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This paper reviews key characteristics and requirements for Two-Way Satellite Time and Frequency Transfer (TWSTFT) links. The outline and perspective of two concepts are described: 1. The classical bend-pipe transponder, without special equipment on board, with the potential of 10-17/day frequency transfer uncertainty; 2. The ACES-MWL (Atomic Clock Ensemble in Space Microwave Link) like active on-board equipment, with the potential down to few 10-19/day, using both high-rate modulation and carrier-phase. An outlook to advanced links is given. The aim of these proposed links is to provide a unique platform for the demanding requirements in the field of time and frequency metrology and to support improved orbit determination and relativistic geodesy for the next decades.
Schäfer et al. (Wed,) studied this question.