Improved Measurement of the Spectral Index of the Diffuse Radio
  Background Between 90 and 190 MHz
  We report absolutely calibrated measurements of diffuse radio emission
between 90 and 190 MHz from the Experiment to Detect the Global EoR Signature
(EDGES). EDGES employs a wide beam zenith-pointing dipole antenna centred on a
declination of -26.7$^\circ$. We measure the sky brightness temperature as a
function of frequency averaged over the EDGES beam from 211 nights of data
acquired from July 2015 to March 2016. We derive the spectral index, $\beta$,
as a function of local sidereal time (LST) and find -2.60 > $\beta$ > -2.62
$\pm$0.02 between 0 and 12 h LST. When the Galactic Centre is in the sky, the
spectral index flattens, reaching $\beta$ = -2.50 $\pm$0.02 at 17.7 h. The
EDGES instrument is shown to be very stable throughout the observations with
night-to-night reproducibility of $\sigma_{\beta}$ < 0.003. Including
systematic uncertainty, the overall uncertainty of $\beta$ is 0.02 across all
LST bins. These results improve on the earlier findings of Rogers & Bowman
(2008) by reducing the spectral index uncertainty from 0.10 to 0.02 while
considering more extensive sources of errors. We compare our measurements with
spectral index simulations derived from the Global Sky Model (GSM) of de
Oliveira-Costa et al. (2008) and with fits between the Guzm\'an et al. (2011)
45 MHz and Haslam et al. (1982) 408 MHz maps. We find good agreement at the
transit of the Galactic Centre. Away from transit, the GSM tends to
over-predict (GSM less negative) by 0.05 < $\Delta_{\beta} =
\beta_{\text{GSM}}-\beta_{\text{EDGES}}$ < 0.12, while the 45-408 MHz fits tend
to over-predict by $\Delta_{\beta}$ < 0.05.
