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The [signal] delay between a transmitter and a receiver. Path delay is often the largest contributor to time transfer uncertainty. For example, consider a radio signal broadcast over a 1000 km path. Since radio signals travel at the speed of light (with a delay of about 3.3 µs/km), we can calibrate the 1000 km path by estimating the path delay as 3.3 ms and applying a 3.3 ms correction to our measurement. Sophisticated time transfer systems, such as GPS, automatically correct for path delay. The absolute path delay is not important to frequency transfer systems because on-time pulses are not required, but variations in path delay still limit the frequency uncertainty.
NISTIR 8323 from NIST T&F Glossary - Adapted