Roughly, they used gps signals to synchronize two accurate clocks, one each at the source and the destination. For what it's worth, this part of the first pape

Roughly, they used gps signals to synchronize two accurate clocks, one each at the source and the destination. For what it's worth, this part of the first paper is pretty accessible - at least, as to how they did it - it's more difficult to poke holes in how accurate it was :)