Time and frequency requirements for geodetic space techniques

Precise geodetic space techniques as Very Long Baseline Interferometry (VLBI), Laser Ranging to artificial Satellites or to the Moon (SLR/LLR) and the geodetic application of Global Navigation Satellite Systems as GPS, GLONASS and in the future GALILEO are based -in general- on measurements of time and time intervals. The epochs of the global observations are referenced to the timescale UTC. The offsets to UTC have to be known in the domain of some nanoseconds. The observations require absolute frequencies which are generated by precise frequency standards. To avoid scaling errors the frequencies offsets should be less than 10^-12. Moreover e.g. VLBI demands short term frequency stability which active H-Masers are capable to provide today.