Effect of background velocity on the perturbed velocity caused by an example radial mode (l = 0, n = 20, ω_nl/2π = 2.90 mHz) on June 6, 2011, at 00:00 when the B₀ angle is close to 0. The left panel shows the line-of-sight component of the background velocity due to the satellite motion and solar differential rotation. The middle panel shows the amplitude (normalized by μ₀), and the right panel corresponds to the phase (in degrees) of the perturbed velocity computed using the HMI algorithm. An animation over one day of observation is available online, and it illustrates the variations caused by the changing spacecraft motion.