Since the 1980s, the use of reference sources has been studied in application to adaptive optics systems aimed at studying the formation of laser beams in the atmosphere. The atmospheric turbulence, refraction, and thermal blooming are usually considered as the disturbing factors. The reference source is a coherent laser beam propagating backwards with respect to the principal radiation through the same inhomogeneities of the medium. The angular and spatial detuning of the reference and initial beams within the range of the isoplanatism of the propagation path is possible. It is known, from the general properties of the parabolic wave equation for media with fluctuations possessing the reciprocity, that the elementary spherical wave can be reconstructed ideally by the operation of phase conjugation only in the case that the operation is applied within an infinite initial aperture. I consider the possibility of using extended diffusely scattering objects and large-size mirror targets as the reference sources. A diffusely scattering surface (or volume) forms a reflected wave with some interesting properties, which allow the formation of an undirected reference source along almost any direction. Such a source can be a rather efficient reference source for a wide initial beam.