Little Big Horn
![The Calibration Horn Antenna at Green Bank](https://public.nrao.edu/wp-content/uploads/2013/09/gallery-images-largeCalibration-Horn-1967_large.jpg)
The Calibration Horn Antenna, nicknamed the Little Big Horn, at Green Bank in 1967. As its name implies, this 120-foot long horn antenna was used to measure the intensity of radio waves coming from the skys strongest non-solar radio source, Cassiopeia A. The Calibration Horn measured a total power output for Cas A at a frequency of 1.4 GHz (a wavelength of 20cm), and thus provided astronomers with a standard reference point on the sky against which they could measure other sources.
Credit: NRAO/AUI/NSF
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