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 dept
h 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.
An instrument for altimetry that transmits and receives pulses of microwave radiation. Satellite radar altimeters (including ERS-1 and 2, Envisat and others) typically operate in the Ku band (13.5 ghz
; 22 mm wavelength) and were designed primarily for oceanographic monitoring. Because of their relatively large footprint (several km), they are best suited for measuring elevationsof gently-sloping regions of the ice sheets. Steep or undulating terrain produces complex waveforms and difficulties in achieving accurate estimates of range (i. E. Distance). Surface and volume scattering also affect the radar pulse and create uncertainty in the effective depth of the reflecting horizon. Surface dielectric properties and roughness that cause scattering are time-varying and introduce errors in calculations of elevation change. Recent radar altimeters use synthetic aperture processing (see insar) that increases resolution and decreases slope errors relative to earlier radar altimeters. The Shuttle Radar Topography Mission (SRTM) used a C-band radar (5.6 ghz; 54 mm wavelength) and synthetic aperture processing to obtain an accurate map of surface elevations with near-global coverage. Cryosat-2, launched in April 2010, will also use insar to map glaciers and ice sheets.