A Global Positioning System (GPS)- reflection/occultation interferometry was examined as a means of altimetry of water and ice surfaces in polar regions. In GPS- reflection/occultation interferometry, a GPS receiver aboard a satellite in a low orbit around the Earth is used to determine the temporally varying carrier-phase delay between (1) one component of a signal from a GPS transmitter propagating directly through the atmosphere just as the GPS transmitter falls below the horizon and (2) another component of the same signal, propagating along a slightly different path, reflected at glancing incidence upon the water or ice surface.
The integer-cycle phase-difference ambiguity is resolved by noting that both signal components eventually collapse into a single component, representing zero phase difference. From the phase difference and the known positions of the two spacecraft as functions of time, an atmospheric correction obtained as the main data product of the GPS-receiver mission, and basic geometry, the difference in length between the direct and reflection signal paths and the altitude of the effective specular-reflection point can be calculated. This method yields altitude at about 0.7m precision with horizontal resolution of a few kilometers.