In order to design improved spatial compounding ultrasound scanning systems it is necessary to determine the correlation of speckle patterns as a function of aperture translation. The authors have conducted experiments measuring the speckle correlation as a function of lateral aperture translation for linear phased array pulse-echo ultrasonic imaging systems. Results are presented for variable frequency, range, transducer length, focus error and reflecting material. Tests were conducted on two commercially available and one research imaging system. The measured rates of correlation coefficient decrease are independent of frequency, reflecting material, and target range when the target is in the focal zone. The experimentally determined correlations are used to derive the optimal spatial separation of images for speckle reduction, and the benefits of spatial compounding for a fixed aperture system discussed