1 An apparatus for tracking lateral movement of an object having a reference mark with at least two symmetric lines, comprising: a first probe beam projector to project and to scan a first probe beam repeatedly on said reference mark of said object, wherein said first probe beam is scanned perpendicularly to a first symmetric line of said reference mark, wherein said first probe beam projector also produces a first reference pulse for each scan of said first probe beam; a second probe beam projector to project and to scan a second probe beam repeatedly on said reference mark of said object, wherein said second probe beam is scanned alternatively with said first probe bean and perpendicularly to a second symmetric line of said reference mark, wherein said second probe beam projector also produces a second reference pulse for each scan of said second probe beam; a photo-detection means receiving scattered light from said object to produce a scattered-light signal consisting of a sequence of symmetric-profile signals, of which each symmetric-profile signal corresponds to a scan of said first or said second probe beam and indicates with the timing center of said each symmetric-profile signal the instant position of a symmetric line perpendicular to said scan; and an analyzing electronics coupled to said first probe beam projector, said second probe beam projector, and said photo-detection means to generate a first output signal and a second output signal, wherein said first output signal indicates the position change of said first symmetric line with respect to a first reference position so as to track movement of said object along the scanning direction of said first probe beam, and wherein said second output signal indicates the position change of said second symmetric line with respect to a second reference position so as to track movement of said object along the scanning direction of said second probe beam.