Abstract: The attenuation of electromagnetic waves by a transmission medium is detected or measured by directing a continuous periodic electromagnetic wave toward a reflector. The reflector is modulated to modulate the electromagnetic wave as it is reflected. The reflected wave is received and an output signal related monotonically to the power of the modulated reflected wave is detected. The output signal is then compared to the output signal which is or would be obtained under various other transmission conditions.

Abstract: A radiometer on an orbiting spacecraft derives high spatial resolution information from terrestrial and atmospheric regions. N elements or subapertures on the spacecraft transduce electromagnetic energy into electric signals.Many or all of the elements are simultaneously illuminated by electromagnetic energy radiated from the same region. Identical, parallel processing channels are responsive to the N elements. Each of the channels includes a variable gain amplifier responsive to the signal transduced by its corresponding array elements. The gain of each amplifier is controlled as a function of the output difference when the channel is connected periodically to each of a pair of Dicke noise sources, such as resistors maintained at predetermined temperatures.