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Iridium
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Iridium is a mobile-satellite service (MSS) system that was first licensed by the FCC in 1995, and began commercial service in 1998.
Iridium utilizes an FDMA/TDMA Time Division Duplex (TDD) access technology, so that the same frequency band is used for both the user uplink and downlink transmissions, on a time-shared basis. It shares a portion of its spectrum (1617.775-1618.725 MHz) with Globalstar, which has mobile uplinks to its own MSS system in the 1610-1618.725 MHz band.
Each Iridium user uplink/downlink channel has a bandwidth of 31.5 kHz and is separated in frequency by 41.67 kHz to allow for Doppler shift. A single Iridium TDMA frame is 90 ms long, which begins with 22.48 ms guard time, followed by four user uplink and four user donwlink time slots (8.28 ms burst time each, separated by small guard times).
A good discussion of the Iridium system architecture, constellation design, and multi-access scheme can be found in chapter 2 of the thesis by Abdul Jabbar (link below).
In February 2013, Iridium was granted authority by the FCC to provide aeronautical mobile-satellite (route) service (AMS(R)S) in the 1618.725-1626.5 MHz portion of its spectrum, limited to oceanic, polar, and remote regions. According to the FCC, "for purposes of this authorization, we consider oceanic regions to be those beyond 12 nautical miles from the baselines of the coastal states." Authorization for remote areas of other territories is contingent upon completing the agreement-seeking process under 5.367 of the ITU Radio Regulations.
At the end of 2012, Iridium reported approximately 368,000 subscribers. The company plans to launch a new generation of satellites, Iridium Next, beginning in 2015.
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Frequency Bands |
| Band | Use | Service11 | Table |
| 1617.775 - 1626.5 MHz | Iridium (user terminal uplinks and downlinks) | Mobile-satellite | N |
| 1618.725 - 1626.5 MHz | Iridium AMS(R)S | ARMR | N |
| 19.1 - 19.6 GHz | Iridium satellite gateway downlinks | Fixed-satellite | N |
| 22.55 - 23.55 GHz | Iridium inter-satellite links | Inter-satellite | N |
| 29.1 - 29.3 GHz | Iridium gateway uplinks | Fixed-satellite | N |
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