How can a call hop from a cell tower to a satellite?

The same way it moves from one tower to the next. Nothing about the phone changes.

1. Measure

   A phone on a call keeps measuring every cell it can hear: how strong each one arrives, in dBm.

2. Report

   It reports those numbers to the network. A satellite cell is simply one more line in the list.

3. Switch

   When a neighbour is 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.

Past the last tower, the strongest cell in the list is the satellite, so the same rule hands the call to space. In Tower-to-Satellite Lab, with a 3 dB margin, Tower A hands you to Tower B about 5 km out of town, and Tower B hands you to the satellite at about 23 km, where the satellite’s −112 dBm is 3 dB stronger than the fading tower.

Why does the phone accept a satellite at all? AST SpaceMobile’s BlueBirds broadcast on their partners’ own low-band spectrum, the 850 MHz band for AT&T, with the same LTE signal a tower sends. In AST’s film of the test, the phone’s debug screen reads “LTE(SAT)” on band 5, which is 850 MHz. The network does the hard part out of sight: it corrects the satellite’s extra delay and Doppler shift so the phone sees a normal cell, UT Austin’s Todd Humphreys explains.

Because it is so far away. A tower might be 5 km from you; AST’s first BlueBirds fly about 520 km up. A signal spreads out as it travels, so its power per square metre falls with the square of distance. A hundred times the distance spreads the same signal over ten thousand times the area.

The satellite makes up for it with size. Its first five BlueBirds carry arrays of 693 square feet, about 64 m², Space.com reported at their launch in 2024. AST says BlueBird 6 unfolded one of about 2,400 square feet, roughly 223 m², nearly a doubles tennis court’s 261 m². A bigger antenna gathers more of a faint signal and focuses its own.

Speed is the other problem. At about 7.6 km/s the satellite shifts its signal’s 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 instead of a few hundredths of a millisecond from a nearby tower. The network corrects both, and steers the satellite’s beams onto fixed cells on the ground as it passes.

Very likely, because your phone has to shout. A 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 everywhere, and under 2.2% even for the busiest 5% of rural phones.

To reach a satellite, it runs at or near the maximum: the debug screen in AST’s film of the test shows 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. No carrier has published how much battery a satellite call costs, so the tool shows the transmit power, not a battery percentage. Airplane Mode shows the same climb to full power as towers fall out of range.

On 21 October 2026, outside Granite Falls, Washington, AT&T says it moved a phone’s connection from its ground network to AST SpaceMobile’s BlueBird satellites, which the companies call the industry’s first handoff between a cellular network and a direct-to-device satellite network. Ericsson supplied radio and core equipment and Nokia the radio and mobility software for the satellite side, and the phones were standard and unmodified, AT&T says. Ericsson says its automation cuts transition times from seconds to milliseconds.

The release does not say which satellite was used, whether a voice call was carried, or how long the switch took. The call in our film and tool is the model’s, not a replay of the test.

1. 1 See

   Open Tower-to-Satellite Lab and tap Downtown. Strongest tower reads −68 dBm from Tower A and Serving cell reads Tower A. The satellite is already in the list at −112 dBm.

2. 2 Change

   Tap Edge of coverage. Tower B has faded to −112 dBm, level with the satellite, and still serves you: the network waits for a 3 dB lead. Now tap Deep wilderness. The tower is down to −123 dBm and Serving cell reads Satellite.

3. 3 Understand

   Tap Satellite low in the sky. The satellite drops to −120 dBm, 29° up, the weakest usable signal, and Satellite delay · Doppler reads 3.2 ms and +17 kHz. Raise Handover margin to see the network hold on to a fading tower longer.

Every signal level in Tower-to-Satellite Lab is illustrative. Tower signals fade with the Okumura–Hata distance slope from a level we chose for open country. The satellite’s power per beam is not public, so it is set to −112 dBm straight overhead, close to the −109 to −112 dBm on the test phone’s screen. Real signals swing by 10 to 20 dB with terrain, trees, buildings and your body.

The model has one satellite on one pass straight overhead, usable for about four minutes, and no Earth rotation. A real network flies many satellites and hands calls between them. It also uses timers, thresholds and priorities, not just a margin, and the carriers have not said how they tune the satellite layer. The uplink, from your phone up to the satellite, is usually the harder direction; the tool shows its transmit power but does not model the link.

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, satellite, signals and numbers come from the model.