As an orbiting receiver watches a GPS signal set (or rise from) behind the earth, the signal must pass through the atmosphere in a limb-grazing geometry.  The atmosphere bends and delays the signal, increasing its effective path length and thus the number of cycles along the raypath. At the surface the increased path length caused by the atmosphere can reach 2.5 km — or more than 12,000 cycles at the shorter wavelength. By counting the extra cycles we can measure this "excess delay" with a precision of about 0.005 cycle, or ~1 mm — better than one part in a million. This accounts for the extraordinary performance of GPS-RO in recovering atmospheric profiles.