Remote sensing with an Active-microwave sensor or a passive-microwave sensor. At frequencies between about 1 ghz and 40 ghz, microwaves are capable of penetrating clouds, and orbiting sensors can measure surface properties in all atmospheric conditions. Corrections must be made for scattering resulting from atmospheric and ionospheric variations. At frequencies below a few ghz, the depth of penetration beneath the glacier surface becomes great enough to permit Active-microwave imaging or profiling of the subsurface from the surface or from aircraft. The terms microwave and radar are often used interchangeably. This is mainly because the boundary between the lower-frequency radio and higher-frequency microwave regions of the electromagnetic spectrum is fixed differently, between 0.3 and 300 ghz (wavelengths of 1 m to 1 mm), by different authorities.