integrated contribution of extragalactic continuum sources, or the CMB. Second, for similar reasons, it is difficult to avoid confusing any systematic effects in the measured spectrum due to the instrument or environment with received signal from the sky. The severity of these problems is exacerbated in single-antenna measurements by the intensity of the Galactic synchrotron emission. Unlike interferometric observations, a single antenna is sensitive to the large-scale emission from the Galaxy, providing a 200 to 10,000 K foreground in the measured spectrum.

Determining the $\sim25$ mK redshifted 21 cm contribution to the radio spectrum requires separating the signal from the foreground spectrum at better than 1 part in 10,000. This can be accomplished by taking advantage of the differences between the spectra of the Galactic and extragalactic foregrounds and the anticipated redshifted 21 cm contribution. As discussed by , Galactic synchrotron emission is the dominate component of the astrophysical foregrounds below $\nu<200$ MHz, accounting for all but approximately 30 to 70 K of the foregrounds at 178 MHz [@1967MNRAS.136..219B]. Its spectrum is very nearly a power-law given, in temperature units, by $T_{gal}(\nu) \sim \nu^{-\beta}$, where $\beta\approx2.5$ is the spectral index. The spectral index is generally constant over the frequencies of 