All objects naturally emit thermal radiation. Measuring this natural radiation is called radiometry.  In passive mode, AVTIS captures the scene temperature by switching off the transmit signal and carefully recording the amount of natural radiation collected by the antenna.


Spectral Brightness distribution at a temperature of 300K




The spectral brightness distribution of radiation emitted by an object is described by the well known Planck radiation law. Unlike thermal imaging of heat in the infrared, mm-waves do not require a Planckian blackbody correction since they lie in the Rayleigh-Jeans region. This makes MMW radiometry a great deal simpler in principle: the power captured at the antenna is directly proportional to brightness temperature.

The most difficult part of radiometry is calibrating the receiver output. Fluctuations in the amplification chain can easily mask the tiny thermal signal of interest. AVTIS uses a two stage calibration process: the internal amplifiers are switched between the antenna input and an known electronic reference source to remove short term fluctuations in receiver gain. Over a longer timescale AVTIS performs an occasional reference scan of the known sky temperature distribution to achieve absolute calibration.