The present invention relates to a method and a device for time synchronization of an RBS which has lost its GPS signal. The method in the time synchronization device comprises retrieving from the first radio base station a first timing advance value (710) used by the user equipment to adjust its transmission timing before the handover, and a measurement of a reception timing (720) of a random access preamble. The random access preamble is transmitted from the user equipment to the second radio base station during synchronization. The method also comprises retrieving (730) from the second radio base station a second timing advance value used by the user equipment to adjust its transmission timing after the handover, and determining (740) a time synchronization offset between the first and second radio base station based on the retrieved first timing advance value, the second timing advance value, and the measurement of the reception timing.