SARAS measurement of the Radio Background at long wavelengths
  SARAS is a correlation spectrometer connected to a frequency independent
antenna that is purpose-designed for precision measurements of the radio
background at long wavelengths. The design, calibration and observing
strategies admit solutions for the internal additive contributions to the
radiometer response, and hence a separation of these contaminants from the
antenna temperature. We present here a wideband measurement of the radio sky
spectrum by SARAS that provides an accurate measurement of the absolute
brightness and spectral index between 110 and 175 MHz. Accuracy in the
measurement of absolute sky brightness is limited by systematic errors of
magnitude 1.2%; errors in calibration and in the joint estimation of sky and
system model parameters are relatively smaller. We use this wide-angle
measurement of the sky brightness using the precision wide-band dipole antenna
to provide an improved absolute calibration for the 150-MHz all-sky map of
Landecker & Wielebinski (1970):subtracting an offset of 21.4 K and scaling by
factor 1.05 will reduce the overall offset error to 8 K (from 50 K) and scale
error to 0.8% (from 5%). The SARAS measurement of the temperature spectral
index is in the range -2.3 to -2.45 in the 110 to 175 MHz band and indicates
that the region towards the Galactic bulge has a relatively flatter index.
