Repeated-Voyage Measurement of Cellular–GEO Satellite Complementarity and Buffering Implications for Maritime IoT Backhaul

Reliable ship-to-shore backhaul is essential for maritime Internet of Things (IoT) data delivery, yet cellular and satellite connectivity varies by route, operating phase, and qualification criterion. This study analyses 606,625 georeferenced monitoring records collected during a nine-month, 12-voyage campaign aboard a container ship operating between Korea and Southeast Asia, with the cellular–geostationary Earth orbit (GEO) satellite analysis limited to the period of active very small aperture terminal (VSAT) service. During sailing, cellular attachment was reported for 70.3% of valid cellular state records, but only 28.0% satisfied the adopted technology-specific received power criteria, with leg-level qualified fractions ranging from 77.1% (Incheon–Busan) to 17.3% (Shanghai–Ho Chi Minh). Within the joint-analysis window, either the cellular criterion was satisfied or a valid GEO probe response was observed in 99.4% of records, although the residual gap reached 3.1% on Laem Chabang–Ho Chi Minh. Under retrospective cellular-first allocation, raising the candidate VSAT signal-to-noise ratio (SNR) threshold from 6 to 7 dB increased the store-and-forward share from 6.7% to 25.7% without reducing the median round-trip time (RTT) of the retained GEO records, and the longest buffered interval grew from 2.02 to 9.86 h. These results show that route segment, operating phase, state definition, and threshold selection materially influence link allocation and buffering implications; live traffic steering and application-level availability were not evaluated.