A direct multilateration target tracking system is provided with the TOA time stamp as an input. A technique of tracking targets with varying receiver combinations is provided. Methods of correlating and combining Mode A, Mode C, and Mode S messages to enhance target tracking in a passive surveillance system are provided. A direct multilateration target tracking system is provided by TOA tracking and smoothing. A technique for selecting best receiver combination and/or solution of multilateration equations from a multitude of combinations and/or solutions is provided. A technique for correcting pseudorange values with atmospheric conditions is provided. A technique for improving height determination for regions of poor VDOP in a 3D multilateration system is provided.