A '''passive electronically scanned array''' (PESA), contrary to its active counterpart [[AESA]], is a [[phased array]] which has a '''central''' [[radiofrequency|radiofrequency]] source (such as a [[magnetron]], a [[klystron]] or a [[travelling wave tube]]), sending energy into (usually digitally-controlled) [[phase shift module]]s, which then send energy into the various emitting elements in the front of the [[antenna (radio)|antenna]]. AESA devices, in contrast, have each of their elements contain its own radiofrequency source. A PESA radar is therefore simpler to construct than an AESA.
Most phased array radars in the world are PESA. [[Microwave Landing System]] uses PESA transmit-only arrays.
