The entry window is just the period of time while on orbit where the OMS burn will provide the proper deceleration to the shuttle's orbit so that the track/altitude will take the orbiter into the atmosphere (entry interface), and to the selected landing site.
BTW the double sonic boom is due too the fact that there are two shock waves on any supersonic traveling object, most know of the bow front, but you also have one from the rear, also trailing back, in the case of the Shuttle, it's speed over land is such that you can hear the two quite distinctly.
[ 04 August 2001: Message edited by: RW-1 ]