This came up in a game yesterday. My opponent has 4 ships all with the same PS. On his turn before deciding which of his ships will attack first and which ship of mine he will attack, he checks all his arcs and ranges.

My understanding was that I need to pick a ship to activate, and then I can only check that ships firing arc. That is how I played all my games, and mentioned it to him, but he said he was allowed because they are all the same PS.

The core rule book and FAQ talk about the "active" ship being able to measure range. They define the active ship based on pilot skill and initiative, but never distinguish explicitly what happens when you have more than one ship of the same PS.

My understanding was that I need to pick a ship to activate, and then I can only check that ships firing arc.

This is the way it is.

You select a ship first. After that you start the combat sequence. Then you declare a target during the declare target step of the combat sequence. Next, still during the declare target step, you measure if the declared target is in range and arc. If it is, continue with roll attack dice. If it is not, declare a different target.

Dvor has it correct, for the most part. During the combat phase, you declare an active ship. You may then check range from that active ship to any enemy ship within the active ship's firing arc for range. You may then decide which of those enemy ships that active ship is declaring as it's target.

For reference, check out the following section from page 10 in the Rulebook:

1. Declare Target

During this step, the attacker (the active ship) must declare its target (the ship he wishes to attack). The target ship must be inside the attacker’s firing arc and within range. A player may measure to verify that these conditions are met before declaring a target.