Baro Altitude Necessary For IRS Alignment?

[QAVION]

From previous readings on the subject, I seem to recall that IRS's need to know how far away from the centre of the earth the aircraft is both during alignment and subsequent navigation. As I recall, a "distance from the centre of the earth to an average sea level height" is added to airplane baro altitude to estimate the airplane distance from the centre of the earth. In the absence of ADC altitude data during alignment, will the IRS's use some kind of default altitude or can they roughly compute altitude themselves during alignment from the earth's movement?

If they can't align themselves without ADC data, can the IRS's (with FMC's) still be considered a "stand-alone" navigation system?

Thanks.
Rgds.
Q.