A passive electronically scanned array (PESA), also known as passive phased array, is a phased array radar which has a single central radio frequency source (such as a magnetron, a klystron or a travelling wave tube), sending energy into the multiple phase shift modules, which then send energy into the individual emitting elements in the front of the antenna. This contrasts with active electronically scanned array (AESA) devices, which have a separate radio frequency source for each phase shifter/emitting element. A PESA radar is therefore simpler to construct than an AESA.