Abstract: Velocity ambiguity is an important problem in pulse Doppler radar.In the absence of other information sources,the range information measured by a radar can be used to resolve velocity ambiguity.The outputs of the range loop and velocity loop represent the range and velocity of the same target,and there is a certain relationship between them.The least square method used for velocity ambiguity resolution is derived,and a novel method which is called the invariant imbedding method to resolve velocity ambiguity is proposed.The theoretical analysis and simulation prove that the invariant imbedding method is effective in resoluting velocity ambiguity,and this method is the least square method in substance.The invariant imbedding method is equivalent to a compound system consisting of range loop and velocity loop in improving the range accuracy.