Topocentric frequency detections (top, Hz) and frequency detections reduced to a common phase centre – geocentre (bottom, Hz). Jumps in frequency at 03:30 and 11:30 UT are due to the uplink frequency changes at transmitting ground stations. Mean geocentric frequency was converted to phase and applied to the spacecraft signal at correlation to avoid a frequency smearing at high spectral resolution. Station two-letter codes as in Fig. 2.