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This article presents a description of the computer program APAS, a sophisticated phased-array simulator software package for the design and analysis of a wide variety of planar phased-array antennas. It offers accurate, high-speed computation of the far-field patterns of planar phased antennas, including advanced far-field analysis capabilities and comprehensive two-dimensional and three-dimensional visualization of the simulated results. The program was written in MATLAB. Execution time is minimized by coding the program as matrix operations and using two-dimensional FFT (fast Fourier transform) techniques for calculating the far-field patterns. A unique feature is the possibility of synthesizing user-defined low-sidelobe tapers for both sum and difference patterns. APAS can handle aperture configurations with the array elements sited along rectangular or triangular lattices. The aperture may consist of subarrays. The program allows the calculation of the noise figure and the third-order-intercept point of both the array and the applied transmitting/receiving module. For tracking antennas with monopulse capability, there is the ability to determine the tracking slope for both the azimuth and elevation plane. The paper addresses an assessment of the program, details how the computations are carried out, and presents numerous simulations to demonstrate the main capabilities of APAS. A demonstration version is available on request.