Approximations for Converting Geodetic to Cartesian Coordinates

Methods are investigated which apply to the conversion of latitude, longitude and height to cartesian coordinates in a plane tangent to the earths surface in connection with onboard missile targeting. The criterion for the usefullness of the method is the error in the north and east coordinate with reference to Clarkes spheroid 1866. This error is determined as a function of vector length and azimuth for the spherical earth model referenced to the mean geodetic latitude at White Sands Missile Range using a program prepared for a programmable electronic desk calculator. An approximation of geocentric latitude is used in the program. It is explained how the error inherent to the spherical earth model can be reduced applying a certain correction. An expression for computing the reference coordinates in a plane tangent to the earth spheroid is derived which requires less numerical effort than the standard procedure.