A passive electronically scanned array (PESA), contrary to its active counterpart AESA, is a phased array which has a central radiofrequency source (such as a magnetron, a klystron or a travelling wave tube), sending energy into (usually digitally controlled) phase shift modules, which then send energy into the various emitting elements in the front of the 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.