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Iridium Global Satellite Phone System Fault Management




Iridium is a satellite-based global phone system.  Subscribers with handheld phones communicate directly with the satellites.  From there, communication goes between other satellites and/or back to ground stations on the way to traditional phone systems. Or, if the person at the other end of the call is also an Iridium subscriber, that subscriber also connects through the satellite network.  Satellite-to-ground links shown in the diagram only last about 6-8 minutes.  That’s caused by the orbital movement of the satellites.  Also, the satellite-to-satellite links break and are re-formed with different satellites as they all cross the poles in their polar orbits.  This movement is the opposite of the situation with ground-based wireless systems:  in effect the base stations are moving fast and the customers are standing relatively motionless.  Calls must be handed off from satellite to satellite every 7 minutes or so. 

Business Problem

Iridium SatelliteIridium posed a severe challenge for network management, because the network is so dynamic. There was concern that fault and performance management might be too complex to even be practical.  Traditional management solutions all assumed a fairly fixed network, especially at its core.  But here, even the “backbone” was moving. A flexible system was needed, that could analyze fault propagation to diagnose faults while the network structure was changing.

The overall system communications are summarized in the following diagram:


Iridium System Sketch



With such dynamic links between the nodes, it was necessary to build a system based on a very dynamic, real-time model that could account for the up-to-date status of the links.  The CDG technology makes a clear distinction between the domain model (satellites, communications nodes and links, etc.) and the fault models.  It automatically performs diagnostics based on the domain model, fault models, the current system state, real-time events,  and events generated by the performance monitoring system. (Please see the white papers on the CDG technology).


The fault management system was under successful development until Iridium ran out of funds, and cancelled Fault Management.  Ironically, rumor has it that among the 4 or so satellites that were lost just after launch, some of them might have been saved if the Fault Management system had been fully implemented, and problems were noticed before the satellites lost control to the point that command communications were impossible (e.g., a fault in a thruster starts a satellite spinning.  Unless corrected quickly, antennas can’t be aligned for stable command & control.)

CDG technology was productized by us (at Gensym) up to beta software level as a byproduct of our work (while at Gensym) on the fault management system for Iridium.  This was incorporated in the overall Integrity Product Line.


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