Precise Measurement of the Absolute Sky Brightness at 60 to 350 MHz

We present a precision measurement of radio sky brightness over 60-350 MHz. Accurate knowledge of the radio sky over these frequencies is essential for modelling foregrounds in experiments targeting cosmic dawn and the epoch of reionization. Measurements below 1 GHz are also needed to understand the Galactic cosmic ray electron spectrum, constrain nanojansky radio source populations and dark matter models, understand origins of the diffuse radio background, and improve calibration of long-wavelength radio telescopes. Here we show that current all-sky maps and the 2016 Global Sky Model (GSM2016) require substantial corrections over 60-350 MHz. GSM2016 has an offset exceeding 100 K below 100 MHz and must be scaled up by a factor increasing from 1.2 below 200 MHz to 1.5 at 350 MHz. This significantly increases previously inferred excess radio background, motivating review of faint source populations and dark-matter decay models. Our measurement used a new receiver architecture that self-calibrates its noise contribution and bandpass in situ while connected to an antenna. We used a single, accurately modelled, log-periodic SKALA4.1 antenna on a 40 m diameter SKA-Low station ground mesh. Sky models scaled to our measurements can set the absolute flux-density scale for SKA-Low and other low-frequency radio telescopes.