route
ntpsec	Parrot (its security edition pulls it in), the field laptop	Hammunition sets ntpsec up to follow the GPS itself: see GPS time (D-058). Do not install chrony here: it would remove ntpsec, and Hammunition refuses to.
systemd-timesyncd	Debian 13, Ubuntu 24.04, Linux Mint 22.3; Kali, whose kali-linux-core pulls it in (a Kali whose kali-linux-default came first may have ntpsec)	It cannot read a GPS. Replace it with chrony, below.
chrony	Ubuntu 26.04	Already the right daemon. Install the unit below; it adds the GPS and changes nothing else.

The steps below are for a machine with systemd-timesyncd or chrony.

1. Plug in the GPS and confirm gpsd sees it

cgps

Wait for a fix: the screen shows latitude, longitude and a time. A receiver indoors can take minutes. gpsmon shows the raw sentences if nothing appears. The GPS receiver page covers which receivers work and how they appear.

2. Install the chrony unit

On a machine with systemd-timesyncd, remove it yourself first. Hammunition never removes a package you did not ask it to (D-022); if you skip this step, the install refuses and prints the same command.

sudo apt-get remove systemd-timesyncd     # only where it is installed
hammunition install chrony --dry-run      # read what it will do
hammunition install chrony
sudo reboot

The chrony unit installs chrony and writes two files:

• /etc/chrony/conf.d/hammunition-gps.conf, one line: refclock SHM 0 refid GPS poll 2 delay 0.2, the time gpsd publishes for the first receiver it opens. Every target's chrony.conf reads that directory.
• /etc/systemd/system/gpsd.service.d/hammunition-gps.conf, which starts gpsd with -n. Without it gpsd reads the receiver only while a program is connected, and chrony gets no time at all (measured: no samples without -n, one a second with it).

Reboot rather than restart gpsd: restarting a running gpsd in place, tried in a container, left two gpsd processes and no time. After a reboot gpsd starts with -n from the beginning.

chrony keeps using the network's time servers whenever there is a network, so nothing is lost. The GPS line tells chrony its time is good to about a tenth of a second (delay 0.2), which is what NMEA over USB gives and plenty for FT8.

3. Optional: calibrate the offset

NMEA sentences arrive a little after the second they describe, by an amount that depends on the receiver. With the network up, after a few minutes:

chronyc -n sourcestats

The GPS line's Offset column is how far the GPS sits from the network-disciplined clock. To take it out, add offset with that value in seconds to the line in /etc/chrony/conf.d/hammunition-gps.conf (for example refclock SHM 0 refid GPS poll 2 delay 0.2 offset 0.12) and sudo systemctl restart chrony. How large it is on any real receiver has not been measured yet.

A receiver with a PPS output wired to the computer is accurate to microseconds instead; chrony's own configuration examples and gpsd's time service HOWTO cover it. Most USB pucks do not provide PPS, and FT8 does not need it.

4. Check it with the network off

Disconnect from the network, wait a minute, then:

chronyc -n sources

The line starting #* is the source in use. With the network off it should be GPS.

Undoing it

Uninstalling the unit leaves the two files, so remove them by hand:

sudo rm /etc/chrony/conf.d/hammunition-gps.conf \
        /etc/systemd/system/gpsd.service.d/hammunition-gps.conf
sudo systemctl daemon-reload
sudo apt install systemd-timesyncd        # if you want it back; removes chrony
sudo reboot

Position

Your position is yours to share or not.

• Your grid square is what the digital modes send. Set it once with hammunition station set --grid-square FN31pr (your own, not this placeholder); Your callsign in each program lists where each program wants it.
• Moving position comes from gpsd. Pat can post a position report from it (pat-winlink position), APRS clients read it directly, and Offline navigation shows it on a map.
• gpsd is local only. It listens on localhost:2947; nothing on the network can read your position from it unless you change that.

What was measured

Which time daemon each target offers and pulls in, that the three conflict, and the chrony and gpsd behaviour above were measured in containers of all seven targets on 2026-09-29 and 2026-09-30; the tables and commands are in Time daemons, with what was not measured. The gpsd-to-chrony path was run with a script standing in for the receiver. It has not yet been run with a receiver on the field laptop, which runs ntpsec and so takes the D-058 route; the GNSS module planned for it is the bench for that.