Re: IRS question
There are two sources, or more correctly, sets of sources for ground speed within the IRU itself. The primary is the set of 3 laser gyros themselves and the other is an equivalent set of 3 accelerometers. The software can thus differentiate when the IRU is stopped without an external (GPS or air data) source although most IRUs take in air data for windspeed & direction calcs.
Some modern IRUs (e.g. Honeywell Laseref V) can now realign in flight using GPS-aiding - something that unheard of outside some military applications a few years ago.