I was researching potential calibration sources for our orbiting
receivers (where we need to line up GNSS signals with HF signals) and
after looking at the usual suspects like WWV, we came across another one.
Ionosondes - they're all over the place, and these days, they're fairly
accurately timed (how accurately? I don't know.)
Timing wise, since wide band and oblique sounders are popular, they must
be fairly well controlled, since the transmitter and receiver are not
co-located. A traditional vertical sounder drives the transmitter and
receiver off the same clock, so they don't care so much about what time
it is.
I think these things are designed so they have resolutions in "meters"
or "tens of meters" which implies sub microsecond accuracy at worst.
There's several kinds:
The Oblique/QVI sounder - 100 watts into an omni(-ish) antenna - 2-20
MHz chirp at 100kHz/second, for 180 seconds total sweep. They do the
chirp once every 12 minutes.
Wide Sweep Backscatter Ionogram (WSBI) sounder
20 kW(!) into a 2 element log periodic curtain pointed in the general
direction of an over the horizon radar. 5-28 MHz over 282 seconds, also
at a 12 minute cadence.
They have some of these in Vieques PR, New Kent VA, and Corpus Christi
TX. I would imagine the Australians have some associated with JORN
(their OTH radar network). There are plenty of other sounders around, too.
There's a USRP implementation of a receiver for various sounders from
Juha Vierinen
http://www.radio-science.net/2019/04/oblique-ionograms-between-sodankyla-and.html
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