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 ~ { } \textgreater~ { } \beta~ { } \textgreater~ { } -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 ( 34 ) 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 ( 14 ) and with fits between the ( 17 ) 45 MHz and ( 19 ) 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 } { \color { black } = \beta _ { \text { GSM } } - \beta _ { \text { EDGES } } } < 0.12 , while the 45-408 MHz fits tend to over-predict by \Delta _ { \beta } < 0.05 .