  We present Global MOdel for the radio Sky Spectrum (GMOSS) -- a novel,
physically motivated model of the low-frequency radio sky from 22 MHz to 23
GHz. GMOSS invokes different physical components and associated radiative
processes to describe the sky spectrum over 3072 pixels of $5^{\circ}$
resolution. The spectra are allowed to be convex, concave or of more complex
form with contributions from synchrotron emission, thermal emission and
free-free absorption included. Physical parameters that describe the model are
optimized to best fit four all-sky maps at 150 MHz, 408 MHz, 1420 MHz and 23
GHz and two maps at 22 MHz and 45 MHz generated using the Global Sky Model of
de Oliveira-Costa et al. (2008). The fractional deviation of model to data has
a median value of $6\%$ and is less than $17\%$ for $99\%$ of the pixels.
Though aimed at modeling of foregrounds for the global signal arising from the
redshifted 21-cm line of Hydrogen during Cosmic Dawn and Epoch of Reionization
(EoR) - over redshifts $150\lesssim z \lesssim 6$, GMOSS is well suited for any
application that requires simulating spectra of the low-frequency radio sky as
would be observed by the beam of any instrument. The complexity in spectral
structure that naturally arises from the underlying physics of the model
provides a useful expectation for departures from smoothness in EoR foreground
spectra and hence may guide the development of algorithms for EoR signal
detection. This aspect is further explored in a subsequent paper.
