Abstract: A differential GPS system for precise time and/or location tagging of an event. Reference and rover units cooperate to compensate for delayed availability at the rover unit of error-correction data transmitted via a two-way, high-latency communication link. A reference unit calculates and stores error-correction data. A rover unit collects and stores uncorrected GPS fix data, and transmits a message demanding error-correction data. The message preferably includes a time stamp indicating the time for which the error-correction data is to be valid. In reply to a demand message from a rover unit, the reference unit retrieves stored error-correction data and transmits it to the rover unit via the high-latency communication link. The reference unit includes a time stamp indicating the valid time of the error correction data if the valid time is not implicit in the reply. The rover unit uses the stored, uncorrected GPS fix data and the received error-correction data to compute a differentially-corrected GPS fix.