There are some applications where it becomes necessary, at the distant end of a satellite link, to provide a high-stability G.703 timing reference for timing equipment connected to the modem. For example, in cellular backhaul applications, the BTS equipment may require such a reference even though the satellite link itself may be operating at a data rate other than 1.544 Mbps or 2.048 Mbps. This is sometimes accomplished by adding a specialized GPS receiver at the distant end, which then provides the G.703 synchronizing signal. However, with the G.703 clock extension mode this may become unnecessary, as our modems with this feature – operating at either end of the link, where the local modem has access to a high-stability G.703 signal – can provide an almost perfect copy of this signal at the distant end. The presence of Doppler shift on the link is the only factor affecting the overall accuracy. If Doppler shift were not present, the copy of the clock would be perfect.