What frequency does the Iridium satellite system use?

The Iridium satellites are Low Earth Orbiting (LEO) satellites that are owned by Iridium LLC based in the USA. Iridium LLC is a global satellite communications company, providing voice and data services across the globe.

The company was originally started in the 90's to provide a global satellite phone system. However, they went bankrupt and then resarted a few years later. The company has restructured and rebranded itself for more specialized applications like providing global coverage for voice and data (although at a low rate). It is also certified for airborne use and along with many other roles, the Iridium satellite phone system can provide useful communication services to many people.

The Iridium Satellite System uses several microwave frequency bands for communication including L-Band (1 to 2 GHz), K-band (18 to 26.5 GHz) and Ka-band (26.5 to 40 GHz). The Iridium satellite system uses these frequency bands to provide voice and data coverage to satellite phones and other equipment with global coverage.

The Iridium satellites provide communication services via Iridium terminals and Iridium gateways. The terminals are wirelessly linked directly to the satellites, while Iridium gateways serve as hubs for telephone networks to use the Iridium band for communication. The Iridium frequency bands are used for communication between two iridium satellites, between an iridium terminal & an iridium satellite and between a gateway or earth station & an iridium satellite.

The Iridium satellite system uses K/Ka-band frequencies for inter-satellite links and to communicate with gateways. The gateways on earth that are linked to the Iridium satellites and form the backhaul of the network use the frequency bands 29.1 to 29.3 GHz for uplink and 19.1 to 19.6 GHz for downlink communication. The 66 active Iridium satellites communicate with each other using frequency bands between 22.55 and 23.55 GHz.

Iridium uses the L band frequency ranging from 1616 to 1626.5 MHz to communicate directly with ground-based users (subscribers). There are 240 main channels between 1616 MHz and 1626 MHz which are spaced 41.667 kHz from each other as this is the minimum bandwidth needed for receivers to properly receive Iridium signals. This user-satellite link uses a total of 10.5 MHz starting at 1616 MHz. This also allows for a channel bandwidth of 31.5 kHz together with a suitable guard band to accommodate inter-modulation and Doppler shifts caused by the satellite velocity relative to the ground stations.

In addition to this subscriber-links band, a 12-frequency access band is reserved for the simplex (ring alert and messaging) channels. These channels are located in a globally allocated 500 kHz band between 1626.0 MHz and 1626.5 MHz.

Iridium uses a Differentially Encoded QPSK (DEQPSK) modulation technique with an occupied bandwidth of 31.5 kHz. To support many users, the satellite system uses a scheme where the different users can be managed so that they may gain access to the satellite system without interfering with each other. While transmitting,  Iridium satellite technology uses both FDMA (frequency division multiple access – where users are allocated different frequencies) and TDMA (time division multiple access – where users are allocated different time slots in a transmission). The transmitter duty cycle allows for burst transmission every 8.28 ms out of 90 ms, or 9.2%, at a rate of 50 kbps, with a symbol rate of 25000 bd. For voice communications, Iridium uses a 2.4 kbps Advanced Multi-Band Excitation (AMBE) vocoder developed by Digital Voice System Inc. (DVSI). This vocoder is tailored to the Iridium communication channel.

 The system was originally aimed at providing a global satellite phone system in the days when terrestrial cellular-based systems were not as widely used. Now the company has restructured and rebranded itself for more specialized applications like providing global coverage for voice and data (although at a low rate). It is also certified for airborne use and along with many other roles, the Iridium satellite phone system can provide useful communication services to many people.