Most of us are familiar with the way mobile phone networks work, with multiple interconnected cellular towers. As you move from one cell to another, the system automatically hands off your call seamlessly to the next tower. Iridium’s satellite network functions in much the same way. The satellites are the towers, orbiting the Earth and handing off calls to each other as they pass overhead.
Iridium’s constellation consists of 66 cross-linked operational satellites, plus six in-orbit spares. The satellites operate in near-circular low-Earth orbits (LEO) about 780 km (483 miles) above the Earth’s surface. There are 11 satellites in each of six orbital planes and their orbits “crisscross” roughly over the north and south poles. The low-flying satellites travel at approximately 17,000 miles per hour, completing an orbit of the Earth in about 100 minutes. It is a function of latitude/longitude and beam coverage, but it typically takes about ten minutes for a satellite to cross the sky from horizon to horizon.
Each satellite can project 48 spot beams on the Earth’s surface. The size of each spot beam is approximately 250 miles in diameter and the satellite’s full 48-beam footprint is approximately 2,800 miles in diameter. All spot beams and satellite footprints overlap.
The network is considered a meshed constellation of interconnected, cross-linked satellites so that each satellite “talks” with the other nearby satellites in front, behind and in adjacent orbits. Thus, the satellite network – much like a cellular network – hands off voice or data communications automatically from one spot beam to another within the satellite footprint, and from one satellite to the next as they pass overhead. The call is relayed from satellite to satellite around the constellation without touching ground until it is downlinked at an Iridium gateway and subsequently patched into the public switched telecommunication network (PSTN) for transmission to its destination. And this all happens in fractions of seconds and is completely seamless to the end user.
This architecture is unique to Iridium, and it provides inherent advantages in performance and reliability over other mobile satellite services providers:
In summary, one of the keys to Iridium’s enviable record of network reliability is the design of the satellites themselves and the unique cross-linked constellation that provides a constantly moving canopy of low-flying satellites with visibility over the entire face of the planet.
In our next Network Reliability Report, we will turn our eyes from the skies to the ground infrastructure.
Iridium’s cross-linked, low-Earth orbit (LEO) satellite constellation is supported by an extensive ground infrastructure that provides terrestrial connections for satellite voice and data calls, as well as network command, control, monitoring and technical support.
The ground infrastructure plays an important role in ensuring the network is reliable, dependable and ready.
The SNOC is the nerve center of the Iridium space and ground network. The SNOC is connected via a dedicated terrestrial fiber-optic system, comprised of a Multiprotocol Label Switching (MPLS) cloud and out-of-band satellite links, which directs and carries data to remote antennas and all other ground sites. It provides 24/7 monitoring and control over all network elements, including the satellites, ground sites and interconnections. The SNOC team conducts trending and performance analyses to ensure Quality of Service (QoS) requirements are met, and produces daily and weekly mission planning and orbit determination reports. The SNOC also provides real time satellite and network engineering support.
The Gateway is the landing point for commercial voice and data traffic via the satellites, providing connections into the Public Switched Telephone Network (PSTN) and the Internet cloud. Network Operators and Engineers constantly monitor the highly sensitive alarm and notification system with a direct, secure link to the SNOC and other ground stations.
The Ground Stations serve as the primary landing points for Iridium OpenPort® communications, and as a backup landing point for commercial traffic, which is back-hauled via dedicated fiber-optic links to the primary gateway for processing. They can also serve as Tracking, Telemetry and Control (TTAC) sites.
The TTAC sites route satellite health and safety information to the SNOC, provide upload commanding from the SNOC to the satellites, and serve as a delivery mechanism for satellite telemetry of key space network data. They also provide the precise timing signals, which are vital for network operation. In addition, they have Iridium-dedicated tracking and data reception antennas to augment visibility and command of the satellite constellation.
The Technical Support Center is managed and staffed by engineering and technical personnel with in- depth understanding of all aspects of the Iridium network. This team provides systems integration testing of network systems and subsystems, service monitoring and performance analysis, system anomaly response and resolution, system over-the-air testing, software development for satellites and ground infrastructure, Iridium partner product certification support, and other vital technical functions. They have access to on-the-ground, fully functioning Iridium satellites, the Gateway and TTAC subsystem test stations, which are used to test and validate software prior to operational upload to the systems and to analyze and troubleshoot any anomaly detected in the satellite and ground networks.
The ground infrastructure provides the following extensive back-up systems to ensure continuity of operations in the event of any subsystem failure or anomaly: