Iridium

What Is Iridium?

Iridium is a low Earth orbit (LEO) satellite communications system consisting of a constellation of 66 active satellites that together provide voice and data coverage over the entire surface of Earth, including the poles, oceans, and remote regions beyond the reach of terrestrial cellular networks. Each satellite orbits at an altitude of approximately 781 kilometers in one of six near-polar orbital planes, completing a full orbit in roughly 100 minutes. Unlike geostationary systems, which use a small number of satellites at 36,000 km altitude, Iridium's LEO orbit produces signal propagation delays of only 6 to 8 milliseconds to a satellite, significantly lower than the roughly 240 ms round-trip delay of geostationary links.

The system was developed in the late 1980s and early 1990s by Motorola and takes its name from the element iridium, as the original design called for 77 satellites, matching that element's atomic number, though the final constellation used 66. The first-generation commercial system launched in 1998, went through a bankruptcy and restructuring in 1999 to 2000, and was relaunched as Iridium LLC. The second-generation constellation, Iridium NEXT, replaced the original satellites between 2017 and 2019.

Constellation Architecture

The 66 operational satellites are distributed evenly across six orbital planes of 11 satellites each, with adjacent planes separated by 30 degrees of right ascension. At any point on Earth's surface at least one satellite is above the horizon, and in mid-latitudes several are typically visible simultaneously. Satellites are replaced by spares held in lower parking orbits when failures occur. An IEEE overview of the Iridium LEO satellite system describes the orbital mechanics, coverage geometry, and handoff procedures that maintain continuous service as satellites move across the sky. Because each satellite footprint moves at roughly 26,000 km/h relative to Earth, user terminals must hand off calls between satellites and between beams within a single satellite approximately every minute.

A distinctive feature of the Iridium architecture is the intersatellite link (ISL) network, in which each satellite maintains Ka-band crosslinks to its two neighbors in the same orbital plane and to satellites in adjacent planes. This mesh of ISLs allows user traffic to be routed from satellite to satellite and reach an Earth gateway without passing through an intermediate ground station. The implication is that a single gateway can serve global traffic, reducing the infrastructure cost compared to systems that require ground stations distributed around the world. Call routing through the ISL network is computed on-board using distributed algorithms that account for the rapidly changing topology as satellites orbit. An IEEE operational performance analysis of the Iridium system evaluates throughput, handoff reliability, and network behavior under degraded constellation conditions.

Communications Services

Iridium provides short-burst data (SBD) for machine-to-machine telemetry, low-rate voice at 2.4 kbit/s encoded with the AMBE codec, and broadband data service at up to 704 kbit/s through the Iridium Certus platform introduced with the NEXT constellation. The system uses L-band spectrum in the 1,616 to 1,626.5 MHz range for user links, with TDMA and FDMA access schemes. Iridium also operates a passive listening payload called NEXT ADS-B that collects aircraft Automatic Dependent Surveillance-Broadcast messages globally, filling gaps in ground-based radar coverage over oceans and remote areas. The Iridium network technical overview describes the Certus service tiers and the ADS-B payload mission.

Applications

Iridium provides communications in a range of environments and sectors, including:

  • Maritime voice and data for vessels beyond coastal cellular coverage
  • Aviation safety services including ADS-B tracking over oceanic routes
  • Military and government personnel communications in remote and polar regions
  • Machine-to-machine telemetry for pipelines, environmental sensors, and remote assets
  • Emergency response and disaster relief communications when terrestrial networks fail