Just as a way of updating the radio position of the aircraft navigation systems. The aircraft will use probably GPS if available, DME/DME for very accurate position check, then VOR/DME, finally VOR/VOR. It will autotune using the FMC/FMGC etc. The aircraft will also have the IRS position, individual and mix. A Kalman filter is used (if memory serves) to integrate these various positions to give the best known position to other systems eg. Navigational display. The freqs are pulled from a Navaid database.
The aircraft whilst flying a SID will be using the FMC/FMGC computed position to fly it in managed nav mode (or LNAV Boeing style), not raw data (which you can cross check against or fly if the FMC dies.
If you lose GPS and VOR/D the last bias from the calculated mixed IRS position can be used to give a good position. Basically the IRS will drift, by remembering by how much compared to known position (bias) it can give you a good position until the radio update is done.
Obviously for modern aircraft with full avionics.
Last edited by tom775257; 14th Dec 2011 at 17:57.