Abstract: The paper is in 2 parts. The first part is the analysis of why the low altitude is limited in frequency modification continuous wave Doppler navigation radar. It demonstrates that the radar works at zero altitude when J₁ (M₁) side frequencies are adopted. Here, J₁ (M₁) is the first order of Bessel function. Otherwise, there is a limitation in minimum altitude. The calculation method for this minimum working altitude is deduced. The second part is the analysis of why there is a low altitude limitation in pulse Doppler navigation radar and a simple and direct method for calculating the minimum working altitude is deduced. This method is used to calculate the minimum working altitude of AN-APN-220C Doppler navigation radar. It is found that the result is in agreement with the value measured by TELEDYNE RYAN Co. in U. S. A.