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 along the intervening media or in the receiving 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 wavelength $\lambda$. The unit vector $\mathbf{s}$ indicates the direction toward the source, and the projected baseline is perpendicular to it. The signal arrives at the reference antenna to the right first and then at the other antenna after a time delay $\tau_g$ related to the physical path length difference $\Delta l_g$ because

$$\tau_g = \frac{\Delta l_g}{c} = \frac{b\sin\theta}{c}$$

in seconds, which is related to the number of cycles ($\Delta l_g/\lambda$) and the phase difference $\phi$: