Passive sensing involves only the use of receivers, with no transmitters involved. The signals sought by these receivers are the background signal (i.e., noise) floors that occur naturally, usually at very low power levels, and which contain essential information on the physical processes under investigation. Of interest are peaks in the background signal indicating the presence of specific chemicals, or the absence of radiation from certain frequencies indicating the absorption of the frequency signals by atmospheric gases. The strength or absence of signals at particular frequencies is used to determine whether specific gases (moisture and pollutants being obvious examples) are present and if so, in what quantity and at what location. Additionally, the brightness patterns over the Earth’s oceans and land masses provide key information on fundamental dynamic processes driving ocean circulation and diurnal land interactions with the atmosphere above it.