NAL Research Corporation (451-93156-002A)
104
APPENDIX F: DESCRIPTION OF THE IRIDIUM NETWORK
F.1 Description of the Iridium Network
The Iridium satellite network is owned and operated by Iridium Satellite LLC (ISLLC). It was constructed
as a constellation of 66 satellites in low-earth orbit, terrestrial gateways and Iridium subscriber units (ISU).
An ISU can either be an Iridium satellite phone or any of NAL Research’s A3LA series modems. The satellites
are placed in an approximate polar orbit at an altitude of 780 km. There are 6 polar planes populated with
11 satellites per orbit constituting the 66 satellite constellation. The near polar orbits of the Iridium
constellation provide truly real-time and global coverage from pole-to-pole.
The Iridium is designed to operate in the band of 1616 to 1626.5 MHz although the exact frequencies
used depend on the local regulating authorities and issued licenses in any particular region. Each satellite
projects 48 beams on the surface of earth, which may be viewed as providing coverage cells on the ground
similar to terrestrial systems. Each beam is approximately 600 km in diameter. The 66-satellite constellation
has the potential to support a total of 3,168 spot beams; however, as the satellite orbits converge at the
poles, overlapping beams are shut down. The satellite footprint is ~4,700 km in diameter. Under each
footprint, a satellite is power limited to ~1,100 simultaneous circuits.
The Iridium network uses a time domain duplex (TDD) method and transmits and receives in an allotted
time window within the frame structure
.
Since the system is TDD, the ISU transmit and receive in the same
frequency band. The access technology is a FDMA/TDMA (frequency division multiple access/time division
multiple access) method whereby an ISU is assigned a channel composed of a frequency and time slot in
any particular beam. Channel assignments may be changed across cell/beam boundaries and is controlled
by the satellite. The system will provide an average link margin of 12 dB.
Constellation
Satellite
Gateway
Satellite Network
Operation Center
(SNOC)
