Greetings all:

I have been using the GPS data via an old Garmin OEM receiver for some time
now to trigger accurate time chirps on a LDE experiment I run here.

First you program the receiver to send only this data stream, GPRMC.

Then you look for the date and time of the second before you want to begin
transmitting.  When you see the next "$" (dollar sign character) drop into
the transmit sequence.  The offset, a small number of Milliseconds, will be
a constant offset.

For one or more minute transmission periods the error is trivial.

I have had no problems with the GPS receiver being bothered by high power
RF, nor have I had many periods when the GPS dropped out of sync even though
part of the sky at my transmit location is obscured.

The OEM GPS receivers were quite inexpensive a while back, but I see new GPS
receivers with a data connection now for under $100. US$.

Good Luck

Larry
VA3LK