Stereoscopic helioseismology. Numerical simulations showing the spatial sensitivity of helioseismic holography to the meridional flow at latitude 75° and radius 0.7 R_⊙. The resolution is poor when using only observations of solar oscillations in the equatorial plane (left panel, data coverage indicated in red). The resolution approaches the diffraction limit of λ/2 ≈ 38 Mm when combining the previous data with observations from a LOS inclined by 35°. The noise in the measurements may be very high; it depends on the total duration of the observations. See Gizon et al. (2018) for a discussion of signal and noise in helioseismic holography.