  Cross-power spectrum is a quadratic estimator between two maps that can
provide unbiased estimate of the underlying power spectrum of the correlated
signals, which is therefore used for extracting the power spectrum in the WMAP
data. In this paper we discuss the limit of cross-power spectrum and derive the
residual from uncorrelated signal, which is the source of error in power
spectrum extraction. We employ the estimator to extract window functions by
crossing pairs of extragalactic point sources. We desmonstrate its usefulness
in WMAP Difference Assembly maps where the window functions are measured via
Jupiter and then extract the window functions of the 5 WMAP frequency band
maps.
