In radio interferometry, multiple radio antennas or apertures (elements) are used for observation in order to increase angular resolution. Without interferometry, radio telescopes would have resolutions comparable to the size of the sun and the moon. Let us begin our discussion by understanding a standard two-element interferometer in one dimension.

Consider a simple source in the far field generating plane waves at a single frequency $\nu$, two identical antennas receiving only one polarization at $\nu$, without any distorting effects either in the intervening media or in the reception system. The geometric configuration is shown below.

The baseline vector $\mathbf{b}$ between the phase centers of the antennas is written $\mathbf{b}_\lambda$ when expressed in terms of the wavelength $\lambda$.