The positional arcs produced by INMARSAT have been developed using original source data and a deep understanding of the equipment and specifications. Ultimately it is a pure guess, but a guess based on the best interpretation that can be made by people at the peak of their careers and with experience and access to data that has not been publically released.
And it would be appropriate for the data to be released to qualified individuals or agencies for independent verification of InMarSat's findings.