Please can anybody tell me how the Heads Up Display in an airliner know where the runway is situated so that it can "paint" a representation of the runway on the screen ?

Does the HUD calculate the position from GPS data ?
What is involved in certifying an existing HGS system for landings in a country where the HGS is not previously certified ?

Also can Infra Red be overlaid on older HGS systems or must one purchase an entirely new HUD system for I/R ?