How does direct-to-phone satellite messaging work?

A satellite text crosses up to three kinds of link. The 3GPP, the body that writes mobile network standards, calls the hop between the phone and the satellite the service link, and the hop between the satellite and a gateway on the ground the feeder link. Some systems also pass traffic between satellites over inter-satellite links before it comes down.

What the satellite does with the signal depends on its payload:

• Transparent, or bent pipe. The satellite has no on-board processing. It changes the carrier frequency, filters and amplifies the signal, and sends it back down, acting like an analogue radio repeater. The base station sits on the ground at the gateway.
• Regenerative. The satellite demodulates, decodes and routes on board, so it carries the functions of a base station itself.

Once the message reaches the gateway it is in an ordinary ground network and travels on like any other text. Apple’s service is a concrete case: its satellite network is provided by Globalstar and its affiliates or other network providers, and Emergency SOS texts go either to emergency service providers or to an Emergency Relay Center that passes the content on.

The distances are the reason. The 3GPP’s overview gives typical orbits:

At those heights the signal takes longer to travel. The overview puts propagation delay on terrestrial networks at usually under 1 ms and on geostationary links at up to a few hundred milliseconds. Distance also weakens the link, especially the uplink, because a handheld phone can only transmit so much power. Satellites in low and medium orbits move fast relative to the ground, which shifts the frequency (Doppler shift) and makes coverage cells move.

Apple’s support page turns this into practical advice. Satellites “move rapidly, have low bandwidth, and are located hundreds of miles away from Earth”. In ideal conditions with a direct view of the sky and horizon, a message might take 30 seconds or more; under light or medium foliage, at higher latitudes or in poor local conditions it can take several minutes; under heavy foliage or other obstructions the phone might not connect at all. Apple also says the service might not work above 62° latitude. The iPhone guides the user to turn left or right to keep the link.

Google describes a different geometry for Pixel: the phone connects to satellites in fixed positions above the earth, and the user faces the right direction and tilts the phone to point at one.

Apple. Emergency SOS via satellite works on iPhone 14 and later when a call or text to emergency services cannot connect and there is no cellular or Wi-Fi coverage. The user answers a short questionnaire, and the iPhone shares those answers with responders along with location, including elevation, remaining battery and, if set up, Medical ID. Messages via satellite, from iOS 18, sends iMessage and SMS texts to friends and family, in the United States, Canada, Mexico and Japan at the time of writing. Apple’s page lists what it leaves out: photos, videos, audio messages, stickers and group messages. SMS via satellite depends on the carrier, and Apple notes that carriers may also provide their own satellite features.

Google Pixel. Satellite SOS is available on Pixel 9 and later, except Pixel 9a. The user dials the local emergency number, fills in a questionnaire, and follows prompts to aim the phone. Google says the emergency service provider should reply by text within a few minutes, and the phone needs Google Messages as its default messaging app.

Android as a platform. Android 15 and higher has platform support for satellite connectivity, which Android also calls non-terrestrial networks. The Android Open Source Project describes the services the platform routes over these networks: contacting emergency services, SMS, MMS and RCS messaging, location sharing, and IP data, with background data on constrained networks limited to core services and allowlisted apps. Apps do not use constrained satellite networks by default; they opt in through their manifest.

Apple’s and Google’s features are services each phone maker runs over a satellite network. Separately, the 3GPP has brought satellites into the mobile standards as non-terrestrial networks (NTN). Release 17, whose ASN.1 was frozen in June 2022, was the first release with normative NTN requirements.

The Release 17 radio work rested on a few assumptions: transparent payloads only, FDD operation in frequency range FR1 (410 MHz to 7,125 MHz) with two new bands, n255 in L-band and n256 in S-band, support for handheld devices of power class 3, and devices that have satellite positioning. That last point does real work. The network broadcasts each satellite’s orbit data (its ephemeris) and timing parameters, and the phone uses its own GNSS fix and the satellite’s position and speed to correct its own timing and Doppler shift before it transmits. A phone without a valid position or valid ephemeris does not talk to the network until it has both again. Release 17 also covered LTE-based satellite access for IoT devices, using NB-IoT and eMTC.

A satellite link is a narrow, slow path to the outside world, not a substitute for a normal connection. Android’s guidance for constrained satellite networks says they work best when apps use the network in bursts and stay idle the rest of the time, that text messaging is the reliable case, and that high-bandwidth work such as uploading uncompressed HD video may stall or fail.

For an app, that points to small payloads, a queue that holds messages until a link appears, and clear status for the user while a message waits. How does store-and-forward messaging work? covers that queueing pattern. Satellite reaches people far away from places with no towers. For people close to each other, local radio links such as Bluetooth and Wi-Fi do not depend on a view of the sky.