In a coding system that uses complementary codes, the decoding process favors the geophysical signal and other radar returns which have been coded by the radar system, while interference signals may be attenuated. The attenuation is relative to the geophysical signal and is dependent on the spectral characteristics of the interference signal and the code used. An interference suppression factor (ISF) is defined as the ratio (in dB) of the power gains of the interference to that of the geophysical signals, as both traverse the coherent integration and decoding stages of the radar signal processor. ISF values of codes, in and around the zero Doppler frequency band, are obtained analytically, while the corresponding values for other frequency bands are estimated from simulations performed using the sinusoidal interference signal. The results show that codes having an equal number of opposite elements have superior performances over other codes as they have better values of ISF in and around the zero Doppler frequency band, a segment of the spectrum where the geophysical signal is expected. This is confirmed by the observations made using the middle and upper atmosphere radar in Japan, where other experiments intended to study the effectiveness of different codes have also been performed, and the results are reported.