The Military uses of the Palantiri.

[Inziladun]

Originally Posted by Rumil

One question is why Sauron didn't spot the Rohirrim appearing on the flank of the Pelennor Fields. However it seems that you must know where and when to look, and his attention was probably taken by the encounter at the gate. Also, the Rohirrim rode through the forest on their march, so likely not spottable, and even if Sauron had immediately seen them forming up, he would pesumably have had to send a Nazgul or other messenger to Gondor, which takes some time even for Winged Beasts.

I think the primary reason Sauron was blindsided by the Rohirrim was Aragorn. He had revealed himself by using the Orthanc-stone before leaving the Hornburg. That caused Sauron to ignore everything else and focus on Minas Tirith.