GPSd & NTP Time Synchronization

Accurate system time is critical for allsky camera operation: solar, lunar, and planetary ephemeris calculations, coordinate transformations, and satellite pass tracking depend upon precise UTC timestamps.

For remote observatories without reliable internet connectivity, a USB or serial GPS/GNSS receiver paired with gpsd and ntpsec provides stratum-1 time synchronization.

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1. Install & Configure gpsd

Ensure your USB GPS receiver (e.g. u-blox, MediaTek) is connected to the host system.

Debian Package (.deb) Installations

sudo indi-allsky-ctl setup-gps

Source / Git (setup.sh) Installations

./misc/setup_gpsd.sh

The script automatically detects the GPS serial port, installs gpsd and diagnostic utilities (gpsd-clients), configures /etc/default/gpsd, and enables the systemd service.

Verify GPS Reception

Test satellite fix and NMEA sentences using cgps or gpsmon:

cgps -s

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2. Configure NTP Time Synchronization (ntpsec)

To sync the Linux kernel system clock directly from GPS shared memory (SHM):

Install ntpsec

sudo apt-get install -y ntpsec

Configure NTP Shared Memory Driver

Edit /etc/ntpsec/ntp.conf:

# Shared Memory driver (unit 0) for GPSD
server 127.127.28.0 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.0 refid GPS stratum 1

# If operating completely offline without internet fallback NTP pools:
tos minclock 1 minsane 1

Restart the time service:

sudo systemctl restart ntpsec

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3. Validation

Query NTP synchronization status with ntpq:

ntpq -p

Expected output format:

     remote           refid      st t when poll reach   delay   offset   jitter
===============================================================================
*xSHM(0)          .GPS.            1 l   14   16  377   0.0000   0.0242   0.8120
+time.cloudflare. 10.0.0.1         3 u   97 1024  377  12.1493   1.4558   0.8710

An asterisk (*) or plus (+) alongside SHM(0) indicates the kernel clock is actively synchronizing with the GPS receiver.