Technical Details

An in-depth look at the technical aspects of Apple's satellite communication system

Satellite Communication Radio Frequency System - Illustration showing different modulation techniques and frequency bands

Satellite Network Specifications

Apple uses the Globalstar satellite network for its satellite communication features. This network has specific characteristics that influence how the communication system is designed.

Satellite Constellation

Total Satellites: The Globalstar constellation consists of 48 satellites, with an additional 4 spare satellites.

Apple Usage: Apple currently uses 28 Globalstar satellites for its service.

Orbit Type: Low Earth Orbit (LEO) at an altitude of approximately 1,414 km.

Orbital Period: Approximately 2 hours, making about 12 turns around Earth per day.

Satellite Age: The oldest satellite in use was launched in May 2007, while the newest was launched in February 2013.

Ground Station Infrastructure

Ground Stations: Multiple ground stations around the world receive signals from the satellites and connect to Apple's servers.

Bent-Pipe Architecture: Satellites act as "mirrors in space," forwarding signals from the iPhone to a ground station and vice versa.

Coverage Requirements: Both the iPhone and a ground station must be within range of the same satellite to establish communication.

Internet Connectivity: Ground stations are connected to the internet, allowing interaction with Apple's services.

Radio Frequency Specifications

The satellite communication system operates on specific frequency bands and uses specialized modulation techniques to optimize transmission in challenging conditions.

Frequency Bands

Uplink (iPhone to Satellite): L-Band (1,610 MHz to 1,626.5 MHz)

Downlink (Satellite to iPhone): S-Band (2,483.5 MHz to 2,500 MHz)

Channel Spacing: 200 kHz, with 180 kHz used for transmission and 20 kHz as guard space

Channel Numbers: Seven specific channels are used: 262,336, 262,338, 262,340, 262,342, 262,344, 262,346, and 262,348

Modulation Scheme

Modulation Type: Single Carrier Frequency Division Multiple Access (SC-FDMA)

Implementation: Single Physical Resource Block (1-PRB) SC-FDMA, similar to Narrow-band Internet of Things (NB-IoT)

Optimizations: Reduced Cyclic Prefix (CP) due to minimal multi-path effects when pointing directly at a satellite

Synchronization: Transmission starts with a training symbol to enable synchronization

Advanced Modulation Techniques

Apple's satellite communication system employs sophisticated modulation techniques to maximize data throughput while maintaining reliability under challenging signal conditions.

SC-FDMA

Primary Uplink Modulation: Single-Carrier Frequency Division Multiple Access is used for the uplink (iPhone to satellite) transmission.

Power Efficiency: SC-FDMA offers a lower peak-to-average power ratio (PAPR) compared to OFDM, making it more power-efficient for battery-operated devices.

Implementation: Uses a 12-subcarrier configuration with QPSK (Quadrature Phase Shift Keying) modulation for the underlying symbols.

QAM

Adaptive Modulation: The system can switch to Quadrature Amplitude Modulation under favorable signal conditions.

Constellation Points: Uses 16-QAM in optimal conditions, falling back to QPSK when signal quality degrades.

Spectral Efficiency: QAM allows for higher data rates by encoding more bits per symbol when channel conditions permit.

OFDM

Downlink Application: Orthogonal Frequency Division Multiplexing is used for the downlink (satellite to iPhone) transmission.

Multipath Resistance: OFDM's inherent resistance to multipath interference makes it suitable for the downlink where power efficiency is less critical.

Subcarrier Configuration: Uses 15 kHz subcarrier spacing with a specialized pilot pattern optimized for satellite channels.

Signal Transmission Details

The transmission of data via satellite follows a specific pattern optimized for reliability and efficiency.

Each transmission consists of multiple bursts, with each burst containing different types of data. The system includes automatic retransmission mechanisms to ensure reliable delivery even under challenging conditions.

Under optimal conditions, a complete transmission takes approximately 20 seconds, though challenging conditions may extend this time to 90 seconds or more.

Fig. 1: Spectrogram of physical-layer uplink signal

Protocol Flow

The satellite communication protocol follows a specific sequence of steps to establish a connection, authenticate the device, and transmit data.

Protocol Steps

Setup Phase: iPhone sends orientation updates and configuration data to the baseband chip
Security Configuration: iPhone sends the EPKI and shared secret to the baseband
Registration: Baseband registers with a satellite using the activation information
Security Confirmation: Ground station confirms registration and provides a Master Session Key
Message Transmission: iPhone sends encrypted application-specific content
Confirmation: Ground station acknowledges message reception
Teardown: Baseband deactivates the service to save power

Fig. 2: QMI messages exchanged during satellite transmission

Signal Processing Algorithms

Apple's satellite communication system employs sophisticated signal processing algorithms to overcome the unique challenges of satellite links:

Adaptive Equalization

Channel Estimation: The system continuously estimates the satellite channel characteristics using pilot symbols embedded in the transmission.

