In this article, performance of estimation of signal parameters in PAssive Synthetic Array (PASA) is studied, focusing on the background of airborne platform intercepting narrowband microwave signals. Cramer-Rao Bound(CRB) of bearing is derived in case that none of bearing, frequency and amplitude is known. And a synthetic aperture selecting method is given to achieve the desired estimation precision. In addition, Maximum Likelihood(ML) estimator is derived for PASA. It is demonstrated that ML estimating error can converge to CRB quickly with increasing synthetic aperture length and Signal to Noise Ratio(SNR), but a threshold effect exists. The result is validated by computer simulation.