A Doppler navigation system includes an altimeter section with increased accuracy resulting from closed loop operation of signal processing portions for reflected signals including phase shift information. Quadrature processing of the reflected signal includes compensation for differences between quadrature processing channels, wherein the reflected signals are periodically processed by alternate ones of the channels. A maximum amplitude signal is forwarded by the signal processing portions to a frequency tracker of the navigation system. Periodic calibration of the altimeter section corrects for delays introduced in the signal processing portions. Effects of FM to AM conversion in a Gunn oscillator are reduced by connection of the signal processing portions to a short circuit during the calibration periods.