These are incredibly cheap these days!  There's little excuse not
to run a timing system to open up a whole new realm of weak signal
operation, LF and VLF.  There's a big green field out there.

I use a ublox LEA-6 which is quite an old one now probably.
I get less than 100nS timing jitter on recorded signals, and
usually it is 50nS or less.

There's lots I like about the ublox GPSs, they work well for
a start, and their documentation on their 'UBX protocol' for
serial/USB commands is excellent.

Whatever you buy, make sure it has a 1PPS output (and nice
if its frequency is programmable), and it talks either NMEA
or UBX protocol (probably they all do both) via RS232 serial
or USB serial (ACM or FTDI).

--
Paul Nicholson
--