Geometric dilution of position (GDOP) in position determination through radio signals

In a spherical or hyperbolic system of position determination, geometric dilution of position (GDOP) is a term commonly associated with errors in position, caused by the relative location of the radio sources and the platform. It is shown that excessive errors occur in two-dimensional position determination if and only if the platform is collinear with a pair of radio sources. The concept of collinearity can be expanded to include a global system in which flat earth approximations cannot be used.