Impact of the transponder configuration on the Asia-Europe TWSTFT network

Two-way satellite time and frequency transfer between Asia and Europe has been performed for many years first by a two-transponder and later by a one-transponder configuration on the IS-4 and the AM-2 satellites, respectively. A diurnal phase instability was observed in links established through IS-4, while such instability has not been observed when AM-2 was used. It could be shown that the diurnal variation are site-independent. An on-board path asymmetry due to the use of two transponders seems thus to cause it.