Numerical Example Astronomical Radiation from CN

Doppler-independent heterodyne radiometry, on the other hand, requires only a 

We have described a selective heterodyne radiometer potentially useful in the detection of remote species such as pollutants and interstellar molecules. The system operates on the basis of the difference frequency between two radiated lines which, for closely spaced lines, is relatively insensitive to Doppler shift. This allows for the sensitive detection of known species moving at unknown velocities. The two frequencies may be obtained from individual transitions or 

Disadvantages of the system include the lack of Doppler information, the difficulty of observing absorption lines and continuum radiation, and the added complexity. Use of a calibration load is also more complicated than in the conventional case. Finally, there will be an uncertainty that the detected difference-frequency signal can be properly identified, in analogy with the identification problem for the Doppler shifted signal in the conventional configuration. Thus, the system should be used for the application in which it is most effective the search for a known emitting weak remote species with an unknown Doppler feature. 

Finally, we draw attention to a variation of this scheme, called heterodyne correlation radiometry [7.114] that should be useful for the sensitive detection of radiating species whose Doppler shift is known, but whose presence we wish 

Acknowledgement. I am grateful to the John Simon Guggenheim Memorial Foundation and to the National Science Foundation for generous financial support. I am also indebted to Rainfield Y. Yen for permitting me to present many of the calculations (in Sec. 7.3) that he carried out as a part of his Ph.D. thesis, and which we later published jointly. 

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