Classification of EM Systems

  1. Waveform transmitted
    1. Frequency domain systems or continuous wave systems transmit sinusoidal currents at one or more fixed frequencies
    2. Time domain systems transmit time-dependent sequences or pulses which individual contain many simultaneous frequencies
  2. Active vs passive systems
    1. Active systems - control of transmitter location
      1. Tilt-angle methods - mapping orientation of major axis of polarization ellipse (VLEM)
      2. Phase-amplitude measuring systems (HLEM)
    2. Passive systems - utilize EM fields from distant sources
      1. VLF - tilt angle method using naval communications antennae
      2. AFMAG - tilt angle method using EM energy (sferics) from distant thunderstorms