Echo power spectra of asteroid Atira obtained at Arecibo Observatory on 2017 January 20. The OC signal (solid line) is considered to be a single reflection by a surface that is smooth in the wavelength scale; the SC signal (dashed line) is considered to be caused by multiple scattering or asymmetric single scattering by wavelength-scale or large particles. The ratio of these signals, SC/OC = μ _c , is 0.213 ± 0.011. The echo from the secondary is superimposed upon the primary's echo, seen as the large spike near 0 Hz. Δf is the Doppler frequency resolution.