Impact of atmospheric non-reciprocity on satellite two-way time transfer

In the two-way time transfer method signals are exchanged back and forth, via a satellite transponder, between the master and slave stations. Because of the two-way signal exchanges in path delay cancels out in the relevant calculations, leaving only the difference in the clock readings at the master and slave stations. However, the cancelling of path delay is critically dependent on the assumption that the signal path delay from master to satellite to slave is equal to the delay from slave to satellite to master-that is, the path are reciprocal. In actual practice, the uplink signal frequencies from both master and slave to the satellite differ from the downlink frequencies from the satellite to the master and slave stations, so that the reciprocity assumption does not hold over the ionospheric portion of the path. Thus one must make calculations to see if the lack of reciprocity is significant in relation to the degree of timing accuracy required. It is possible, under some circumstances, to measure the degree of nonreciprocity directly because the signals relayed by the satellite are generally available at both the slave and master stations simultaneously