How can your phone hop from a cell tower to a satellite mid-call?

On 21 October 2026, outside Granite Falls, Washington, AT&T says it moved a phone’s connection from its ground network to one of AST SpaceMobile’s BlueBird satellites. The two companies call it the industry’s first handoff between a cellular network and a direct-to-device satellite network, and the phone was an ordinary one. So how can a call hop from a cell tower to a satellite?

▶ Try it yourself in Tower-to-Satellite Lab: walk the phone out of range, move the satellite across the sky and change the handover margin, and watch which cell serves your call.

Just another tower

The phone isn’t doing anything special. A BlueBird sends the same kind of LTE signal a tower does, on the carrier’s own spectrum: the AT&T and Verizon leases cover the 850 MHz band. To the phone, the satellite is one more cell in its list. In AST’s film of the test, the phone’s debug screen reads “LTE(SAT)” on band 5, which is 850 MHz.

The handover rule

A phone on a call keeps measuring every cell it can hear and reports back. When a neighbour becomes stronger than the serving cell by a set margin, the network tells the phone to switch; in the 3GPP standards that report is event A3. It is how your call moves from tower to tower on a drive. Past the last tower, the strongest cell in the list is the satellite, so the same rule hands the call to space. In the tool, with a 3 dB margin, that happens about 23 km out of town, where the satellite’s −112 dBm beats the fading tower by 3 dB.

The hard part is up there

A tower might be 5 km away; AST’s first BlueBirds fly about 520 km up. A hundred times the distance spreads the same signal over ten thousand times the area. AST answers with size: its first BlueBirds carry 64 m² arrays, and it says BlueBird 6 unfolded one of about 2,400 square feet, roughly 223 m², nearly a doubles tennis court. The satellite also moves at about 7.6 km/s, which shifts its frequency by up to about 18 kHz while it is usable in the model, more than the 15 kHz between LTE subcarriers, and the signal takes 1.7 to 3.4 ms each way. The network corrects both so the phone sees a normal cell.

And yes, it costs battery

Your phone sets its own transmit power from how weak the cell sounds, up to 23 dBm, 0.2 W, for an ordinary LTE phone. Near towers it rarely needs much: in Ericsson’s measurements of about 7,000 phones on Swedish LTE networks, average output stayed under 1% of the maximum. To reach a satellite it runs at or near full power; AST’s test phone showed 23 to 24 dBm. Transmitting harder drains the battery faster: in a measured LTE phone, the transmitter drew about 100 mW more for every extra decibel above roughly 16 dBm.

The tool’s signal levels are illustrative: towers fade with the Okumura–Hata distance slope, one satellite passes straight overhead, and real networks add timers, thresholds and many satellites. The film uses two short pieces of AST SpaceMobile’s own footage, credited on screen: the test phone’s debug screen (@AST_SpaceMobile on X) and the array of BlueWalker 3, its 64 m² test satellite (AST SpaceMobile on YouTube). The towers, the satellite, the signals and the numbers are drawn from the model.

Sources: AT&T, AT&T and AST SpaceMobile: engineering the connected world, one handoff at a time (2 Nov 2026); AST SpaceMobile via Business Wire, BlueBird 6 unfolding (10 Feb 2026); UT Austin Radionavigation Laboratory, AST SpaceMobile in cellphone satellite communication; P. Joshi et al., Output power levels of 4G user equipment (IEEE Access, 2017); M. Lauridsen et al., An empirical LTE smartphone power model (Intel Technology Journal, 2014).