OK here we go something like this:
Geoscape = A4/A3 map of the globe with the base indicated on it and "fuel zones" (like time zones except indicates the amount of fuel needed to travel there).
Active alien ships indicated with Red markers (marked with sizes), Xcom ships in Blue (marked with type), crashed ships in white and terror in another colour....
1. After each mission after the players have determined reasearch, selling and production the Games master rolls to see how many alien ships there are, their current fuel zone, the direction they are traveling (scatter dice with hit indicating landed) and their size. He places markers on the geoscape (possibility there might be none). (Note there might even be some from last turn still on the map, in which case see section 7)
2. Players decide which ships to attempt to engage.
3. Players move their interceptors to the ships current locations and deduct that much fuel, assuming they have the speed to reach it, otherwise they just move as close as they can.
4. Players roll to attempt to engage the ships (D6, must be lower or equal to hull size, -1 if moving away, +3 if moving toward, 1 always engages).
5. For any engagements players choose their attack stance (the more aggressive the higher the chance to hit and more damage, but that goes for both ships).
6. Shops that have been engaged “fight” each other until one ship is downed, the players withdraw or the Alien ship escapes.
7. Assuming no missions were started the GM starts this sequence again rolling a new direction for any surviving alien ships.
If interceptors are out of the base, they may attempt to engage a new enemy if they have sufficient fuel units / speed to reach it. Otherwise they must return to base and spend a geoscape turn re-fuelling.
If at any point a skyray or equivelent engages a crashed / landed ship or terror mission then complete any other engagements noting thier results if required and begin the mission.