Satellite networks vary greatly in terms of their orbits, architecture and technologies — different numbers of satellites, different coverage areas, different distances from Earth and different communications infrastructures, to name a few. We can debate, at length, the positive and negative aspects of these. Yet when reliability is the gating factor — as it is in the maritime industry, where loss of communications can mean lost lives — a low Earth orbit (LEO) satellite network such as Iridium’s provides a uniquely reliable option.
When it comes to measuring reliability, organizations like the International Maritime Organization (IMO) set the standard of 99.9 percent network availability for providing Global Maritime Distress Safety Services (GMDSS). When you translate that into minutes, it means that there can be cumulative outages of no more than 53 minutes per year. Iridium’s network architecture minimizes the impact outages have on users, ensuring this standard is met even if there may be an issue with a satellite. However, with geostationary satellite networks, that may not always be the case, and service outages resulting from a satellite failure affect a much greater area and potentially last far longer due to how the networks are designed to operate.
The benefits of Iridium’s network have not gone unnoticed by the maritime industry. Iridium’s application to the IMO for providing GMDSS mobile satellite communications represents the first LEO network to receive consideration as an alternative to the current mobile satellite service provider, and recently received overwhelming support by the IMO subcommittee to undergo formal technical and operational evaluation for the provision of GMDSS services.
To provide greater detail, Iridium’s constellation consists of 66 cross-linked, LEO satellites that are 780 kilometers from Earth. These operate as a fully-meshed network, and are supported by multiple in-orbit spares of the same technology as those operating the network. This architecture means that each satellite provides a moving coverage area roughly 2,500 kilometers in diameter on the surface of Earth at any given time, and it takes just minutes for a satellite to cross horizon to horizon, with an adjacent satellite coming into view and overlapping that same area no more than six minutes after the preceding one started that path.
In contrast, in geosynchronous equatorial orbiting networks, satellites are in orbit roughly 36,000 kilometers above Earth. These networks have far fewer satellites, and each covers a much larger geographic area, yet they still have coverage gaps in key oceanic and polar regions. Spares supporting these networks are often from previous generations consisting of much older technology, and have limited capacity. In this type of system a satellite doesn’t move across the sky relative to the vessel — it either works or it doesn’t.
What do these differences mean in terms of a real-life scenario? Let’s assume a large cargo vessel is stranded in the Indian Ocean and needs to make a distress call, and a satellite has an outage (for whatever reason) that lasts for 100 minutes. On the Iridium network, this would mean that (depending on specific location) the vessel would only lose satellite service for a maximum of six minutes — the amount of time it would take before another satellite would crest the horizon and service would automatically be restored.
With a geosynchronous constellation, however, a 100-minute outage of a satellite would mean all vessels in the coverage area of the satellite would lose service for the entire 100 minutes. This could be catastrophic to the coordination of search activity as all parties involved could also be affected. Due to the network architecture, it is uncertain how long or to what extent communications may be restored for users in the region. Due to the nonsymmetric coverage of the “spare” geosynchronous satellites, some areas may not have service restored at all.
Iridium’s LEO network provides other advantages as well — for example, when Iridium transitions to its second-generation constellation (Iridium NEXT) beginning in 2015, existing devices will continue to be supported such that seafarers won’t need to purchase new equipment to continue to receive the same services as today. Additionally, one of the biggest advantages is polar coverage, an area of increasing importance as vessels are able to operate farther north and south throughout the year.
When lost service may translate into lost lives, providing alternative networks and understanding the reliability differences between those networks is crucial. For minimizing outage impact to users — and ensuring reliability — an interconnected LEO network provides a valuable and necessary service option.
Brian Pemberton is executive director for Iridium’s maritime and aviation lines of business.
