Concept for an accurate calibration of long baseline two-way satellite time and frequency transfer (TWSTFT) links via two separated transponders on one telecommunication satellite

Two-way satellite time and frequency transfer (TWSTFT) is the technique which allows time scale comparisons at the nanosecond level. Until now, this uncertainty level is restricted to those institutes which both can exchange signals through the same single transponder of one telecommunication satellite. Only in this case the calibration scheme of circulating a portable TWSTFT reference station is applicable. This condition is not fulfilled in the operational intercontinental links, e.g. between Asia and Europe. Calibration of the TWSTFT link is in such cases achieved by adjusting the TWSTFT results to those obtained with a calibrated GPS link. The uncertainty is then limited by the uncertainty of the GPS link, which is commonly estimated to be at the level mu_B = 5 ns. We propose an experimental configuration which allows determination of all important delay differences in a long baseline link including the differential delays between the operational ground stations as well as the differential transponder delay between the two separated transponders used on the satellite. The latter has never been determined on a flying satellite. We propose to exchange two portable TWSTFT stations and two GPS receivers between the two ground stations to be calibrated. We estimate the overall uncertainty for the calibration to be at a level of 2 ns. We also discuss briefly the ability of calibrating a TWSTFT link as a whole by means of GPS on the same level of uncertainty.