A method of, and by extension an instrument for, detecting and locating objects by sensing radiation transmitted by the instrument and reflected from the objects. See Active-microwave sensor. The depth to which a radio or microwave signal is likely to penetrate ice or snow before being absorbed or scattered depends on the frequency (or equivalently the wavelength). In the case of scattering, the penetration depth also depends on the size of any inhomogeneities in the ice; those smaller than the wavelength of the signal cause less scattering. In glaciology, the lower frequencies (about 2 to several hundred mhz) are the basis for ground-penetrating radar (see also radar method), while frequencies of 1 to 15 ghz, at which effective penetration depths can still reach some metres, are used in radar altimeters mounted on aircraft or satellites (see also insar, Shuttle Radar Topography Mission). Radar is an acronym standing for radio detection and ranging.