Frequency Domain Equalization: Applies adaptive equalization in the frequency domain to compensate for channel distortions.

Doppler Compensation: Sophisticated algorithms track and compensate for Doppler shift caused by satellite movement, which can be up to ±10 kHz.

Error Correction Coding

Turbo Coding: Employs rate-1/3 turbo codes with up to 8 decoding iterations to approach Shannon limit performance.

Interleaving: Bit-level interleaving spreads burst errors across multiple codewords, improving error correction performance.

Hybrid ARQ: Implements a specialized Hybrid Automatic Repeat Request system that combines multiple transmissions to recover data under poor conditions.

Hardware Components

Apple's satellite communication system relies on specialized hardware components in the iPhone.

Qualcomm Modem

Apple partnered with Qualcomm to develop a custom modem supporting satellite communication. This modem handles the physical layer of satellite communication, including modulation and demodulation of signals.

Custom Antenna

The iPhone includes a customized antenna design that boosts outgoing signals to reach satellites hundreds of kilometers away. This antenna is optimized for the L-Band and S-Band frequencies used for satellite communication.

Secure Enclave

The Secure Enclave Processor (SEP) generates and stores the cryptographic keys used for satellite communication. This dedicated security chip provides hardware-level protection for sensitive key material.

Software Components

The satellite communication system involves multiple software components working together.

System Daemons

CommCenter: The core daemon responsible for satellite communication, managing states, regional restrictions, and baseband interactions.

searchpartyd: Handles tasks for Find My in satellite communication, including creating and encrypting location data.

identityservicesd: Coordinates keys for sending iMessages and SMS over satellite.

User Interface Components

SOSBuddy: The user interface for satellite communication, guiding users through the connection process and questionnaires.

Find My App: Provides the interface for sharing location via satellite.

Messages App: Handles text messaging via satellite, including iMessage and SMS.

Data Compression Techniques

To optimize transmission over the limited bandwidth of satellite communication, Apple employs various data compression techniques.

Location Compression

Lite Location Format: Converts latitude and longitude double values into 32-bit fixed-point integers by multiplying them with 10,000,000.

Horizontal Accuracy: Represented as a 1-byte integer.

Total Size: The compressed location requires only 9 bytes of data.

Omitted Data: Elevation, speed, and other details are not included to minimize size.

Text Compression

Language-specific Codecs: Different compression algorithms are applied based on the user's language settings.

Selection Process: Multiple codecs are tried, and the most efficient one is selected.

Fallback: If compression doesn't reduce size, the uncompressed text is sent.

Efficiency: Typical emergency messages achieve a compression ratio of about 2.8:1.

Regional Availability

Satellite services are available in specific countries and regions, with some features limited to certain areas.

Fig. 3: Ground station and country configurations

Coverage Details

As of 2024, satellite services are available in 16 countries, primarily in North America and Europe. The availability of specific features varies by region:

Emergency SOS: Available in all supported countries
Find My via Satellite: Available in all supported countries
Roadside Assistance: Currently limited to the United States
iMessage and SMS: Currently limited to the United States

There are also radio exclusion zones within supported countries where satellite transmission is prohibited, such as near astronomy sites, certain islands, and national border areas.

Satellite Internet Comparison

While Apple's satellite technology is primarily designed for emergency communication and basic messaging, it's worth comparing it to dedicated satellite internet systems.

Apple Satellite vs Internet Satellites

Apple's satellite technology is optimized for low-bandwidth, critical communications rather than general internet access. Unlike satellite internet providers like Starlink, HughesNet, or Viasat, Apple's system doesn't require dedicated equipment beyond the iPhone itself.

The primary difference is in bandwidth capabilities. While satellite internet systems provide multiple Mbps of bandwidth suitable for web browsing, video streaming, and other internet activities, Apple's satellite features operate at just a few kbps—sufficient for text messages and emergency communications but not for general internet use.

This fundamental difference in purpose allows Apple to integrate satellite capabilities directly into a smartphone without requiring external antennas or equipment, making it accessible for emergency use anywhere.

Future Developments

As satellite technology continues to evolve, we may see increased integration between smartphone capabilities and satellite networks. Future developments could include:

Higher bandwidth satellite communication for smartphones
Expanded messaging capabilities beyond emergency services
Integration with global navigation satellite systems for enhanced location services
Hybrid cellular-satellite communication systems
Direct satellite internet access from smartphones without additional equipment

These advancements would bridge the gap between current emergency satellite features and full satellite internet capabilities.