Guide Clock sources

Guide: clock sources

What it is. Two related things carry the word "clock" in OnAir:

1. Sync computer clock. OnAir can measure how far your computer clock is off against a trustworthy time source received over the air, and, if you ask, correct it. The sources are GPS (the satellites' UTC, read in the GNSS mode) and a time station (DCF77, MSF, WWVB, JJY, WWV, BPM, BPC or HLA, read in the Time signals mode). There is also an optional NTP server that shares the corrected time on your LAN.
2. The radio's reference clock. Some radios can run from an external 10 MHz reference instead of their own oscillator.

Sync computer clock

Nothing changes your clock by itself. Only a click on Sync now does anything, and only for a locked, plausible measurement.

Step by step

1. Choose the source. In the GNSS mode open the Clock view (the views are Sky, Map, Channels, Interference, Clock); in Time signals the clock comparison is part of the mode's panel (check the panel for the exact tab).
2. Select GPS or time station with the two radio buttons. The radio runs one mode at a time: switch to the mode of the source and start the receiver.
3. Wait for lock. For GPS: a fix with the date known. For a time station: until two decoded minutes agree.
4. The panel shows measured offset, for example +12.3 ms ahead (+- 4 ms), coloured green below 50 ms, amber below one second and red above. It also shows the source and, for a time station, which station.
5. To correct the clock press Sync now. The system asks for permission: the administrator password on macOS, a UAC prompt on Windows, a polkit prompt on Linux. The button is disabled with the reason when there is no locked measurement.
6. A Command (run as administrator) is shown with a Copy button, so you can run the same correction yourself. On Windows the button runs the same PowerShell command.

How accurate is it

The offset includes the radio's USB and driver buffering (the earliest arrival over half a minute is used), typically 5 to 30 ms, and for a time station also the path delay (a few ms). It tells you whether the clock is seconds or hundreds of milliseconds off, not whether it is within 1 ms. For hobby use only.

Optional NTP server

Act as NTP server (LAN) serves the computer clock, corrected by the measured offset, to other computers on your network on UDP port 123 (1123 without administrator rights). It is off at every start, answers private (LAN) addresses only, and limits requests per client. Clients must use the port shown (chrony: server HOST port 1123) or you can forward UDP 123. The accuracy is that of a USB radio and an ordinary clock, tens of milliseconds.

The radio's reference clock

Some radios show a Reference clock setting in Radio settings: Fobos SDR, LimeSDR (with a clock input), USRP and radios opened through SoapySDR when the driver lists clock sources. For Fobos the choices are Internal (the default) and External (10 MHz); with no signal on the clock input an external setting makes the radio not run. For the LimeSDR you also enter the external frequency, and it must match. Use an external reference (a GPS-disciplined 10 MHz source) when you need frequency accuracy for narrow modes or time-stable measurements. For converters with a TCXO, see converters.

Troubleshooting

• "none: open the GNSS mode, start the receiver and wait for a fix with the date known": no GPS measurement yet. GNSS needs an active antenna with bias-tee on; see GNSS.
• "none: open the Time signals mode...": nothing locked. Time stations are low-frequency or HF; see Time signals for antenna and band advice.
• Sync now does nothing: the administrator prompt was declined, or the offset is outside the limit the system allows for the one-shot correction.
• Offset keeps changing: it is an average with USB jitter; wait for the value to settle.

Related

GNSS | Time signals | Tutorial: time stations | Privacy