A detailed post of mine about how to use INMARSAT satellite coverage to narrow down the location of signals has not appeared yet after I posted it.
However, a brief extra point is that, even with just one ACARS satellite communicating with you (which is the case over most of the Indian Ocean and subcontinent), you can determine distance (from signal to satellite) quite accurately from ping times (as there are usually several time-stamped messages going each way).