Abstract: Incoherent scattering radar use complicated data process, among which the calculation of auto correlation function (ACF) is very important, to achieve many high precision ionospheric parameters. We deduce the formula that includes ACF with ambiguity function, and emulate 13 bits barker code, integer and fraction of 16 bits alternating code (AC). We achieve two results. The first is for barker code:barker code only use lag0 to calculate ACF. The second is for AC code:fraction way has more delay points than integer way, so fraction way calculates ACF more precise with wide power spectrum. This is important for improving ionosphereic detection and studying ionosphere.