Also known as: Iridium, Iridium NEXT
Iridium is a satellite communications constellation that provides voice and data coverage over the entire surface of the Earth, including the poles, using 66 active satellites in low Earth orbit.1 Unlike a navigation GNSS, Iridium is a two-way communications network: its satellites relay QPSK bursts to and from handsets in the L-band and pass traffic between one another over inter-satellite cross-links, so a call can transit the whole constellation without touching the ground.
Overview
Iridium’s 66 operational satellites (plus in-orbit spares) fly in six polar orbital planes at about 780 km — far lower than the ~20,000 km medium-Earth orbits of the navigation constellations. That low altitude means each satellite races across the sky in minutes, so the network continuously hands a call from one satellite to the next, and the ground segment needs only a few gateways because most switching happens in space. Services range from satellite telephony to low-rate Short Burst Data used by IoT trackers and maritime devices.
Technical characteristics
| Property | Value |
|---|---|
| Orbit | 780 km LEO, six planes, 66 active satellites |
| Band | L-band 1616–1626.5 MHz (user links) |
| Access | Combined FDMA and TDMA, ~50 ms frames |
| Modulation | Differentially-encoded QPSK, bursty |
| Cross-links | Ka-band inter-satellite links |
| Doppler | Tens of kHz swing per pass |
The low orbit produces a large Doppler shift — the carrier can swing by tens of kilohertz as a satellite rises and sets — which both the handset and any receiver must track. Iridium’s frames combine time- and frequency-division access, and its downlink bursts (including the periodic ring-alert and broadcast channels) use differential QPSK so a receiver need not perfectly recover absolute carrier phase.
History
The original Iridium system, conceived at Motorola in the late 1980s, launched its constellation in 1997–1998. The operating company went bankrupt in 2000 but the network was rescued and continued in service. A full replacement fleet, Iridium NEXT, was launched between 2017 and 2019, adding higher data rates and hosted payloads (including an aircraft-tracking ADS-B receiver).
Deployment
Iridium is used for satellite phones, maritime and aviation communications, remote IoT telemetry, and government services, valued for its true pole-to-pole coverage that geostationary systems cannot match. Its Short Burst Data service underpins many remote asset-tracking and safety devices.
Decoding it with GopherTrunk
GopherTrunk does not decode Iridium. It is a land-mobile trunking scanner, and satellite communications are out of scope. Iridium is, however, a notable software-defined-radio subject: its ~1.6 GHz downlink is strong enough that hobbyists with an L-band antenna and tools such as gr-iridium and iridium-toolkit can capture and analyze the unencrypted signalling bursts, contending mainly with the fast pace of satellites and the large Doppler shift. None of that is part of GopherTrunk; its only connection to satellites is using a GNSS receiver as an external timing reference.
Sources
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Iridium satellite constellation — Wikipedia, for the 66-satellite LEO constellation, the polar orbits and cross-links, the L-band QPSK air interface, and the Iridium NEXT replacement fleet. ↩