Chrony with GPS fallback; robust antenna supply; all GNSS constellations

Time: chrony replaces timesyncd, with NICT/JP-pool NTP primary and gpsd
SHM as an offline-only refclock; makestep any time and fake-hwclock
cover the battery-less RTC.

Antenna: the modem cleared the supply again after the service set it,
so the service is now idempotent, configurable (ANTENNA=active|passive,
passive keeps it off) and re-checked by a 30 s timer.

gpsd: a gpsdctl udev hook spawned a stray gpsd without -n at boot that
blocked the real service; gpsd.service now binds to the NMEA device.

GNSS: CGNSSMODE=15 (GLONASS, BeiDou, Galileo, QZSS) and all supported
NMEA sentences; both persist across a modem reset. Adds sim7600-at.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/impl/reports/TIME-SYNC.md

@@ -8,10 +8,11 @@ date: 2026-10-08
 
 | Item | State |
 |------|-------|
-| Network time | `systemd-timesyncd`, primary `ntp.nict.jp` (stratum 1), synced over whichever uplink holds the default route (Ethernet / WiFi / LTE) |
+| Network time | **chrony** (since 2026-10-09; was `systemd-timesyncd`), primary `ntp.nict.jp` (stratum 1) + JP pool + Ubuntu pools, over whichever uplink holds the default route |
 | Timezone | `Asia/Tokyo` (was `Asia/Yakutsk`: same +09:00 and no DST either, but the wrong place) |
 | Clock across power-off | **Not kept.** The RTC has no backup battery (see below) |
-| GPS time | Not yet. Waiting on a GNSS fix (see [SIM7600 report](SIM7600-SETUP.md)) |
+| GPS time | chrony refclock `GPS` (gpsd SHM 0), fallback only; used when offline with a fix |
+| Boot clock without network | `fake-hwclock` restores the last saved time (replaces timesyncd's saved timestamp) |
 
 ## RTC loses time when powered off
 
@@ -41,11 +42,33 @@ FallbackNTP=ntp.ubuntu.com 0.jp.pool.ntp.org 1.jp.pool.ntp.org 2.jp.pool.ntp.org
 
 `sudo systemctl restart systemd-timesyncd`, then check with `timedatectl timesync-status` (server, stratum, offset).
 
-## Later: GPS as a time source
+## Chrony with GPS fallback (2026-10-09)
 
-timesyncd only speaks NTP. Once GNSS has a fix, replace it with `chrony`, configured with:
+Replaced `systemd-timesyncd` (removed by the `chrony` package) so GPS can serve as an offline time source.
 
-- the same NTP servers as above, which remain the default whenever any uplink exists, and
-- a gpsd refclock (`refclock SHM 0`), which takes over when offline.
+```
+sudo apt install chrony fake-hwclock
+```
 
-NMEA over the USB serial port carries no PPS signal, so GPS time alone is only good to tens of milliseconds. That is plenty for log timestamps.
+`/etc/chrony/conf.d/10-yukart.conf`:
+
+```
+server ntp.nict.jp iburst prefer
+pool 2.jp.pool.ntp.org iburst maxsources 2
+refclock SHM 0 refid GPS precision 1e-1 offset 0.0 delay 0.5 poll 3
+```
+
+- **NTP stays primary.** The GPS refclock's generous `precision`/`delay` give it a large root distance, so any reachable NTP server wins.
+  It steers the clock only offline with a fix. NMEA over USB has no PPS, so GPS time is good to ~tens of ms, plenty for log timestamps.
+- **`offset 0.0` is a placeholder.** Tune it once NTP and a GNSS fix are both available: `chronyc sourcestats` shows the GPS offset
+  against NTP.
+- **`makestep 1 -1`** in `/etc/chrony/chrony.conf` (was `1 3`): step the clock whenever it's > 1 s off, not only in the first three
+  updates, because with no RTC battery a stale clock may only meet its first source (a late GPS fix) well into an offline drive.
+  chrony's config format has no inline comments; the explanation sits on its own line above.
+- **`fake-hwclock`** saves the time hourly and at shutdown and restores it at boot, so an offline boot starts near the last shutdown
+  instead of the RTC's reset date. This replaces what timesyncd's `/var/lib/systemd/timesync/clock` did.
+- gpsd must run with **`-n`** to feed SHM without a client. See the SIM7600 report: a stray gpsd started by a `gpsdctl` udev hook ran
+  without it, now fixed.
+
+Check: `chronyc -n sources` (`^*` = selected NTP server, `#` = GPS refclock), `chronyc tracking`.
+At setup: synced to `ntp-a3.nict.go.jp` (stratum 1); `GPS` listed with no samples (antenna detached indoors).