Direct light propagation delay To measure a distance to 1 mm requires timing accuracy of 3x10-12 seconds(3 picoseconds) Timing accuracy needs to be maintained over the flight time. For satellite at 1000km distance, this is 3 millisecond Clock stability needed 3ps/3ms=10 A clock with this longtime stability would gain or lose 0.03 seconds in a year(10-9*86400"365) (Clock short term and long term stabilities are usually very different --Characterized by Allan Variance) 02/25/02 12.540Lec0602/25/02 12.540 Lec 06 5 Direct light propagation delay • To measure a distance to 1 mm requires timing accuracy of 3x10-12 seconds (3 picoseconds) • Timing accuracy needs to be maintained over the “flight time”. For satellite at 1000km distance, this is 3 millisecond. • Clock stability needed 3ps/3ms = 10-9 • A clock with this longtime stability would gain or lose 0.03 seconds in a year (10-9*86400*365) • (Clock short term and long term stabilities are usually very different -- Characterized by Allan Variance)