Fundamentals of two-way time transfers by satellite

The principles of two-way time transfer are discussed with emphasis on its use with commercial communications satellites. The limitations imposed by the atmosphere, the equipment, and the rotating, noninertial reference system are discussed on the assumption of reciprocal paths. The advantages include: (1) operation is on a leased space segment rather than on specialized space hardware; (2) satellite location is required only to point antennas and not to compute distances; (3) effects on accuracy of time transfer by the ionosphere and troposphere are subnanosecond without modeling; (4) locations of the clocks in the ground do not need to be known better than as provided by a geodetic map-precise positions are not required; (5) simple averages of 100 1-s measurements yield about 300-ps resolution-no elaborate data analysis is required; and (6) equipment delays are easily calibrated through the use of a standard earth station acting as a transfer standard.<>