Microwave sensing is important in the monitoring of permafrost because the factors that stand in the way of visible sensing are not impediments to sensing in the microwave spectrum. Microwave sensing is effective in detecting presence or absence of ice, land temperature, and freezing or thawing of soil surface. Microwave sensors are divided here between passive and active sensors. Passive sensing measures the radiation naturally emitted by an object. Active sensing sends stimuli down to Earth to collect data.