Head towards the exit from the partially destroyed Sionis Steel Mill #1. After getting outside, Batman will speak with Alfred and Oracle #2, commenting Hugo Strange's latest actions. In order to stop the madman, you will have to get inside the Miracle Tower, though first you have to gain the access codes.
Before you do anything, a few words should be said about the TYGER helicopters that are flying above all of Arkham City #1. Up until now, if one of them has detected Batman, it only informed Hugo Strange about it and nothing more happened. Now you will have to take into account that they will attack you upon detection #2. In such situations I'd recommend using the Grapnel Gun and quickly move towards any direction, hoping to lose the helicopter. Luckily the helicopters won't be searching for Batman specifically, so as long as you dodge their searchlights you can pretty much complete this mission without being detected.
The primary objective of this mission is scanning helicopter and each time you have to wait for it to get nearby Batman #1. In order to improve this process, you can of course reduce the distance to them by using towers, roofs and other high points. Scanning can be done by pressing LB and filling up the bar that displays on screen #2.
The first scans will of course end with failure, but don't get discouraged too soon and keep scanning further machines #1. It's also worth pointing out that eventually you will have to leave the industrial district, as not every helicopter flies above this part of Arkham City. Keep repeating this until you find the proper helicopter and receive further instructions #2.