A remote-sensing technique in which surface altitudes (elevations) are estimated as a function of the travel time of a pulse of electromagnetic radiation transmitted from and received by a precisely located altimeter. Altimeters are mounted on either satellite or aircraft. Satellite altimeters use on-board Global Positioning System (GPS instruments and star trackers to determine orbital position and altimeter pointing angles. Aircraft systems measure the altimeter trajectory using GPS and inertial navigation systems. Accurate altimetry measurements, especially those acquired from space, require corrections for variations in atmospheric and ionospheric conditions, and for variations in orbital position of the sensor. Altimeters are either laser altimeters or radar altimeters. Each of the two radiation bands has strengths and weaknesses with respect to footprint size and ability to sample through atmospheric obstructions such as clouds. Altimetry measurements are compared with surface elevations obtained at identical points in horizontal space at an earlier time to calculate elevation changes which can then be used to compute volume changes. The earlier elevation measurement is commonly obtained from a previous altimetry pass, but can also be derived from other sources such as topographic maps. A mass balance is obtained from knowledge of the ice-column density usually supplied by Sorge's law.