A Variegated Look at Direct-to-Cell Satellites in the Wild

Direct-to-cell satellites aim to deliver ubiquitous cellular network connectivity for our regular phones from space. Starlink, as the most successful direct-to-cell satellite operator, has already provided messaging and data services to unmodified smartphones across multiple countries, proving valuable in emergency communications, disaster response, and outdoor activities. However, how do Starlink direct-to-cell satellites behave in the wild? This work presents the first in-depth cellular measurement study of Starlink's direct-to-cell satellites by collecting a two-month full-stack dataset. Our analysis reveals that Starlink strives to mitigate the satellite's high latency and extreme mobility through infrastructure-side mechanisms, including reasonable function split, Doppler and delay offset compensation, and rapid cell switching strategies. However, its backward-compatibility requirements prevent cellular protocol and signaling modifications, yielding unresolved issues including frequent access failures, ping-pong handovers, and extensive retransmissions. These findings expose fundamental limitations in adapting legacy terrestrial 5G/4G to space. In the long term, these issues cannot be resolved simply by allocating more spectrum resources or deploying more satellites. We further conduct data-driven emulation using 3GPP non-terrestrial network (NTN) protocol stacks and satellite channel simulators to characterize how these issues could be resolved through protocol modifications implemented in 5G NTN and beyond.