SpaceX Sends First Text Messages Via Its Newly Launched Direct to Cell Satellites
On Monday, January 8, the Starlink team successfully sent and received its first text messages using T-Mobile network spectrum through one of its new Direct to Cell satellites launched six days prior. Connecting cell phones to satellites presents several major challenges to overcome. For example, in terrestrial networks cell towers are stationary, but in a satellite network they move at tens of thousands of miles per hour relative to users on Earth. This requires seamless handoffs between satellites and accommodations for factors like Doppler shift and timing delays that challenge phone to space communications. Cell phones are also incredibly difficult to connect to satellites hundreds of kilometers away given a mobile phone's low antenna gain and transmit power.
Starlink satellites with the Direct to Cell payload are equipped with innovative new custom silicon, phased array antennas, and advanced software algorithms that overcome these challenges and provide standard LTE service to cell phones on the ground. As the global leader in rocket and satellite launch and manufacturing, SpaceX is uniquely positioned to rapidly scale its Direct to Cell network and will rapidly launch a constellation of hundreds of satellites to enable text service in 2024 and voice, data, and Internet of Things (IoT) services in 2025.
On January 2, 2024, SpaceX launched to orbit its first six Starlink satellites with Direct to Cell capabilities. Launch and early tests of the technology were all completed without issue. On Monday, January 8, less than 6 days after launch, text messages were sent and received to and from unmodified cell phones on the ground to these new satellites in space using T-Mobile network spectrum. This validates that the link budget closes, and the system works!
Image description: The top six Direct to Cell Satellites stacked and ready for launch.
The Direct to Cell network will expand Starlink's vision by providing ubiquitous connectivity and seamless access to text, voice, and data for LTE phones and devices across the globe. Text service begins this year, followed by voice, data, and Internet of Things (IoT) services in 2025.
Developing a satellite network that connects to unmodified cell phones presents several new technical and regulatory challenges beyond those Starlink has already overcome in deploying a 5,000+ satellite constellation in a few years, serving more than 2.3 million customers across the globe.
The first challenge is transmitting sufficiently strong radio signals to and from cell phones that were not designed to connect to satellites, with very low gain antennas and transmit power (max 0.2 Watts).
Technology and System Architecture
SpaceX's team developed custom silicon onboard the satellite that is optimized for this application and reduces power and cost on the satellite. They also developed large 2.7 m x 2.3 m advanced phased arrays that use extremely sensitive radio receivers and high-powered transmitters for communicating with cell phones from space. The antennas were designed to ride on the Starlink v2mini satellites and are reliably launched and deployed to low-Earth orbit every few days by the Falcon 9 rocket. SpaceX is uniquely positioned to rapidly scale out its first Direct to Cell constellation in mere months, due to its unique and unprecedented vertical integration controlling both launch and satellite production in addition to operations. In the future, Direct to Cell satellites will launch on Starship to improve the service even further and increase launch cadence.
The vision is to connect everyone using their existing phones without needing a new phone, upgrades, or a special app, utilizing standard LTE/4G protocols. Starlink satellites orbit hundreds of kilometers above the Earth's surface at speeds of 7.7 km/s. As a result, compatibility with LTE timing, Doppler, and latency constraints are extremely challenging. To perform like a true cell tower in space, handoffs between satellites and on the ground must be completely seamless to the user. To accomplish this, the system was architected including satellite altitudes, beam size and placement, elevation angles, and number of satellites, operating at the edge of physics where LTE is achievable and reliable. An advanced LTE modem onboard each satellite operates like a cell tower in space.
Diagram description: Direct to Cell satellites function as cell towers in space, routing traffic to partner operator's cores via Starlink's integrated space and ground systems.
Partnerships and Global Rollout
The Direct to Cell network leverages the infrastructure built for Starlink over the past several years. Direct to Cell satellites integrate into the existing Starlink satellite constellation via laser backhaul, enabling services anywhere with regulatory approvals without requiring dedicated ground infrastructure. The system also utilizes existing Starlink networking, ground stations, and Points of Presence (PoPs). Direct to Cell data travels over Starlink's core directly to the operator's core, providing seamless integration.
In August 2022, the first partnership was announced with T-Mobile. Since then, deals have been announced with Rogers in Canada, Optus in Australia, One New Zealand, KDDI in Japan, Salt in Switzerland, and Entel in Chile & Peru. These operators provide critical LTE spectrum in the 1.6-2.7 GHz range used for transmitting satellite signals. This allows Starlink to integrate like a standard roaming partner with operators, enabling direct and seamless service provision to their customers. Operators in the network gain reciprocal global access, allowing their users to access the service when traveling to partner countries.
There is incredible demand and high interest in this program, with handset providers and mobile operators alike eager to test and participate in a successful rollout. SpaceX worked closely with the Federal Communications Commission (FCC) to obtain initial authority to launch and test satellites in record time and will continue to collaborate with them to find innovative ways to connect people and save lives in emergencies. SpaceX is also working closely with regulators worldwide to bring this service to their countries as soon as possible.
This year, SpaceX looks forward to expanding testing to include greater coverage, launching hundreds of satellites to enable the text constellation, working toward the voice, data, and IoT constellation in 2025, and expanding its global footprint. Interested individuals can contact SpaceX here or look for opportunities to join the team here.
