Abstract: A novel coherent detection spatial tracking system based on a highly sensitive angular discriminator was designed. The incoming signal beam and the local oscillator (LO) beam was split into two paths in order to estimate the azimuth and elevation angles of the incoming signal beam. The optics design in each path comprised of an electrooptically tuned Fabry-Perot Interferometer (EO- FPI) and two photodetectors. The only difference in the two paths was the orientation of the EO-FPIs. In this paper, the azimuth and elevation angles of the incoming signal beam are calculated given that the incident angles of the signal beam at the two EO-FPIs are measured by the FPIs. It will be shown that the solution is obtained at the intersection of two ellipses centered on the principal axes of the reference coordinate system. Each of the ellipses is the locus of all possible angular direction of the signal beam with respect to the normal of the EO-FPI. This exact solution can be approximated by the intersection point of two straight lines. Each straight line is tangent at the point of the ellipse intersecting the principal axis. When the angular bias of the EO-FPIs deviates from its nominal value, the location of the ellipses will be rotated. This will cause the estimated signal beam direction to be angular biased from its true direction. The effect of the deviation of the EO-FPI's angular bias must be corrected in order for the coherent detection spatial tracking system to work properly. !7
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