Passive TWSTFT: a cost-effective alternative for robust UTC(k) dissemination

Abstract Two-Way Satellite Time and Frequency Transfer (TWSTFT) is among the most accurate techniques for comparing clocks over (inter)continental baselines, exchanging bidirectional signals through a geostationary satellite —but the cost and complexity of an active (Tx & Rx) terminal restrict it to a handful of UTC(k) laboratories. We show that a passive, receive-only user can reach those same laboratories by reducing a one-way pseudorange for the satellite-motion, atmospheric, and equipment delays. Using active-network observations over 130 days, the satellite ephemeris is reconstructed by orbit determination. A portable terminal —a commercial TV dish and a software-defined-radio receiver— was calibrated in common-clock mode at INRiM and deployed to the Côte d’Azur Observatory (OCA). Over a 40-day campaign, time transfer to three UTC(k) laboratories agreed with independent GNSS and active-TWSTFT links at the nanosecond level; despite larger short-term noise from solar-radiation-pressure and wet-tropospheric mismodelling, the overlapping Allan deviation reaches ≈10^{−14} at one day, matching GNSS performance. The accuracy is bounded by an ≈ 7 ns (1σ) budget, and an INRiM–OCA–INRiM closure sequence confirmed robustness to relocation and traceability to UTC(k). Passive TWSTFT thus offers a scalable, interference-resilient, and low-cost route to UTC(k) dissemination, with future work targeting automated orbit determination for quasi-real-time operation.