Retaliation battleships can
be shot down using an Avenger at the target base! Your Hyperwave Decoder at the target base (or any other nearby base) must detect the battleship at a distance far enough out for your Avenger to do something. If the Hyperwave detects the battleship too close, your Avenger will be helpless to stop the attack.
Ok, let me explain this a little. Your Avenger is perfectly capable of out-running any alien ship in the game in a side-by-side race. Avenger: 5400 knots, Battleship: 5000 knots. When you send your Avenger out to intercept a UFO that is heading to your base, the Avenger first must reach the UFO. Then it must turn around and pursue the UFO until it is within Standoff range. Only then may the Avenger attack - never in a head-on game of chicken!
Because the intercept/pursuit process is taking place while the battleship is heading towards your base, precious time (and distance) is wasting away. If your detection system detects the battleship inside the boundary necessary for your Avenger to get into standoff (and ultimately attacking) range, it will assault your base without your Avenger even firing a shot. I am not sure what this range is, but it is definitely within 1000 nautical miles from your base.
Avengers from other bases can attack this battleship also. First and foremost, it must be able to reach the UFO and get into standoff range. Next, your Avenger must be sent from a base that is within a 180 degree arc directly in front of where the battleship is heading. For example, if the battleship is heading directly north to its target base, your second base must lie within 90 degrees of either side and to the rear of the target base, and also ahead of the battleship. So between W and E on the compass rose. Too far south or off to either side and your Avenger is reduced to a 400 knot catch-up craft - which never works!
Also, shooting down the battleship and then successfully fighting the aliens on the ground does not
stop the invasions - they just keep coming, and coming, and coming... Damn those aliens!
Just a question: why does the Hyperwave Decoder sometimes miss a UFO inside the detection radius? It should pick up all UFO's, within range, all the time!