Many aircraft today rely on the Global Positioning System (GPS) to provide accurate position data for navigation.  This is particularly helpful over large distances, such as ocean crossings, where there are no ground-based radio navigation sources.  If GPS is not available, aircraft also use inertial navigation systems to estimate their position over time.  However, the position error of an inertial navigation system alone will drift over time, and its drift rate will increase over some period of time due to inaccurate estimation of platform velocity.  The position error can increase by many kilometers over the time of flight across an ocean.  The drift can be reduced by using a Doppler navigation system, which provides an independent source of velocity information. 
Conventional Doppler navigation systems require narrow radar beams aimed in multiple directions to achieve accurate measurement of Doppler frequency shift in each direction.  The Doppler frequency shift is proportional to the velocity in a particular direction.  Measuring velocity in three or more directions allows a Doppler navigation system to create a velocity vector, which can be integrated over time to compute the change in position of an aircraft.  This velocity vector can also be used as an additional velocity estimate input to an inertial navigator. 
Narrow radar beams used in Doppler navigation systems require large antenna surfaces and/or higher transmit frequencies.  The larger antenna surfaces are especially undesirable for small aircraft.  Higher transmit frequencies make Doppler navigation systems more susceptible to signal loss through air or clouds.