New Horizons' radio occultation of Pluto

Explanation: Historic occultation techniques exchange the locations of transmitter and receiver, but experiment fundamentals remain the same. Earth DSN transmits a monochromatic plane-wave of 7.2 GHz to New Horizons as it flies behind Pluto. The signal passes through Pluto's atmosphere and refracts, resulting in a small change of direction, exaggerated for clarity in Figure 1. The direction change manifests as a Doppler shift in frequency, therefore REX measures a signal rate slightly different from the original 7.2 GHz. The direction change, or bending angle, is proportional to dielectric constant in the atmosphere. Given prior knowledge of Pluto's atmospheric composition, we use dielectric constant to calculate temperature and pressure in the atmosphere as a function of altitude.

Pluto

Image credit: NASA/JHUAPL/SwRI/William Woods