developed using original source data and a deep understanding of the equipment and specifications.
I admit that Inmarsat and academic scientists know their systems inside out, but my question on the accuracy of their prediction is based on my own knowledge of aircraft systems and equipment that interface with the outside world [40+ years as an avionics specialist on Boeing 707, 737, 747, 757, 767 and currently on 787].
The MOPS for a Mode S Transponder (which includes Modes A and C functions) requires a very precise 8 microsecond interval to identify an interrogation, and trigger the response. The response is then either a 56 microsecond or a 112 microsecond burst of data. The 120 microsecond reply is very finely calibrated. A recent Airworthiness Directive (2014-05-07 for those who are interested) directed at a particular manufacturer's Mode S transponder requires them to be sent for bench testing and re-alignment every 4 years to detect and correct age related alignment drift in the electronic components. Such drift cannot be noted by crew or maintenance monitoring. DME Interrogators require similar calibration accuracy, but DME indications are directly monitored by the aircew and drift is quickly noted.
Now, compared to the precision calibration of ATC transponders and DME interrogators, there are no such requirements for Satcom systems. A one microsecond variation within the airborne equipment's processing circuits could easily result in a 2,000 km error in the position calculation. There is nothing in the equipment specifications that makes that unlikely - the equipment is just as likely to suffer age related drift as any other avionics equipment. Unlike transponders, Satcom equipment is maintained purely "On Condition" and there is absolutely no way that the scientists producing their range predictions, based on analysis of "handshake" responses that were never intended for range determination, could know the internal condition of the airborne components of the Satcom system. I have been sceptical of the predicted range arcs since they were first promulgated and nothing I have seen or heard since has changed my opinion - these Satcom range arcs are of questionable accuracy.
Sadly, they are the only thing the search coordinators have to go on.