TIME TRIGGER UNIT
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We designed and built a GPS based time trigger unit that sent Closure (locked to GPS PPS) and NMEA "§GPGGA" telegram every 20. second. It worked satisfactory; on average one missing 20-second block per day (which is acceptable), and no overlaps.
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The design has one side effect which is easily overcome: The timestamp in the NMEA telegram lags 1 second behind true UTC time. When 20-second blocks in SEG2 format is concatenated into, let's say, 1-hour blocks, change the timestamp of the final file. Timestamping will of course depend on choice of final file format - SEGY, or MiniSEED, or something else ....
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