Iridium

Iridium is a MSS system providing global L-band voice and data communications using a constellation of LEO satellites. Unlike most MSS systems, including Globalstar, Iridium employs on-board digital switching and inter-satellite links, allowing calls and data to be routed through space without immediate reliance on ground gateways.

The Iridium constellation provides seamless global coverage using 66 active satellites in near-polar LEO orbits at an altitude of approximately 780 km, with an inclination of 86.4° and an orbital period of 100 min 28 s. The one-way propagation delay to an Iridium satellite is approximately 2.6 ms, compared with roughly 124 ms for a geostationary satellite.

The satellites are distributed across six orbital planes spaced 31.6° apart in right ascension, each containing 11 satellites. Orbital planes 1 and 6 are separated by only 22° to maintain seamless global coverage. The name Iridium was originally chosen because early designs envisaged 77 satellites, corresponding to the atomic number of iridium and evoking the image of electrons orbiting an atomic nucleus. Although the final constellation required only 66 satellites, the name was retained rather than adopting the element with atomic number 66 (dysprosium).

The first-generation Iridium constellation was deployed between 1997 and 2002 by Iridium SSC, financed primarily by Motorola. The satellites had approximate dimensions of 1 × 3.5 m and dry masses of about 556 kg. Although the system was technically successful when commercial service began in 1998, it failed commercially due to limited demand at the offered price points, handset size, and call quality issues. Iridium filed for bankruptcy in 1999 and was subsequently restructured as Iridium Communications, which refocused on niche markets including maritime, aviation, emergency services, and military users operating beyond the reach of terrestrial and GEO systems.

Although the original satellites were designed for an 8-year lifetime, no replacements were launched after 2002. By mid-2016, multiple failures had reduced operational availability to 64 satellites, resulting in degraded coverage in some regions.

The second-generation Iridium-NEXT constellation was deployed between January 2017 and January 2019 using SpaceX Falcon 9 launch vehicles as part of an approximately US$3 billion replacement program. Services were fully transitioned to the new satellites in February 2019. In total, 95 first-generation satellites (of 99 built) were launched in 22 missions, and 75 Iridium-NEXT satellites were launched in eight missions.

Currently, 81 satellites are in orbit, including 66 operational satellites and on-orbit spares held at approximately 666 km altitude for insertion as required, with additional ground spares maintained.

Iridium-NEXT satellites are based on Thales Alenia Space’s ELiTeBus-1000 platform. Each satellite has a launch mass of up to 860 kg and measures approximately 3.1 × 2.4 × 1.5 m in its stowed configuration. Average electrical power is about 2.2 kW on a 28-V bus. The spacecraft are three-axis stabilized using star trackers, Earth and Sun sensors, inertial measurement units, reaction wheels, and torque rods, with hydrazine monopropellant thrusters for orbit maintenance and attitude control.

Three L-band phased-array antennas operating in the 1.6160–1.6265 GHz band generate 48 spot beams per satellite. Each phased-array panel forms 16 fixed beams using right-hand circular polarization. The satellite footprint is approximately 4,700 km in diameter. Each satellite can support up to 1,100 simultaneous voice circuits at 2.4 kbps.

Iridium-NEXT enables the Iridium Certus service, offering global broadband with up to three simultaneous high-quality voice channels and IP data rates of up to 704 kbps.

Downlink access to user terminals employs a hybrid FDMA/TDMA scheme. FDMA channels are spaced at 41.666 kHz, with occupied bandwidths of 31.5 kHz to accommodate Doppler shifts. Each channel contains four TDMA time slots per direction, with 8.28 ms QPSK slots within a 90-ms frame. Forward error correction uses code rates of 4/5 and 2/3, with a target bit-error rate of 10^–7.

Traveling at approximately 7.5 km s^–1, a satellite remains in view of a user terminal for roughly seven minutes, after which calls are automatically handed off. Each satellite supports four inter-satellite links: two to neighbors fore and aft in the same orbital plane, and two cross-plane links to adjacent planes using gimballed antennas. To limit Doppler effects, hand-offs occur only between satellites traveling in the same orbital direction. Even so, Doppler shifts of up to ±45 kHz are observed at L-band.

Inter-satellite links operate in Ka-band (22.55–23.55 GHz) at data rates of approximately 25 Mbps using 8PSK modulation with a 2/3 code rate. RF cross-links provide sufficient capacity and robustness without the complexity of optical links.

Iridium employs three types of hand-off: 1) Channel/time-slot reassignment within a beam. 2) Beam-to-beam hand-off as the satellite passes overhead. 3) Satellite-to-satellite hand-off when a spacecraft sets below the horizon. The final hand-off may cause a brief interruption of up to 0.25 s.

Calls between Iridium users may be routed entirely through the space network. Calls to external networks are routed via feeder links operating at 19.5 GHz (downlink) and 29.2 GHz (uplink) to terrestrial gateways. Although up to 250 gateways are supported by the architecture, the current operational gateways are located in Tempe (USA), Fairbanks (USA), Svalbard (Norway), and Punta Arenas (Chile).

Iridium-NEXT satellites also support hosted payloads of up to 54 kg, with approximately 90 W average power (200 W peak) and data connectivity up to 1 Mbps. All Iridium-NEXT satellites host ADS-B receivers, enabling global real-time aircraft surveillance for air navigation service providers via 1090-MHz Mode-S transmissions.

Iridium also provides the U.S. government’s intelligence and military communities with a global, secure connectivity through a dedicated gateway and communications infrastructure in Hawaii. Since 2019, the Iridium Enhanced Mobile Satellite Services (EMSS) program provides access to global unlimited secure and standard narrowband voice, broadcast, push-to-talk, and select additional services to an unlimited number of US DoD-approved subscribers.

In May 2025, in its latest support for the EMSS program, Iridium Communications announced a special US government version of the Iridium 9603 module built for Enhanced Short Burst Data (ESBD). ESBD is a low-latency and secure messaging service designed to provide optimal transmission security (TRANSEC) for the US government and approved allies. Iridium 9603 ESBD can deliver secure one- or two-way messaging with an increased payload capacity for command and control (C2), image data transfer, and asset tracking.