A flexible solid-state phase-tracking system designed for use on ionospherically propagated radio signals is described. The output is basically digital in form and suitable for use with autoiuatic data proceseing systems. The unit provides an output equal to the rate of change of phase or Doppler frequency of the incoming signal and Is sufficiently flexible to be used on VLF through HF signals. The design is described and specifications of the prototype unit included. Some examples of results obtained using ihe equipment are given.